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.



1,161 thoughts on “Ask Ted!





    • The moisture is very likely coming from inside the house, not outside since the mold is forming on that beam. I wrote extensively on this in a recent article that you can find on my homepage.
      Humidity at the ceiling is higher so that 47% in the living space is higher at the ceiling. If the beam gets cold at night, water can condense on it.
      You could try blowing fans at the ceiling to circulate air and speed up evaporation. If you have a ceiling fan, use it to help air circulation.

  2. I have a metal roof over a vaulted ceiling. Condensation has created mold. After having company remediate and restore, the mold is back. I am told I need ventitation. The roof is tongue and groove; rafters are 16″ apart. Is it possible to ventilate without an attic? What do you recommend?

    • Again, apologies for the delay in responding.
      If there’s nothing underneath the T&G (between that and the insulation), I hate to say it, you’re in trouble.
      T&G provides zero resistance to moisture from the house getting up into the ceiling cavity. As such, even adding ventilation above it will do little to improve the situation. In fact, it can make it WORSE because the ventilation will increase the flow of humid air from the house into the ceiling cavity.

      The real solution is to pull down the T&G and install a vapor barrier between the rafters and the T&G ceiling. My personal preference is installing a layer of sheetrock, or better, foil-faced poly-iso foam board, across the rafters, taping the seams with foil tape.

      Also, if you have any recessed lights, those can allow a lot of moisture to flow up through them and into the ceiling cavity. It’s best to have an airtight ceiling and use fixtures that are fully inside the ceiling – i.e. no holes cut through the ceilings.

  3. Hi Ted,
    I have been trying to get rid of a musty smell in my daughter’s house in the deep south for several years, unsuccessfully. The smell gets in your clothes very quickly. Everything smells after an overnite stay. Vinegar in the washing machine removes the smell. It is a 1940s 1400ft2 conventional foundation house. Painting over the years has stuck most windows shut. She has 2 indoor cats. I built a trap door connected through a window to an outside cage to allow litter boxes to remain outside the house. She has washed walls with chemicals. Air conditioning system is new although ducts are not. I installed a whole house dehumidifier 6 months ago to keep humidity about 50% (used to be about 75%). No change so far. I tried an ozone generator – it gets rid of the smell for a day or less. As my next step I am considering an energy recovery ventilator to compensate for the closed windows. Do you think such a ventilation system might take care of the problem? Do you have alternative ideas? This is driving me nuts. Thanks.

    • Apologies again for the long delay.
      Smells like that can be maddening and difficult to track down.
      The key is to track the source of the musty smell, which is usually mold somewhere.
      Is there a crawlspace under the house or some other space that could be getting moldy? Often the odors will flow up from moldy crawlspaces and into the house.

      The dehumidifier was a great idea – you definitely want to keep the humidity low. Keep using it. 75% is well in the “danger zone” (anything above 60% makes mold happy).

      The ventilation system will certainly help make the house air fresher, even without the musty smell, it will help with those windows that can’t be opened. And it seems like a good idea as long as you keep searching for the source of the smell. The reason is that fresh air is a band-aid. The mold, assuming that’s what it is, will keep working. And, if it is mold, it could also be indicating rot. I’ve seen floor joists and wall studs completely rot out leading to homes that were structurally unsound, so you definitely want to find the source!
      Hope this helps!

  4. Hi Ted.
    I had closed-cell insulation sprayed on the underside of my new roof 4 years ago. I had ice-and-water shield installed as well completely covering the outside of my roof. The insulation ended up not being directionally stable and pulled away from some of the beams leaving some small gaps allowing humid air to hit the underside of the cold roof, condense, drip down, and cause some small water stains on the cathedral ceiling. Almost all of the problem seems to be at the peak of the ceiling

    The insulation installer came back with a thermal gun to find the gaps and drilled several holes in the ceiling. He then shot OPEN-cell insulation through the holes. He chose that OPEN-cell because it travels better to fill in the gaps and would be less likely to crack the ceiling. I questioned that choice because water vapors can penetrate that type of foam and I have ice-and-water shield which cant be penetrated so water vapor could still be an issue. He said that he wasn’t worried about that because the gaps were small and there wouldn’t be enough water vapors to cause a lot of damage. I went along with the plan because he agreed to cover the whole repair with his own money.

    That was a year ago. Over the last year, I have constantly monitored the humidity in my house and consistently ran dehumidifiers. I also had a timer put on my top floor bathroom vent so it kicks on for 10 minutes every hour. I’ve done all I could and the problem still returned. I’m assuming my insulation installer will return to take another look but I’m afraid we won’t have a great solution. I obviously don’t want a complete tear out as it would be very disruptive. I want things to be done correctly but in the easiest way possible. I also would love to not have to check my humidity levels 10 times a day. It seems like something I shouldn’t have to worry about.

    Do you have any ideas, Ted? I would LOVE to hear them.

    Thanks for reading this.

  5. Hi Ted,

    I stumbled upon your page on moisture problems with cathedral ceilings and am wondering if you would mind commenting on my situation.

    I bought a home about 6 months ago in Charleston, SC that was built around 1850. There is a cathedral ceiling on the second floor on which mildew started growing (and later, drywall started peeling) at the top of the ceiling around mid-March of this year. There are a few things I think might be causing the problem. First, there is no return HVAC register upstairs, so hot humid air is rising and stagnating near the top of the ceiling. Relative humidity upstairs is between 60-70 and temps so far get up to the low 80s, even with the central air on (although it wasn’t warm in March when the problem became visible). Second, the ceiling fan is poorly sealed and the hot humid air is escaping into the ceiling cavity. I didn’t realize how much of a problem this is until reading your article. Third, it’s an old house and I’m sure some air is getting in and out around windows, doors, and walls although the home is in good shape overall.

    I don’t have any knowledge of the roof design or insulation levels up there unfortunately. The ceiling is drywall and the roof is metal.

    I’ve had several contractors out who have given me different recommendations. One recommended tearing out all the drywall and adding spray foam insulation under the roof. Another recommended adding a return HVAC register upstairs…the problem is, there is no room in the walls for the necessary ductwork, so I would have to install one or more minisplit units. One guy just recommended buying a dehumidifier and seeing if that fixes the problem.

    Any of these courses of action is going to require a substantial investment (except the dehumidifier) so I’d like to make sure the one I choose actually fixes the problem. Do you have any recommendations for how to proceed? Or recommendations for experts to consult in my area (googling building scientists in Charleston unfortunately got me nowhere)? Do you consult remotely?


  6. Hi Ted. Hoping you can help us! We have 3 littles at home (including a newborn) and are worried with a smell in our house. We’ve have our ducks cleaned fall 2018. Our house is newly built 2015, developed bsmt. We do not have a HRV but a air ventilation system that runs in isolation from the furnace. On a timer 4x a day. On two occasions both in the evening this past month a musty stale overwhelming odor came only when the air ventilation was running not when the furnace was on. We can not determine the source and why there isn’t a smell all of the time. We have set the system to off now but still am worried it poses a health risk. Please any ideas would be appreciated

  7. Ted,
    I have a puzzle for you. On the front on my house I have a 3′ overhang with soffit venting and ridge venting at the peak. I know roof venting isn’t super important as long as your attic floor and any penetrations down into the living space are well sealed so you don’t have conditioned air and humidity leaking into the attic, which requires exhausting via soffit and ridge vents. Unfortunately, most of the general public doesn’t know this, so should I ever sell my house, appraisers and home inspectors would scream ROOF VENTING!!
    This summer I would like to add a covered porch onto the front of my house, extending the current roof line, but at a lower pitch. At some point I may screen this space in as more of a breezeway, or possibly even end up moving my living room wall out into this space to create more interior space. Either way, this space would have a ceiling at some point and be closed to the outdoor air, but I also would be enclosing the soffit venting that is in my current overhang. Due to the lower pitch I would probably like to spray foam this area, but what about the venting? Would you suggest I just extend the current venting via some sort of chase until it’s exits into the new overhang?

    As always, thanks in advance. Your insight is appreciated!

    • I’ve seen a product for exactly this type of case. Ventilating underneath the shingles on top of the roof line and into spaces like this. I’ll send you a link when I locate it.

      • Smart Vent or The Edge Vent shingle over is our solution in the midwest. There is another type that is rigid plastic and has larger openings, (Lomanco and GAF 4′ types) and we have found that on certain applications as it weathers and lifts, the vent will have enough space for the bats to get in.

  8. I bought a bungalow built in 1970 in Summerside PEI, My plan is to do a bunch of energy efficiency updates. One is building another 2×4 wall in the interior. I am using 2′ on centre and no bottom or top plate going to rabbet the 2x4s on the top and bottom to put boards at the top and bottom to avoid thermal bridging. Should I remove the existing gyproc? As well would air sealing and gyproc with primer and paint giving a Class 2 vapor barrier be acceptable?

    • This depends on your climate. What you want to avoid is that inner surface becoming cold enough for condensation formation. If your new interior insulation is much greater than the old exterior insulation, then the wall, now interior, will probably get cold enough to cause problems if you live in a cold climate and moisture gets into the inner cavity. PEI is cold enough in winter that I’d be concerned.
      If you could be 100% certain that the moisture cannot get to that wall, then you could get away with it. For example, if you framed the interior wall, then spray foamed over the interior wall and behind the new framing, you wouldn’t have the opportunity for moisture to condense on it. You could also ‘laminate’ it with sheet foam, sealing the foam around the edges and at seams. Then build your inner wall. You’d have a great thermal break and protect it from moisture.
      If it were my home, that’s the approach that I’d take.

  9. Hi Ted,
    Our single-story 1500 sq. ft. ranch home in Phoenix, Arizona was built in 1995 and has it’s original Air Conditioning system in the attic. We’re getting ready to have an HVAC company replace the 24 year old Trane electric heat pump and air handler. The ductwork is Insulated Flexible R4.2 Duct (wire reinforced with a foil/mylar appearance) and is also original (24 years old). There are about 14 supply registers in the ceiling and about half or them have noticeable dirt/dust streaks/stains coming from them. If I remove a register and run a damp finger into the dirtiest ones I get a dirty finger. We’re reluctant to have the flexible ducts cleaned for fear of damage to them. So we are debating whether to replace the entire ductwork with new flex duct. However, all 3 of the HVAC companies (who came out and gave us bids on the AC units) said new ductwork is not necessary – but, when pressed, reluctantly quoted bout $3000 for new ductwork. Only one offered professional duct cleaning instead. What are your thoughts? What is the life expectancy of this type of duct? Some websites say 10-15 years, while the Air Duct Council website says “Flexible Air Ducts should last for the life of the dwelling.” Would you replace the ducts? p.s.. We are seniors and plan to stay in the home indefinitely.

    • Air duct systems shouldn’t get overly dusty unless there are leaks in the system that permit dust to be sucked in, bypassing the air filter. OR if filters don’t filter sufficiently small particles.
      In the old days, we all used those loose fiberglass filters. They kind-of worked, but still let too much dust through. Today, higher MERV filters (MERV 8 or higher) are available which do an excellent job at filtering out very small dust particles. MERV 11 or higher will filter out things as tiny as mold spores, but you have to keep in mind that higher MERV filters resist the flow of air through the system. I personally have been happy with MERV 8 filters in my own home.
      This is a long winded way of saying that some dust around the registers isn’t unusual, but you can greatly minimize the amounts of additional dust in the future by using good filters and ensuring the new system is well sealed.
      As for cleaning – most don’t recommend it for the reasons you mention. Especially with an older flex duct which may have become brittle.
      It’s easy to tell if the ducts have become compromised over time by simply handling them. I’ve seen flex duct so brittle that it breaks with the slightest touch. I’ve also seen decades old flex that is fine. If your HVAC people are telling you that the duct is good, then you’re probably fine re-using it as long as it has been performing well over the years.

      The big thing I would suggest is ask the installers to be extra careful sealing the system for air leaks and to have a duct test performed after installation. There are tightness standard that should be met and testing is required in many areas now. Poor duct sealing will severely compromise efficiency and cause excess dust around the house.

      One other thing to have them check (or do it yourself if you’re handy) is to pull the registers off all the air supplies and returns in the house and ensure that the connection between the “boot” and the ceiling/wall/floor is sealed. Most installations are not sealed properly, allowing the passing air to suck dust in from around the boot where it meets the back of the sheetrock/ceiling/wall/floor material.

      Hope that helps. Good luck with your new installation!

  10. Ted, I have read your description of how to insulate an attic and cathedral ceiling, and many of your related blog entries. Thanks for a good resource. My builder installed T&G directly onto the rafters of the cathedral ceiling, with only a partial and imperfect vapor barrier at that. During the course of the winter when the outside gets really cold and then warms up, I get random dripping water here and there in the room, especially around the chimney, but also in some other random locations as well.

    To fix it, I will be removing all the T&G and insulation. Treat the mold that I find, then putting it back together right. To do it right, I want to put 2″ of XPS spaced 2″ from the roof deck. Then fiberglass insulation up against that. Then, to air seal it, I will staple plastic sheeting to the rafters, and put rubber tape over the staples along the length of the rafters. Finally, I will put the T&G back up, nailing through the rubber tape into the rafters for a good seal. I can send you a diagram of my strategy if you’d like to see it.

    Is this a good strategy, or am I missing something?

    • Michael, sorry to hear of your T&G ceiling issues. It is dismaying that builders continue to take the same old failed approach to constructing these types of ceilings, forcing homeowners to have to reinvent the wheel and re-do the building of their homes.
      The potential issue I can see with your solution is trapping moisture in the region filled by the fiberglass. The XPS and the plastic are slow vapor retarders, so any moisture that gets past the plastic is going to get trapped in the fiberglass without any way of getting out. A safer solution would be to place the fiberglass under the roof deck, with a gap for air flow, then put XPS under that (towards the inside of the house). Then a thinner layer (maybe 1/2″) of XPS or poly-iso foam covering everything (imagine replacing a sheet-rock ceiling with foam). This creates a durable, moisture resistant barrier.
      Then, you can put whatever aesthetic ceiling in place you’d like. The T&G could be applied directly at this point.
      With this construction, any moisture that slowly works its way up to the fiberglass layer will easily be flushed out. Moisture that passes the T&G would be mostly stopped by the thin foam layer, which will be essentially at the temperature of the inner ceiling, so condensation risk is minimal. And since there will be minimal seams between the large sheets of the thin foam, you can tape these seams for a fully continuous barrier.

      • Hi Ted,

        One other consideration here is whether the space between the roof deck and the T&G is actually ventilated. If it isn’t then this solution will create the same problem that the OP’s solution creates: the fibreglass is sandwiched between two vapour barriers (the XPS and the roof shingles) with no means of letting moisture escape.

        So for the OP, by ventilated I mean are there soffit vents and roof and/or ridge vents on the roof above your cathedral ceiling?

        Apologies for hijacking this response!


      • Good addition input, thanks Jeff.
        The risk is slightly reduced, even in an unventilated roof assembly. I wouldn’t want much moisture getting in there, but using multiple layers of foam board would reduce the moisture migration to a tiny amount. Any residual would likely be so little that it would diffuse out of the cavity through small air leaks that are likely to exist. But, as Jeff notes, best be safe and ventilate properly with continuous soffit and ridge vents.

      • Ted, I took the wood off the ceiling and noticed that the fiberglass insulation was stapled to the inside edge of the rafters, not the room-side face! No wonder I had so much moisture pouring up through. I pulled all the fiberglass insulation from the rafter bays and the undersides of the roof plywood are all blackened with mold maybe? Yes, the cavity is vented mostly by soffit vents and ridge vents in answer to Jeff’s concern. Another issue is that the insulator installed the foam spacers tight against the facia board, reducing the ability for them to vent any air. Also he didn’t vent side-ways above and below the sky-light cavities. So there is no possibility for those portions of the roof to vent. My thought is to drill some 1″ holes sideways into those cavities above and below the skylight from neighboring rafter bays.

        I have some questions which maybe you can help out.

        After I spray the blackened areas with a mold-eraser (TM), do I have to paint over it with a primer to seal the mold from the air, or once it is killed and I prevent further moisture, it won’t come back again?

        If I do as you suggest, the XPS will be right against the wood ceiling material. I read on the XPS sheet that you need a fire or thermal barrier between XPS and living space to be compliant to building codes. Is the wood layer enough of a “thermal” or “fire” barrier between the XPS and the living space, or should I put a layer of sheetrock between the wood and the XPS?

        Is the XPS really worth my effort, or will it cause more harm than good? I originally thought to add it because I wanted to increase my R value from a paltry R-30 to R-38. Still not to code, but a lot better than it was.

      • Hi Michael!

        One other option you might want to consider is bringing that ‘attic’ space (i.e. your rafter bays) into the conditioned space of your house. This requires air sealing the roof deck. This means blocking off your soffit/ridge vents (plywood with the joints sealed with acoustic sealant to provide an air seal would do the trick). You’d also want to make sure the skylight frames are air sealed as well (spray foam works).

        You can then pack the rafter bays with batts. I’d use rock wool instead of fibreglass; higher R value for rock wool, don’t deform after being wet as fibreglass batts do, plus they are friction fit into stud bays (or rafter bays in your case). If you use fibreglass, you will have to fight to hold them in place against the roof deck while you try to staple them in place… no need for your staple gun with rock wool batts!

        You could then drywall over the rafters to air seal the rafter bays from the interior then install your T&G over that.

        I would definitely not put plastic sheeting under the drywall. For starters, you’re creating the dreaded vapour barrier sandwich (shingles on one side, plastic sheeting on the other). Secondly, if you do get a roof leak, you want the water to come straight through so you can detect the leak as early as possible.

        If you’re concerned about moisture migrating through the T&G and drywall, consider that generally when moisture condenses in a wall space, it’s not migrating through the drywall, it’s entering via interior air which seeps in around electrical outlets, window frames, etc. If you air seal your rafter bays (i.e. with the drywall), you block this direct moisture infiltration. Also, by not putting plastic sheeting over the rafter faces, you are giving the opportunity for any potential moisture build up (which would be negligible) to dry to the interior on low humidity days. If you are still concerned, a layer of a vapour retarder like Tyvek beneath the drywall or even a second layer of drywall would further reduce the possibility of vapour diffusing into the rafter bays.

        Another benefit of making the space below the roof deck unvented is that you won’t need as much insulation. The large R values you see listed for attic insulation are for vented attics. The reason they are so high is because you have exterior air flowing through a vented attic, which will penetrate the top few inches of attic insulation: fibreglass, cellulose, rockwool… all of which are porous to air.

        If you do go with an XPS solution, consider that certain species of insects, particularly ants, have been known to nest in foam. If you have vented rafter bays, that’s a direct conduit for these insects to enter your attic space and find your foam boards.

        Anyway, that’s what I would do. Bear in mind that I don’t do this for a living! Building science is just a hobby of mine. I’d definitely recommend doing some research into the pros and cons of unvented attic spaces though…


    • Ted, you’re right of course that there is a minimal chance of moisture being introduced from the interior of the home through multiple layers of XPS board (in fact, I find there is a lot of fear mongering with regards to how much moisture transmission there actually is through most construction materials in most real world scenarios).

      I always side with Murphy’s Law in these situations though… for example, what if there is a roof leak? Now there is water between the roof deck and the T&G with no chance of escape. So you’re suggestion of ensuring there is a ventilated space below the roof deck is the safe bet in my opinion.

      Again, sorry for jumping into this discussion. I just found this situation an interesting one.


      • Jeff, thanks for the thoughts. However, I have read about the dangers of non-vented cathedral ceilings in a northern environment (ice dams and wood rot). So, I am going to stay with a vented design.

      • Michael, I get it. I think they can work, but it’s your house and you have to be comfortable with your solution.

        To create your vents, you can fasten lengths of 2×2 to the tops of your rafters (i.e. against the roof deck) then fasten plywood to the 2x2s to create the floor of your vent. You can then airseal the joints between the plywood and the rafters (with sprayfoam, acoustic sealant, whatever). Now you can add your batts. Instead of drywall against the rafters, maybe the 1″ of XPS that you both were talking about is a better choice… much lower permeance than drywall (and at 1″ thickness, not quite a vapour barrier, which is what you want), which will go even further to minimise moisture diffusion into your ‘attic’ space. Then your T&G on top of that.

        Whatever you do, I think the most important step is to airseal the insulated segment of your rafter bays.

        Anyway, good luck with your project!

      • Jeff, I am wondering if I need anything between the living space and the XPS foam for fire protection, besides the wood ceiling material. The XPS material from Corning said that I need a fire or thermal barrier, depending on the local codes. Is the wood enough of a fire or thermal barrier, or do I need a layer of gypsum between the XPS and wood T&G?

      • The other question is, I already have a full vapor barrier on the room side of the rafters. Why do I need to worry about sealing the roof side? Shouldn’t I leave that open to moisture so it doesn’t trapped in the insulation layer? We just want to stop wind from interacting with the fiberglass insulation, because that would reduce its effectiveness as an insulation, but we are not trying to stop the moisture flow, right? If so, the air-gap channel should be made of something that prevents air-flow, but not moisture. Could I staple a layer of cardboard to the 2×2’s that you suggested, instead of plywood. Plywood seems to be a little overkill, and the foam sealant on the edges might be appropriate for moisture sealing which is not necessary, I think. What are your thoughts about that?

      • In my part of the world, 1/2″ gypsum or 5/8″ plywood are generally accepted as sufficient thermal barriers. Your building code may vary, but I suspect not.

        The vents under your roof deck are exposed to outside air/humidity/insects. I wouldn’t put any cardboard in there! The plywood would not be blocking moisture at all; 1/2″ plywood is considered a vapour semi-permeable material, so a vapour retarder, not a vapour barrier. The plywood would be acting as a vapour retarder and air barrier. As well, plywood is sturdy enough to resist any bulk water (as in from a roof leak) and to keep insects out of your insulated attic space.

        Sealing the edges of the plywood with sealant or foam would serve to air seal the attic space; you don’t want any air from the roof deck vents or from the interior of the house to enter your insulated attic space. Air infiltration is the prime culprit in introducing moisture into a space.

        So to summarise: you want zero air infiltration into your attic space. You do not want to enclose the attic space with vapour barriers. It’s key that moisture be allowed to escape through vapour permeable materials (and again, not through air gaps!).

  11. I am trying to figure out if I have mold between hardwood floor and subfloor. Old house with dark floors. What can I do?

    • Unless the floors have been allowed to remain wet, there’s little reason to suspect that they’re moldy. Does the floor feel “squishy” like it’s rotting? Are there any other symptoms besides some discoloration? Wood will naturally discolor from water damage such as under flower pots or repeated animal urination on carpets (trapping the moisture between the carpet and floor).
      If you have significant reason to suspect mold issues, you might have to have someone remove the hardwood floor in the damaged area so they can examine the wood below. However, this is a fairly drastic solution that requires cutting and prying out the boards and should only be done as a last resort

  12. Ted,
    I’m very aware of your disdain for recessed cans due to their flawed characteristic of having a 5-6″ infiltration area into your attic space and unconditioned air. Would you still dislike them as much if I built a box out of insulation board to enclose them in the attic? I know they have the LED retrofits for existing cans now which may help with cutting down on air infiltration, but honestly, I’m not a big fan of the hue of LEDs yet.

    Much thanks!

    • I’ve built boxes in the attic for my pre-existing recessed lights, so it’s certainly a feasible solution.
      However, it was a real pain due to the hardware on the attic side that got in the way of the boxes, requiring me to notch the boxes and fit them into awkward spaces. I made mine out of foil-faced poly-iso board. Foil tape sticks to this very well making construction relatively easy.

  13. Ted,
    Our home is in the Phoenix area and we have 12 interior soffits, with the kitchen being the largest at 18′ x 17′ x 18″ depth none of which has insulation. Should we consider blowing insulation into those soffits or air sealing the attic floor over those soffits with a material such as Astrofoil?

    • I would definitely air-seal the attic floor above them. My personal preference would be to use 2″ thick poly-iso foam board, caulked in place to air seal the areas and then add insulation on top of that to help reduce the heat that will be trying to invade from those hot Phoenix summers.

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