Ask Ted!

If you have any questions you want answered, feel free to drop me a note. If you’ve got a question, chances are, there are lots of others out there with the same question. So ask away!

Note: all comments are moderated unless I’ve approved one of your previous comments. Almost everybody gets thrown off by this, but I moderate comments to avoid spammers. The downside of this is that you won’t see your comments post until I’ve had a chance to review and approve them. Sometimes this can take days (sorry!) Thanks for your patience.


1,266 thoughts on “Ask Ted!

  1. Hi Ted,

    I found your blog to be very informative and wanted to ask a question, have you ever see faulty spray foam cause moisture and mold issues? We have a very severe moisture problem (and hence mold problem) in our house ever since we made our walk up attic a finished space about three years ago. The space has open cell spray foam and knee walls. No dehumidifiers in the attic and the bathroom fan vents through the attic out the roof. An old duct vent also sits in the bathroom and was not closed up by the last contractor going strait into the attic. Further more we have an encapsulated crawl space in the basement which is rather dry and has one of those EZ Breathe Ventilation Systems. The entire house is heated and cooled by Mitsubishi mini split systems. Upon inspection an energy audit company explained that the spray foam job was not done correctly and we have air leaks in in the section between the roof and the knee wall. We are looking into this further but I feel the moisture levels are so high that something else may be the cause. The moisture and mold is interestingly sitting in the stairwell wall cavities which we know due to air testing by our mold remediation company. Last year we had an HVAC, which sat behind the knee wall, over condensate and dump buckets of water into our walls. This was remediated and repaired or so we though until the mold smell came back this summer. We are trying desperately to pin point the root of the issue so that we can get everything fixed and don’t have this problem a 3rd time. Figured it would not hurt to ask here and see if anything immediately jumped out to you as the cause.


    • The higher humidity could be a side effect of the foam but not the fault of the foam per se. Because foam is so effective at air sealing the house, even with some imperfections it is much better at air ceiling than any other method of insulation. What happens is that if there are moisture issues in the house, a foam house is less forgiving because there is less air exchange with the outside. So any excess moisture in the house will be more likely to be trapped in. The big question is what is causing the excess moisture in the house in the first place. Since you are using a mini split, you may wish to run that in its dehumidify mode to pull the moisture out of the air. This may help to reduce the issue during times of high humidity. The other thing that is counterintuitive is you should not leave the house open in the morning because the humidity levels outside in the morning are extremely high and could be making the problem worse. I would just try running dehumidification constantly and keeping the house closed up to see if that helps enough.

      • Thank you so much for your answer! We cannot sort out the cause of the humidity, at least not yet. We switched all the mini-splits to dehumidify a few weeks back. We now have an industrial dehumidifier in our attic and every 3-4 hours it is still pumping out about 5 quarts of water and has been for over two weeks. We have a few specialists coming out in the next few days so hopefully we can locate an answer. In the meantime, your articles have been very informative!

    • A continuation of my reply, now that I’ve had a chance to re-read your original question. I had a migraine and a fuzzy head when I sent my first reply.

      You wrote: “An old duct vent also sits in the bathroom and was not closed up by the last contractor going strait into the attic.”
      I’m not sure I understand this. What do you mean by “sits in the bathroom”?
      I checked out the “EZ Breathe Ventilation System” and am dubious about this for a couple of reasons.
      First, if it sucks air out of the crawlspace and vents it outside, that air has to be replaced by an equal amount of outdoor air, being drawn in somewhere. That outdoor air has all the moisture in it that’s outside, so this is no different than installing a fan blowing humid air into the house. This will almost definitely create moisture problems if it’s running all the time.
      Keep in mind that humid air rises up through the house and collects at the highest points. Hence attic/upstairs rooms tend to be the ones that get loaded with moisture. The ventilation system may be removing humid air from the crawlspace but it’s sucking it in anywhere else it can come in.
      If you want to do an experiment, turn off the EZ breathe system for a week or two and observe what happens in your house as far as moisture levels.

      • Ted,
        I have a curious question for you, but it’s a subject you’re an expert in: condensation. This summer I’ve been working on erecting one of the temporary carports that you see at Harbor Freight, Canadian Tire, and the like. ShelterLogic is another name brand of them. They’re basically a metal tube frame that a tarp is secured over. I reinforced mine with a wood sub-frame to stiffen it up and for when the tarp has served it’s purpose, I can sheath it in another material: corrugated metal, siding, etc.
        My issue is there is a tremendous amount of condensation on the inside of the tarp and metal framework. I know it’s not airtight, and I don’t think there’s really a way to make them airtight. It is warmer inside the tent than the ambient air temperature, so I’m assuming the difference in temperature is what’s causing the condensation? This isn’t a few beads running down the sides, though. This is like someone sprayed the whole inside with a hose. If you take a broom and hit the ceiling it will rain on you. For the floor I put down plastic sheeting and then stone on top of that in hopes that it would keep ground moisture from rising up into the space. Do you think that this is from ground moisture? Should I put another tarp over the stone? Or is this just cooler air migrating into the tent and then condensing based on the change in temperature? What’s the best way to address this? A couple of vents on either end of the structure?
        All the best, Andy

      • Hey Andy,
        Before reading your entire post, the thing that came to mind was ground moisture. In order to get that much condensation, there’s got to be a massive source of moisture.
        I have experience heavy metal objects in my garage getting covered with water when I leave the doors open on a humid day after a cold night when everything inside has cooled off. But given the extent of the moisture that’s accumulating, it really seems like a ground moisture issue.
        I had a wet dirt crawl space under a condo that was rotting out the floor joists above the crawl space. To remedy, I used thick plastic sheeting with all seams overlapped by several feet and taped. Then I put another layer on top of that and did the same thing. Then, I used spray foam to glue and seal the plastic to the foundation walls. It was probably overkill but it stopped the moisture problem instantly. So much water was coming up through the ground that puddles would form under the plastic.
        So that’s my best guess. I think you did the right thing, but you might just have to work extra hard to ensure that the plastic is completely water tight.
        But, it still could just be so humid out that the air is saturated with moisture and that’s causing the issue. But then I would expect the entire outside of your house to be covered in moisture every morning too.

      • Ted,
        I did put heavy plastic sheeting under the stone that I laid and was careful not to punch too many holes in it, other than for the landscape stakes I used to hold it in place. I’m sure there’s always a chance that the stone has poked holes in it. I have noticed that every since I have placed the tarp over the structure that the stone has been a dark color, like it’s damp, compared to before I had the tarp over top, and it would dry out. I didn’t know if the stone was damp because of the condensation dripping on it, or if it was the source of the moisture. We have had lots of rain over the past couple weeks/month, so it’s 50/50 whether it’s humidity in the air, or from the ground.
        Should I try to disperse the existing condensation on the walls and ceiling via squeegee, or would it rectify itself once I solve the ground moisture issue? Should I put a couple vents on the end walls?
        It seems like there’s been constant condensation every since I put the tarp on: warm, cold, dry, raining, it’s always there.

      • That is the weird thing. It seems like you did the right stuff to protect against this happening.
        It sounds like the stone is absorbing moisture from the air and holding it. Since it has more thermal mass, it will promote condensation formation then soak in the water, forming a reservoir. The water could then be coming out of the stones, getting trapped in the structure and exacerbating the condensation issue. Just a guess.
        You might try laying plastic sheeting or tarps over the stones for a week to see if that makes a difference. Anything that can hold moisture in the structure should be eliminated
        Fans can help a lot to flush out moisture and reduce condensation. Ideally, you’d put fans at either end of the structure, one blowing air in and a second flushing it out, to promote air movement through the structure. Then run them when it’s dry outside. But it’s a real battle during spring and fall when it can be so humid outside.
        Let me know if you figure this one out. It’s something of a mystery why the condensation would be so bad.
        As far as the squeegee, that would certainly speed the process. But if the condensation just comes right back, it would be a waste of time.

      • Thanks again, Ted!
        I’m going to get a big tarp, maybe even big enough to double, and place it over the stone on the ground. I don’t have anything in there yet as I’ve still been working on it, and then noticed the condensation issue, so wanted to try and correct that first! I definitely don’t have anything in there that has any thermal mass: a fiberglass ladder and some sawhorses, some leftover lumber, etc.
        I may try getting some vents or cold air returns and putting them at the top of the front and back end walls, just to try and promote cross ventilation.
        I’ll let you know what seems to rectify the issue once I know!
        Thanks again!

  2. Hi Ted,

    I live in East Tennessee and have a house built in 1970. Lots of large trees around but the sun does hit my roof about midday for several hours. My living room has a vaulted ceiling and that room is the only room in the house that is above a crawlspace. The vaulted ceiling is painted sheetrock with wood beams that were covered and trimmed out with wood trim and paneling when the house was built. There are no recessed lights. I have had a terrible humidity problem in this room. Moisture seems to collect along the point of the ceiling along the beam and on the sheetrock At the peak. The moisture then leads to surface mold and a funky odor in the room. When I clean it it appears to be only on the surface and nothing feels soft or damaged. I recently had my crawlspace encapsulated with a dehumidifier and also had a new hvac system installed as well as new windows and doors. It seems to have helped somewhat but not entirely. Additionally, before the encapsulation and new hvac, there is one ac vent on a wall about 8 feet high and it would sweat and water would run down that wall. Then water would form on the opposite wall at the peak of the ceiling and run down that wall. So far that has not happened yet this summer but we are just getting started with the heat and humidity. This mainly all happens in the summer btw. Also, there is a ridge vent on this house which I don’t believe was common practice when it was built. I never got a chance to speak with the previous owner about whether or not she had this problem prior to the ridge vent and she has since passed. The portion of the house that has the vaulted ceiling has soffit vents as well as this ridge vent. The rest of the house has a gable vent on one end and the ridge vent. Another issue is when I get up there to clean the beam in the living room I notice little leaves and seeds stuck between the moldings. I’m thinking this ridge vent is causing this and wondering if it is the source of all my problems. Any insight you could give me would be greatly appreciated!


    • Hi Kip, sounds like there’s a lot of humidity around. Could be coming from the crawl space or just general humidity in the outdoor air. From the actions you’ve taken care of the crawlspace but it’s worth measuring the humidity level in there. You can get a humidity gauge from Amazon for about $10, so it’s worth getting a few. With what you’ve done, your crawlspace should be well below 60% humidity but it’s worth checking. If it’s high, then the mitigation efforts you’ve taken may have to be checked to ensure the encapsulation is complete.
      I’d also measure the humidity at a couple spots in the living room. A few feet off the ground and up near where you’re getting the mold. The humid air in the house will rise, so it accumulates at vaulted ceilings. Any place it condenses will be colder than normal, as you found with the air conditioner vent. The cold air is chilling the vent and it’s acting like a dehumidifier by causing the moisture in the air to condense on the vent. It’s also cooling the ceiling where it’s blowing, causing the condensation to form in those areas.
      If the humidity in the house is at normal levels, you shouldn’t get the condensation on the AC vent or ceiling, so the big question is where is the humidity coming from. There are a few possibilities…
      First, you might be unintentionally letting moisture into your house by opening windows for fresh air. This time of year, allowing the fresh air in brings tons of moisture in with it. If you then close up the house and run the AC, all that moisture has to be removed from the air. But it will also condense exactly the way you’re seeing. The only solution is to keep the house closed up tightly when the humidity levels outside are high. One of those moisture gauges is useful to have outside so you can know when it’s safe to open the house up and when you should keep it closed. I wouldn’t open the house up if there’s more than 60% humidity outside as a rule of thumb.
      If you aren’t opening the house for fresh air, the house could be very leaky. Windows, doors, other areas can let in copious amounts of humidity. You can tell some of this visually, and you’ll know where you have drafts during the winter. So that’s something to check.
      Another very common area for humidity problems involves leaky ductwork for the HVAC system. Especially air return ducts, which suck air back towards the unit (everything on the filter side of the air handler). If there are any leaks, which there always are, they will suck in moisture from the outside. If those ducts run through the attic or other areas outside the enclosed area of the house, they will suck in outside moisture and distribute it throughout the house.
      Given that this is an ongoing problem and there are several possible sources, it would be worth spending the money to have your home analyzed by an energy auditor who can do “blower door” and “duct blaster” tests and visually inspect the home for other issues. They should be able to pin-point the problem areas and suggest remediation methods. Without that, it’s going to be a lot of guesswork and you could spend thousands on contractors without getting anywhere.

  3. sorry another question – we have already ‘painted TnG and it supposedly just started leaking, frankly I believe it was the previous owner who knew it and did not disclose but that is another fight he already screwed us on the roof around the chimney – anyway are you recommending to sheetrock OVER the T&G or demo and then sheetrock this will be our last address want it done right

    • As long as the moisture issue is caused by condensation rather than a roof leak, if it were my house, I would just sheetrock over the T&G. If you wanted to improve the ceiling insulation in the process, you could first screw a layer of PolyIiso foam board to the T&G, taping the seams. Then screw the sheetrock through the foam into the T&G. This would give you a near perfect moisture barrier and really improve the R-value of the ceiling.
      Before doing any of that, again, if it were my house, I would cut out a section of the T&G and inspect the water damage in the ceiling and ensure that there aren’t bigger issues that need to be dealt with first.

  4. Hey Ted, I tried to post this last week so I apologize if it’s a double post. I had some trouble logging in to post my question and I’m not sure if it actually went through or not.

    I’m building a new house, ICF exterior walls, typical rafter roof system, unvented. This question is regarding the roof system. My original thought was I would put 2” of rigid foam above the rafters to reduce thermal bridging, then spray foam between the rafters. I’ve seen this done plenty but hadn’t looked at the details. All the details I can find show, from the inside to the outside, rafters, decking, foam board, then another layer of decking. Having to double up on decking would make this a no-go for me. I was wondering, since the second layer of decking has to be attached through the foam board anyway, is the first layer of decking absolutely required? Why could I not place foam board directly on the rafters, then decking over the foam board, with fasteners going through the foam board directly into the rafters? The only thing I could think of is that with the foam board not securely sandwiched between two layers of decking, maybe moisture could accumulate between the foam board and decking? That got me thinking even more, and I started wondering if instead of foam board, what if I just ran 2×4 purlins every 24” OC, which would space the decking off the rafters 1.5”, then when I spray-foamed from underneath, the foam would fill the cavity between the rafters and the decking. I realize there would still be thermal bridging wherever a purlins meets a rafter, but it seems like it would still be a good system. I would save on cost by omitting the foam board, and the additional amount of spray foam would be pretty small. I have not seen any examples of roofs being done this way, and I was wondering if there’s a reason why. Thanks.

    • I don’t know for sure, but I think the approach of: roof deck + foam board + another layer of wood is probably for structural reason. If you’ve got (for example) 2″ of foam board right on the rafters, then the screws that go through the roof sheathing have to pass through 2″ of non-structural foam. When there’s a shear force, i.e. a heavy snow load, the torque applied to the screws could cause the entire structure to slide down the rafters, breaking the screws.
      However, even if you roof decking screwed right to the rafters, then foam, and then another layer, you’re going to have the same forces, unless maybe they figure that you’d use a lot more screws since you could screw to the first layer decking anywhere? You’d have to ask a structural engineer why it has to be built this way. Sorry I can’t be of more assistance but I wouldn’t want to advise you on something with such structural significance.
      That said, my personal preference would be to put a layer of foam across the inner surface of the rafters. In this location, you’re not affecting the structural integrity at all and you’re putting up a layer that will greatly reduce moisture from getting to the roof deck.
      Since you’re building new, how about SIP roofing panels? They’re designed for this and just drop in place for the roof. My dream house would be a timber frame SIP house built by this builder;

  5. Ted,
    I live in the Mid-Cape area of Cape Cod, Barnstable County, Massachusetts. I have a gambrel style home. The main house, 2″ by 4″ wood frame exterior walls, with a 2nd story was construct
    Your professional expertise would be appreciued in the summer of 1976. An addition with a 2-car attached garage with apartment above was built in the summer of 1988. I have had Cape Cod Light Compact utilizing Rise Engineering as its energy subcontractor, has performed 3 Energy Audits over the last 10 years. In 1988 we added a central air conditioning system, Lennox ducted, serving the entire residence including the apartment area.
    Our entire heating system is a Weil-McLain Gold boiler running on Number 2 home heating oil.
    I am interested in utilizing a basement room, 4 poured concrete walls with a 4′ opening into the original poured concrete foundation basement with a walkout pass door.I was thinking of adding 2″ by 4″ fastened by mastic to the interior foundation and then drywalling same. This room has 2 basement casement Andersen windows that are installed below our wood deck and the room has a masonry fireplace with damper and 8″ by 8″ clay flue liner.
    Can or should I insulate the interior walls with pink Owens-Corning wall insulation before adding drywall or leave space uninsulated. I run a Kenmore dehumidifier year round through my Air Conditioner air exchanger pump in the main basement.
    Your professional expertise would be much appreciated by this retiree and planning to D-I-Y-ere.

  6. Ted, I have the classic T&G ceiling construction: The roof is installed over the rafters. Fiberglass is shoved between the rafters. T&G boards are nailed to the rafters. I do have ridge vents but no soffit vents to speak of. I removed the ridge vents several years ago but encountered condensation problem so I reinstalled them. My issue is leakage. What is the best way to solve this problem and the most economical way? I am certain the best solution is not the most economical way but like to hear from you about the different options available. Thank you for your help.

    • Unfortunately, The best way would be to remove the ceiling material and put up something like drywall or you could probably get by with foam board as a base layer that is airtight. Then you would install the t&g boards over that as an aesthetic layer. Alternatively, you could drywall or foam board over the T&G boards and if desired for aesthetics install new T&G over that. Some people try to just have plastic sheeting underneath the T & G boards but that is prone to getting holes in it or having open seams that allow moisture to travel through so I don’t really like it as a solution. The most economical would be to simply drywall over the existing ceiling and then it would behave like any other cathedral ceiling.

      • I am about to redo my roof. Do you recommend:
        1. Remove roof decking and existing insulation then cover with closed cell foam?
        2. Remove 1 feet or so of decking at peak of roof, remove insulation, then packed space with high density cellulose?
        3. Or any other methods you recommend.

      • That’s lucky, sort of 🙂
        If it were my house, I would pull the roof decking and spray foam directly to the back of the T&G, though you might want to have a layer of something behind it so the foam doesn’t ooze through any cracks and ruin the appearance of the wood. It could be as simple as cardboard. Better would be a layer of poly-iso board foam. It could 1/2, just cut to go in the space, then foam over that. That will completely air seal and insulate the space really well. And, if the roof ever leaks, the water will mostly just drain down the foam and out the soffits.

      • Is the ridge vent still necessary after the closed cell foam insulation is put down? I can’t add enough soffit vents due to how the house is constructed. Thanks.

      • The ridge vent isn’t absolutely necessary much of the time, however inspectors can be pretty rigid about them, so I would probably have one regardless of the soffit venting situation. With the foam, you won’t have the adverse effects that sometimes arise with the ridge vent actually sucking air from inside the house. So the vent will give any residual moisture a chance to diffuse away.

  7. Hi Ted, do you have any thoughts on supply & return vents in an unfinished basement? Most people seem to recommend closing them off perhaps depending on the season, but it makes me wonder if that would lead to moisture problems, or other issues.
    Sorry if you’ve covered this, it seems basic but I couldn’t find anything.

    • Hi Andres, thanks for the question – that’s a good one. I should write an article on it.
      As long as the basement is sealed off from the outside and is just an unfinished part of the house, I think allowing supply/return is beneficial. Here’s why…
      If closed off, the basement will build up humidity. It’s naturally more humid than the house because of moisture coming through the slab and foundation walls. With the vents circulating, the main house air, which is warmer and should be lower humidity, will help flush out the cool, humid air from the basement. It will also keep the basement warmer and closer to the the temperature of the home. This is beneficial as it reduces the condensation potential in the basement.
      This will naturally raise the humidity elsewhere in the house and also distribute whatever is in the basement air so I would encourage keeping the basement relatively clean and free of chemicals or at least make sure containers are well sealed.(lots of people keep nasty stuff in basements. I’m guilty of storing my paint down there.) You might want to install a replaceable filter on the return to minimize dust.

  8. Ted,
    I hope you and your family are well! I’ve begun insulating my concrete basement walls with rigid xps. I have left a 1/2″ gap between the concrete floor and the bottom of the rigid foam. My initial plan was to fill this gap with expanding foam, but then I questioned that plan thinking what if I ever develop a leak in the concrete wall, I would want the water to be able to escape and be a visual representation that there’s a problem I need to address in that location. But my brain also says I should seal that gap to keep indoor conditioned air from getting in between the concrete and rigid foam and possibly condensing or causing other problems.
    What do you think is my best plan of action for this specific issue?
    Thanks in advance, Andy

    • I had to ask “WWJD”? That’s, “what would Joe Lstiburek do?”

      Here’s his page on basement insulation. They seem to recommend a full wrapping of the foundation walls so that no air gets in to add to the condensation. I’ve always assumed that the foundation walls would be wet anyway because of moisture drive from the outside dirt. But when in doubt, follow Joe’s rules!

      • Ted,
        Me again! Thanks for your info on sealing the foam to the wall. I’m going to take your advice and seal the bottom of the rigid foam where it meets the basement floor. I’ve used a handheld temperate gun to take readings on different areas of the surface of the foam and noticed that the temperate would drop where there was an undulation in the concrete wall creating a small air pocket between the foam and wall. I’m assuming this is because air was migrating up from the floor and settling in these pockets.

        Part two: I’m going to insulate my whole basement with the rigid foam. Only about half of my basement will be finished in the traditional sense of a studded wall, electrical ran through the studs, drywall, etc. The other half of the basement is more utilitarian: storage shelves: washer/dryer, furnace, water heater/softener, etc. I know that I cannot leave the foam exposed because it’s hazardous if there’s a fire, but I also don’t want to build a full wall with drywall in this area. I thought about building a stud wall on the flat, reducing the depth from 3.5″ to 1.5″ while still giving me a structure to easily fasten a covering. I could use your advice on what may be an appropriate covering for this area that more than likely will never be finished. I’ve toyed with the idea of either cement board or hardibacker, as they’re unaffected by water and moisture, but very heavy. Thought about faceless drywall or denshield too. I thought maybe polyiso, but it could be easily damaged, and since the walls will have to dry inward, I don’t want to trap any migrating moisture within the wall assembly. I’ve also though about the plastic polywall paneling that they install behind sinks and in wet areas, but I’m afraid that would behave the same as polyiso with no ability to pass moisture. Am I missing any good ideas that you can think of? Thanks for letting me borrow your big brain!

      • Good questions. I seen some people use 1×3 nailers with concrete screws through the foam, then attach drywall to that as the fire-stop. You could likely get by with thin drywall to cut down on the weight. I had no problem hanging bathroom drywall (with the anti-mold facing). I’m not sure if there’s standardized requirements for code regarding the wall material covering the foam. Might be worth contacting the local official to see what their requirements are.

        Good luck with the project. Your basement should be noticeably more comfortable/efficient after that retrofit!

      • Ted,
        What is your opinion on flex duct vs metal duct. Obviously flex is pre-insulated and by name, flexible, which cuts down on elbows and by association, potential leaks, but that could be overcome with mastic and foil tape on a metal duct. And I have to think from a physics or air flow view, metal will beat flex every time. Is the only advantage of flex ease of installation, longer runs, less elbows, etc.? Kind of like the ventilation version of pex?

        Secondly, in replacing existing duct work, if using metal, would you insulate it? My thought was that metal duct work obviously transfers heat well and radiates that heat to some degree. If I were to use metal duct work in my basement and crawlspace, would that heat radiation be enough to somewhat heat the space by proxy, or as supplementation so it wasn’t as cold, or I didn’t need to supply as many vents, which could affect flow throughout the other areas of the house, or even worst case, cause need to upsize my furnace? The basement and crawlspace will be totally encapsulated with rigid foam insulation. I was going to supply a duct and register just for the crawlspace, but if the nature of the metal duct running through the space would be enough to acclimate the crawlspace, that would save me time, trouble, air flow, etc.

        Thank you!

      • In uninsulated spaces, like attics, I’ve found flex is very practical in my own home. As you noted, it’s insulated and one piece, so as long as you secure the ends well, you get no leaks. There are tables that show air flow resistance and you are absolutely correct – metal duct is superior. I just upsized the flex and made sure to use gradual curves. I think for most installations in attics, flex is better if done with a bit of care.
        In semi-conditioned spaces, like you noted, metal ducts will radiate a fair amount of heat. It can be very significant, especially with air losses that almost all of these have. Plus, metal is easier to suspend from ceilings. With flex in hanging situations, you tend to get a wavy runs. So I like your logic – metal makes sense in those situations.
        I’d suggest careful sealing of joints with “duct mastic”. It’s very sloppy but it’s the standard for the best quality/lowest leakage duct installations.

      • Ted,
        Happy Good Friday! I’m getting ready to finally close up the floor in my crawlspace, but before I do, and lose the access I have, I have a question. It’s a 14’x14’x2′ (to the bottom of the sub-floor) space with a single 6″ metal duct running through it supplying the room. It is totally encapsulated in 2″ rigid foam: floor, walls, rim joists, all seams air-sealed with expanding foam. Should I supply a 4″ duct supplying conditioned air to one of the outside corners of the crawlspace facing back towards the room, or do you think the heat radiation from the metal duct would be enough to keep the space conditioned?

        Happy Easter to you and your family!
        Best Regards, Andy

      • Happy Easter to you too. Let’s hope 2021 is a much better year than 2020!
        A small supply duct would certainly help flush out “stale” air from the space and reduce the chances of undesirable moisture issues. You might want some sort of damper on it so you can control the amount of air-flow.
        A small space like that is only ~400 cubic feet and you only need enough flow to flush it with enough air to exchange a couple volumes per hour. Let’s say 1000 cu.ft per hour or about 17 cubic feet per minute – about a quarter of a typical bath fan 🙂 You could probably get that with a two inch outlet on the 4″ duct. But it all depends on the conditions in the space and the air pressure in your ducts.
        To summarize – a little air flow should be sufficient and is better than none.

      • Awesome! I know you always have the answers! I had planned on running 4” flex duct into the space with a damper at the main trunk. In the crawlspace I had planned on terminating to a 2×12” register boot. I really couldn’t find a better type of termination. And the crawlspace is sealed from the basement. Where is the flushed air going to go? I know I’ll end up pressurizing the crawlspace to some degree, which isn’t necessarily bad, but is there any downside to that?

      • The excess air will just find its way out through cracks and holes – there are always some 🙂
        I’ve heard this method recommended many times by building science people and no one has mentioned any ill effects.

      • Yes, here’s another one for you. The floor joists in my crawlspace run perpendicular to the house joists, so as my 6” supply duct comes into the crawlspace, it runs into a 2×10 joist within 16” or less . My original plan was to head off one floor joist so I could use an elbow and create as gentle of an angle as I could until I transitioned under the joists. But what do you think about the heavy duty flexible metal duct that’s available? Would the air turbulence it creates with all those segments outweigh it’s convenience? I did see it’s available at my local home improvement store today in a 6” size which gave me the idea. I would attach a pic but wasn’t sure how.

      • I think I would use a normal metal duct elbow unless you had the space for a very gentle 90 turn for the flex duct (I’d imagine a circle with at least a 2′ radius).
        OTOH, if you’re just talking about the little supply duct for the crawlspace, the airflow requirements are so low that most precautions can be ignored. Also, no need to run between the joists, just drop the duct down and support it under the joists with wide enough hangers so the duct isn’t crimped.
        Here’s a couple of links that might be of use to anyone considering the details of running flex and the impact that bends, and crimps have to air flow

        Click to access Thermaflex-Air-Flow-and-Air-Friction-Brochure.pdf

      • Ted,
        I hope this email finds you and your family healthy and well!
        I’m working on the exterior of my home while the weather is nice and I would like to extend some of my overhangs, or build some awnings to cover my meters and air conditioner and to help keep rain and snow away from the foundation, provide some shade in the summer as well.
        It would be better for me to set the framework for these awnings on the roof over the exterior wall to help carry load, but that is a bit more involved with tying in to the existing shingles and pitch. My thought was to mount a ledger to the wall under the existing overhang within the space above the windows. I probably have close to a foot between the top of the windows and the bottom of the overhang. My concern is I have ridge and soffit venting. I know that if your attic is properly air sealed you don’t need to worry as much about venting, but my concern is in the winter, snow and ice would accumulate on the awning/roof extensions and would more than likely block the gap between the underside of the existing overhang and the top of the awning. Depending on the weather and snow accumulation, it may even be able to grow and push up into the soffit, depending on snow melt and refreeze, etc.
        Do you have any thoughts on how I could avoid this blockage that would more than likely come with winter other than lowering the attachment point of the awning, which I am limited by window-top anyway?
        Always appreciate your insight!
        Thanks again, Andy

      • That’s an interesting building question – never thought of that one before, but it’s certainly a good observation!
        One options might be to build a ventilation channel using something like Cor-A-Vent (see this link: That could go on the ledger board (so as to preserve the strength of the connection between the ledger and the wall). You’d have to leave an air gap so the air could move through the Cor-A-Vent and up to the soffits, and you might want to have some sort of “dam” to stop it from being snow covered.
        Just a thought. Off-hand, I can’t think of any other way around this issue.

  9. hi i just had windows installed and im freaking out because at the bottom corners where a nice wood frame is , if i push on the part closest to the window i can draw moisture/water onto my finger !! i think it starting to rot my wood framing. just on the bottom corners??? what is going ??

    • It’s common during cold weather for windows to have condensation on them, especially over night when the curtains are drawn. Even with high quality windows and good installation, I have to go around with a towel to soak up excess moisture on some mornings. Especially in my BR where the humidity is higher than other rooms and I have thermal shades on the windows.
      If you’re experiencing too much moisture, and you have curtains or other window treatments, you should try leaving them open a bit at night to allow some of the warmer room air to warm the windows. This can help a little, but it won’t solve the problem.
      It’s possible that you have other problems, but condensation is the most common winter moisture issue, so I’d start there.
      One final suggestion, if you’re using humidifiers, use them less or not at all for a while to see if that reduces the problem. Humidifiers usually dump far too much moisture into the air, leading to mold growth, condensation, wood rot etc. If you must use a humidifier, keep the humidity level in the room below 50% during the winter. 40% is better.

  10. Ted
    I wrote earlier today about removing/replacing insulation that has rodent droppings.
    I’ve been re-reading your older articles about air-sealing, and it sounds like the right thing to do.

    But two more questions occurred to me:
    1) I do not have bathroom fans to vent moist air. Should I install them (2 upstairs full baths). Your article said to vent through the roof.
    2) My living room has a cathedral ceiling. In the attic, this is a sloped area. I emailed the rep I talked to, asking him if they air-seal and remove/replace the insulation in that sloped area.

    Once I understand these two questions, I think I am ready to get this job done.
    But as I said, I don’t want to solve one problem only to cause another!

    Your articles have been very valuable!


    • I don’t see my original posting, so here’s a brief summary:

      I had bats/squirrels in my attic a few years ago. I had a wildlife guy put an exclusion device to get the bats out, then he sealed around openings. I haven’t noticed any sound or activity since.

      The attic insulation has droppings, plus the squirrels messed it up a bit.
      I had a rep from a company to evaluate. He said they would remove the insulation/droppings (suited up, house area lined with plastic), vacuum up all droppings/debris, disinfect, air-seal gaps, install new R-30 insulation.
      My original question was whether the air sealing would affect air flow or moisture in the house.
      But then more questions occurred to me as I re-read some of your articles. I sounds like the air-sealing is a “go”. But I had further questions (post above) about the bath fans and I’m asking the rep about the cathedral ceiling area.

      Nancy M

      • Thanks for your questions and notes 🙂

        So, yes on air sealing the area under the insulation when you pull the insulation and clean out the attic.
        Your question on bath fans is timely because it’s easier to do when the insulation is pulled. Make sure the bath fans are air sealed to the ceiling as well – that’s a huge source of moisture damage that almost everyone ignores. A little canned foam from the attic side around the perimeter of the fan box works great.
        The duct from the fan should go straight up (or nearly so) to a vent hood mounted on the roof. This allows the fan to work efficiently (the less duct the less air resistance) and keeping it vertical reduces the chance of water building up inside the duct.
        I’m not sure I understand about the cathedral ceiling. Are you saying that the cathedral ceiling is visible in the attic, like protruding through the floor sloping up? I’ve seen this type of construction a couple of times. If that’s the case, then yes, air seal any gaps (there shouldn’t be any unless you have recessed lights poking through the ceiling) and ensure good insulation covering it, just like you would do the floor of the attic.
        Hope that covers it. My head is a little fuzzy today. Feel free to shoot over more questions.

      • The cathedral ceiling (or just a high ceiling) is above my living room, but below the attic, if that makes sense. About halfway across the living room, the ceiling slopes downward. So from the “normal” attic area, there is a section that slopes downward, which corresponds (as best I can tell) to the sloped area above my living room.
        I emailed the attic guy about whether they remove/seal/replace insulation in that area. I hope to get a response.
        I’ll also ask about the bath fan installation. I just re-read your article about it, and I was thinking just as you said, that it makes sense to do it when the insulation is removed. But I don’t know that I’ll be able to coordinate two jobs that way. I may be stuck doing it afterwards (I don’t want the installer to have to deal with the attic/rodent issues).
        Would have been easier if the original builder had installed bathroom fans!

        Thanks for your response!


  11. Hi Ted,
    I need your help. My exterior back wall is getting blister or bubbles in the paint but I dont have a leak. who do i need to hire to know if the house is having humidity problems? Thank you in advance.

    • Often, though not always, bubbles are indicative of moisture trying to force its way through from the other side. It can generate a lot of pressure which will literally blow the paint off the wall on the inside. It is possible that there is water or moisture behind that wall, which I guess means inside of the wall. Is this a sheetrock wall? One thing you can do very easily is take a sharp knife with a thin blade and carefully push it into the wall to see how much force it takes. If it goes in easily like butter then the drywall is probably saturated with moisture. If it resists and you can’t push the blade in without exerting a lot of force, then the drywall is not wet. This should be a minimally destructive test as it will just leave a small slice in the wall surface That could be easily repaired if it turns out to be nothing. If it does turn out to be wet, then that drywall will probably have to be cut out and replaced anyway.
      It can be hard to find somebody who is good that specializes in tracking down moisture problems. If you Google “building science” and “moisture troubleshooting” in your area, you may have luck finding someone. I would suggest looking at a person or company’s website to see if they have information like mine on there or if they are just trying to sell a product. That should be a start. Feel free to ask more questions if you learn more about your situation and need some more tips.

  12. Your blog has covered all of my questions. As an electrical contractor I see everyone doing just what you say and it sickens me that they get away with doing half the job wasting the clients money. I stand there looking like the a nit picker trying to make sure all of these items are done and I will stand by it to the end. You give my facts another point of meaning now as I will be able to show Clients I am not the only one who does it right. I will now be fixing up the air flow issues just in time for the winter your donation will come when my family sees how much you have helped!

  13. I live in the San Francisco Bay Area peninsula area in an older home with a dirt crawl space, CAN lights in a vaulted ceiling, and fiberglass insulation between the ceiling and the roof. You describe exactly what I have experienced in the Springtime during the past several years. Water drips from the CAN lights onto the floor below the vaulted ceiling (when it is not raining outside and has not rained for many days). There is visible water damage to the ceiling that has accumulated over several seasons. I’d like to have a qualified professional assess my situation before contemplating repairs, but I am struggling to find the right person using the search terms you suggested. Can you recommend some one in the Bay Area who I can work with to better assess my situation and propose solutions? Thanks.

    • I’m afraid I missed your message when you first sent it sorry for the long delay. In the Bay area, there should be many options. You can look for people experienced in building science and moisture troubleshooting. A BPI certification is a good thing to look for. If possible find an independent company, not one that provides services like insulation or HVAC installation. I generally have been more impressed with specialists who focus on building science issues. They don’t have a vested interest in selling you something usually.

    • Thank you! There are a lot of good analysts out in the Northwest. You can try searching for “BPI certified” or “building science” or “home thermal imaging” for your area. The trick is finding people with experience and the right motivations. I typically suggest looking for independent people, not associated with an HVAC or insulation company. If you don’t have luck, email me with your town and I’ll try searching for someone promising.

      • It is great to have the info you provided. I am having water dripping from the light features in the ceiling. I also have fiberglass ceiling. Is there a way to fix the light features without redoing the entire ceiling? Will removing the moisture from the house (for example, seal the crawl space) help?
        I live in San Mateo city, Bay area, California.

      • Water dripping from the ceiling light fixtures is definitely a big problem!

        As you suggested, the first approach would be to greatly reduce the moisture in the house. I’d suggest buying a few of these inexpensive moisture monitors and placing them in a few locations in the house including the rooms where the light fixtures are dripping and the crawl space. This will give you better idea of the current humidity levels in the house and confirm your thought that the moisture is coming from the crawl space. It might be that the crawl space isn’t the source of the problem, which would save you a lot of effort. However, if your crawl space is noticeably wet, then you want to take care of that immediately. I had a townhouse with puddles on the floor from water seepage up from the ground! The previous owners never dealt with the moisture and it rotted out the floor joists under the kitchen to such an extent that the refrigerator was at risk of falling through the floor!

        Given how much moisture is building up in your ceiling, it is very likely that damage has already been done inside the ceiling. The dripping water means that the wood is probably already saturated. Over time, this will lead to mold and rot. At the very least, I would consider replacing those light fixtures with sealed, flush mount LED fixtures. When installed tightly against the ceiling, these will greatly reduce additional moisture from entering the ceiling and will help to reduce additional damage. But ultimately, I think you’ll have to bite the bullet and cut out a section of the ceiling around the light to assess the extent of the moisture in the ceiling.

        Here’s a link to a bunch of these humidity/temperature gauges at Amazon:

  14. I’m having a problem with condensation on my ceilings and bricks of my chimney. I have vaulted ceilings and there isn’t anywhere for venting. I’m at my wits end with this problem and I don’t know where to go next. Is there a way I can talk directly to you or send pictures? Please help!

    • Sure -send me anything you’ve got to
      The more details, the better. What the conditions are when it happens – outdoor temperature and weather conditions. Time of day. What was the day before like? And clear photos showing the area where the condensation occurs and the relative position in the room. It’s also useful to know where you live (city and state) so I can look up weather/temperature history.

  15. I have a 1980’s atrium. Fully HVAC with humidistat and exhaust fan. If I dont manually adjust humidity exhaust almost daily and sometimes hourly, condensation forms in the house.

    • That probably means there’s missing insulation in the areas of condensation. Moisture rises and collects at the ceiling so that’s usually where the problems are worst.

      • Thanks for quick response. The atrium roof is glass and the house surrounds the atrium. It is a t & g, cathedral ceiling on house. Is there way to insulate around atrium from the inside? Its a one story house with asphalt shingles and a 3″ roof deck which in the 1980’s was good enough for insulation.

      • Is the condensation forming on the glass or somewhere else?
        If it’s the glass, there’s nothing you can do other than reduce the humidity in the house or get more warm air movement up near the glass to try to keep it warmer.
        If it’s on other surfaces, like the T&G ceiling, that can be more problematic. Again, any time you get condensation, the first thing is to review your home’s humidity. This can vary greatly from room to room depending on usage. For example, the humidity in my bedroom at night increases considerably because it’s sealed up and we’re breathing all night! Other examples are near showers, if the bathroom isn’t being vented adequately and in areas with lots of house plants that release a lot of moisture into the air.
        If you’re using a humidifier, try not using it for a while and see how that changes things.
        You can get little humidity meters from Amazon for cheap. If possible, put them close to the places where condensation is forming so you can see what the level is right there. Since the moist air rises, the humidity at the ceiling can be significantly higher than a few feet off the ground.
        Here’s one example of the gauges I’m referring to

  16. I have a 2-story property in Huntington Beach, California that was built in 1964. It was renovated in 2017. Starting last December, two sections of the property is showing excessive condensation and moisture accumulation. This condensation problem is mainly on the north facing areas. One area is on the first floor, and the other area is on the second floor, and these areas are not on top of each other. I am using a 50 pint dehumidifier and DampRid closet hangers to absorb the excess moisture continuously, but I want to know what is the cause of this so I can fix it. I greatly appreciate any feedback. Thank you.

    • Huntington Beach has a very mild climate, so it’s surprising that you’re having condensation problems.
      What surfaces is the condensation forming on?
      It’s common for people to develop condensation issues after a renovation due to the improved construction which makes a home “tighter.” The tightness is good, but that also means one has to be careful about uniform insulation and reducing interior humidity sources.
      Condensation will form on the colder surfaces in a house. Often, that’s windows, which is almost unavoidable if there’s excess moisture in colder climates. In your climate, it’s less common.
      I’d do a couple of things
      1 – check for interior moisture sources. Most common is showers that aren’t properly vented. I always recommend checking the bath fan for good suction at the intake. Then, make sure the fan runs during the shower and for at least 15-30 minutes after you finish showering in order to remove the excess moisture.
      2 – monitor the humidity in those rooms where condensation is occurring. You can get these on Amazon so cheaply –
      If you find the humidity in those rooms rises up past 60%, you know there’s too much moisture in the air under most circumstances. During the summer, it can be hard to keep it down.

      I checked the daily humidity numbers for your area in December and they were very high – 60%-80% for many days. If you open the windows, especially at night or early morning when the humidity is very high, the house will fill with moisture which will condense easily. Unfortunately, that’s unavoidable. The humidity drops quickly as the day warms so you can flush out the extra humidity by opening up the house on non-humid days in the afternoon. But it can be a tough battle in your climate.

      I’d start with those little humidity gauges placed throughout the house. See what the indoor humidity is like and if it is associated with any particular usage patterns, like opening windows or showering or cooking. Keep using dehumidifiers when the house is closed up, that might be your only solution during the colder months. During the summer, running the air conditioner will suck the moisture out of the air quickly.

      Hope that helps.

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