How Should You Insulate Your Attic?

One of the hottest topics in energy efficiency and building science is “how should you insulate your attic?” Why? Simply put, the attic has more impact on your efficiency and comfort than any other single part of your home!

Let’s summarize why the attic is so important:

  1. The attic is the hottest part of the house in the summer and is cold in the winter
  2. Hot air rises up to the attic / cold falls drops into the living space
  3. Moisture rises and accumulates in the attic
  4. Central heating/AC systems and ductwork are often in the attic

Attics are very hot during the summer and cold in winter

We’ve all been in attics during the summer – it’s miserable. They can easily be 120F! Imagine how hot your house would be if it was 120 degrees outside. When you put that much heat close to your living space, you have to be very attentive to insulating properly.

One of the first things I look at when inspecting homes with my thermal camera is the upstairs ceilings that separate the living space from the attic. Here’s what I often see:

Thermal image showing poor attic insulation

Thermal image showing poor attic insulation

In this thermal image (aka: thermogram) the temperature is represented by brightness – the brighter, the hotter. So the dark area boxed in is, relatively speaking, cooler than the bright area. As can be seen in this thermogram, there is about six degrees F difference in temperature between the two areas. Imagine what that room feels like, with most of the ceiling radiating heat at 95-100 degrees F all day? For perspective, radiant floor heating is usually set to a temperature of 80 to 90 degrees F to keep a room toasty warm during the winter!

The revers is true in the winter. The attic is cold. With poor insulation, your ceiling might be 40-50 degrees F. That cools the air in your room, which drops down on you with a cold draft.

Hot air and moisture rises up to the attic

Both hot air and humid air are lighter than cool air so they both want to rise up into the attic. In addition, air and water vapor are extremely tiny, able to move through the tiniest crack just as easily as you walk through the garage door. Actually, a water molecule is almost a million times smaller than the size of a hair. So when a manufacturer claims that their recessed light fixture is “air tight” yet it has several 1/4″ holes in  it, you know that it’s bunk.

So any tiny holes between the attic and living space allow your heated air and moisture to flow up into the attic.

Why is this a problem? During the winter, most attics are ventilated, so they get cold. And the roof deck exposed to the attic gets really cold – cold enough to lead to condensation problems if too much moisture accumulates up there. Think about your bathroom mirror – that mirror is at room temperature, maybe 72F. When you take a shower, the water vapor in the air condenses on the mirror to form water droplets. The same thing happens in the attic.

Rotten roof deck

Rotten roof deck

In fact, this combination of cool attic temperatures and excess moisture in the attic is the cause of most roof failures. The plywood of a properly functioning roof should never have to be replaced. The shingles do, but the wood underneath should last the life of your home. If your roof rots out, the most likely cause is excess attic moisture. And that excess moisture is caused by warm, humid air from the house flowing up to the attic.

I encourage everybody to go up to their attic at some point and look around. Do you see anything like the photo above? Black stains on the wood? If so, you better take care of the moisture problem before you have to spend tens of thousands of dollars replacing your entire roof!

Ducts and HVAC systems in the attic

Many homes put their HVAC systems in the attic, and run ducts like a crazy octopus through the attic. This can be a problem on many levels.

Remember that the attic is often 120 degrees F during the summer. That intense heat is heating the ducts (even insulated ones like this). Uninsulated sheet metal, like that used for many duct distribution systems transfers that heat right into the air that’s supposed to be cooling your house. During the winter, the reverse is true, and that metal gets cold. In fact, if you put your hand in front of your ceiling vents during the winter, you probably feel cold air coming out when the system isn’t running.

The other huge problem with attic installed systems is that problems like those shown above can exist for years, robbing you of thousands of dollars of heating/cooling due to many air leaks. I can’t tell you how many ducts I’ve seen falling off, like this one!

The old adage “out of sight, out of mind” applies doubly to attic HVAC systems. Contractors do their sloppiest work where you can’t inspect it. That’s why it’s now building code in most areas for duct systems to be independently verified for new constructions. It’s an incredible problem.

Here’s another reason why it’s a problem – it’s harmful to your health. Any leak in the system allows the dirty air in the attic (filled with fiberglass, dust, mouse droppings etc.) to get sucked in and distributed around your home. Is your home dusty? Chances are good that it’s due to poorly sealed ductwork. Do your children sneeze unexpectedly in some rooms? It could be the nasty air from the attic. Does your house sometimes smell like animal urine? You might even have mice living in your ducts! It’s a huge problem.

How to insulate your attic

Now that you understand “why” you should properly insulate your attic, let’s delve into “how” to do it.

Important note: different climate zones require different insulate amounts and methods. Please refer to this page for some specifics in your area (this is for the U.S.A).

Rather than give you a specific cookbook, I want to lay out the fundamentals so you understand the goals. From their, you can look at your own home and apply these fundamentals in a manner that makes most sense. These goals are listed in priority order with the most important first.

Stop air and moisture from rising up to the attic

Yes – this is the most important thing. Unfortunately, it can be challenging. In order to do it properly, you’ll need to inspect the ducts, any mechanical systems (fans, AC units etc.) and the entire attic floor. This can be a daunting task! However if you skip this step, you could end up creating a well insulated attic that leads to your roof rotting out. Make sure you start with this step!

Tips for inspecting the attic for air leaks

  1. Have an energy auditor do a blower door test and thermal inspection of your house. In a few hours, they’ll be able to locate the trouble spots and prioritize them so you can focus your efforts. Have them test your ducts and locate any air leaks. This is the single most useful thing you can do to track down problems.
  2. Look at the underside of the roof from the attic. If you see anything like photo above with dark staining on the wood, then check directly below this stain for any possible areas where moisture could be rising up from the living space into the attic. Remember, moisture slips through tiny cracks, but usually the problem is a big hole.
  3. Look for fan air outlets above any bathrooms – every bathroom should have a fan, and that fan must vent via a duct out of the attic – best through the roof. Inspect the underside of the roof (in the attic) just above all your bathrooms. If you notice any dark staining, there’s a very good chance you’ll find a leaky bath fan or other cracks that allow bathroom air to rise up to the attic. Fix these ASAP! In no case should bath fans blow air directly into the attic!
  4. Inspect the HVAC system and ducts in the attic. All ducts should be insulated and sealed air tight. Make sure the air handler (giant rectangular fan unit for the AC) is sealed. If it looks like the photo below, it needs work.
    air filter sticking outThis one issue, with an open air filter port, can easily waste 30% of your system’s efficiency and cause countless health and comfort issues.
  5. Visually inspect the insulation on the attic floor – every square inch should be covered. You will often find sections uncovered where someone has removed the insulation to run wires (i.e. the “cable guy”). They don’t care about your heating bills so they leave the insulation off. But did you know that a small segment of missing insulation can waste as much energy as the entire ceiling of a room that is properly insulated? Thank about that for a minute – One panel of missing insulation (like in the photo at the top of the article) may cost you as much in heating and cooling costs as an entire room that is properly insulated. This is also the simplest to fix – just put the insulation back after inspecting the ceiling for holes or other problems noted above.
  6. Check the attic access. Usually this is a pull-down ladder or a piece of paneling that sits on the attic floor. These provide virtually no insulation nor air blocking. Remember, the tiniest crack lets air flow and moisture to rise up to the attic. I’ve used the ESS Energy Products attic hatch kit on several homes and it is, by far, the best I’ve used.
  7. Look around the attic for any “holes” leading into the walls below. For example:
    Duct chase from attic

    Air leaks from the house are filtered by insulation

    Chases like this allow all sorts of air and moisture to flow from the house up to the attic. Notice the discolored insulation by my right hand? That’s a sure sign of air moving up through the insulation. Note that this isn’t mold – it’s dust – the fiberglass acts as an air filter for all the air moving up through it!

This should give you enough to know how to inspect your attic and look for problem areas. In the next part of this series, I’m going to delve into different strategies for insulating the attic.

End of part one of “How should you insulate your attic?”



11 thoughts on “How Should You Insulate Your Attic?

  1. Pingback: Why is HVAC Maintenance Important for an Energy Efficient Home? – Lynn Canal

  2. I’m thinking about turning my attic into a guest suite. Because most of my guests visit during the winter, I’m glad that you mentioned how that space could get cold during that season. Although I definitely plan on using spray foam in the process, your point about keeping air and moisture from rising is definitely something I want to consider as I go about doing that.

    • Hi,
      I am two months’ new to learning about an energy efficient house. This site has been enormously beneficial.
      I would like make my 2 story condo warmer, especially downstairs. I live in California so the climate is more temperate than most. Also, I do have a lot of dust upstairs. I have gotten an offer to do a energy audit but for a high sum, and then he wants to do the work for a price way out of my league. Also, I have had several estimates to get my attic insulated and the contractors want to do different things: air sealing, no air sealing, replace ducts, your ducts are fine. I have been told
      I have an area that cannot be insulated because of high ceilings and they cannot get to that area without knocking out some walls. So does that mean that insulation of my condo is pointless? I have contractors tell me I don’t need to air seal because it won’t due any good since I have a large hole anyway. I have contractors tell me my ducts are fine – even though from pictures I can tell that they are not. Bottom line is I would like to insulate my home but have so much conflicting information I do not know where to start. Any suggestions would be appreciated. Thank you. Seems like the more information I get the more confused I become.

      • hi Linda, thanks for the feedback on the site I’m glad you found it useful. It does sound terribly confusing and it seems like you may have a house that has some complications that require some expert advice. I would highly suggest that you seek out and independent energy auditor. That is someone who does not benefit from the solution you choose. these can be hard to find because most people learned that in order to make a profit, they have to do the analysis and the remediation work. but people like that do exist, I was one of them. For a few hundred dollars, which will be money well-spent, they should be able to do tests and make recommendations based upon hard-science not speculation.
        they may then be able to recommend a reputable contractor or two who can do the insulation and duct repairs.
        in order to find somebody like this look for building science and infrared analysis of homes. someone should pop up. If you tell me what city you live in, I can do a little research and see if I can find somebody who looks like they might be reputable there. good luck.

  3. Hi Ted,
    Thanks for all the info. I have a few questions. I’m installing new bath fans over the shower area. I’ve found that the old bath fan exhaust vents directly to the soffit in the attic. I have continuous vented soffit and the 3in exhaust pipe just goes thru the cardboard attic baffle right to the soffit area (from what I can tell) I’ll be upgrading the fan to a 4in exhaust pipe when I relocate them, but it ok to just keep it like this? I know it’s not ideal, but I live in a 3 story townhouse and have no access to the roof or the eaves. Should I try to install a 90 degree flex pipe on the end and direct down to the soffit area or just use terminate the pipe over the soffit area like it is now? Should I try to cover the soffit vent where the fans exhaust to prevent any intake in that area? My townhouse was built in 2001 and I see no signs of rot on the underside of the roof (knock on plywood). I live in the DC suburbs, it gets hot in summer and cold in winter.

    Also, should there be ANY open holes in the attic? I was doing some early investigative work and I found there’s about a 3.5″ hole thru the bottom of one of the truss “chords” (I think this is what it is called). From what I can tell, it seems to be in between the wall space for my 2 adjacent bathrooms. Interestingly, the plumb vent pipe is only about 3 feet away and seems to be the same diameter. I’m not sure if the builders made the hole first, then relocated the pipe or what. It was just covered with insulation and I found it by chance. Should I cut a piece of drywall, cover the hole, caulk it and cover it with insulation? It looks really suspect, like something was “supposed” to be there… But I don’t want to mess with it, not knowing the purpose.

    • In most situations, I would say absolutely don’t keep it like this. The warm, moist air would likely come right back in the soffit vent (since warm moist air rises PLUS soffits suck in air that should be going out a ridge vent). This has been known to lead so severe mold/roof rot. However, since you aren’t seeing any negative effects, perhaps the dynamics of your setup have worked. I do like your idea of at least covering the area adjacent to where you’re expelling the air, and having a good seal around the outlet so that humidity is less likely to re-enter the attic.

      As for the hole? Highly doubtful that it should be there. As you noted, probably an “oops” from a builder somewhere along the line, or perhaps a moved vent stack. In any case, I would think covering it would be a good idea.

  4. My wife and I have been looking at ways we can increase the overall energy efficiency of our home, and we both thought the attic is somewhere we can start. I didn’t know that we could get an energy auditor to test and inspect our home in order to identify trouble spots. We’ll be sure to have this done so we can determine whether or not insulation in our attic is a good place for us to start becoming more energy efficient.

  5. This is fantastic information!
    I have two questions I hope you can address.
    1. Our attic has soffits and ridge vents…and GABLE VENTS at each end of the attic. AC is also in the attic. We are zone 4, suburbs of Philadelphia. I am considering insulating at the roof deck (with baffles) instead of adding insulation to the attic floor. Our home is a 60 year old split level. Just moved in 6 months ago and doing lots of reno on our own. What should we do with the gable vents? Should we plug them up?
    2. If we do a good job with insulating under the roof deck (with baffles) do we need to worry about leaking into the attic from downstairs?

    • Thanks Jack, glad you find it useful.
      My house in Bucks County is the same – gable vents and the soffit/ridge vents. While not theoretically optimal, it’s typically nothing to worry about. It just means that some of the air that might be pulled up through the soffits will come from the gable. Just remember, not long ago, all our homes only had gable vents and they worked fine. It wasn’t until people started poking lots of holes in their ceilings for recessed light and doing other things that allowed lots of humidity into the attic.
      If you decide to insulate at the roof level, then that is a decision to bring the attic into the conditioned space of the house so you’re correct – the gables would be sealed up and you keep a clear channel for air between the insulation and the roof deck. If you go that route, I’d highly recommend building an air-tight “ceiling” under the batts that you put under the roof. So the attic would look just like a room with a cathedral ceiling. Instead of sheetrock, you can do this with foil-faced poly-iso board foam which would be vastly easier to work with. Just screw it to the rafters and tape up the seams with foil tape. You also want to insulate the gable walls – again, making the attic look just like a finished part of the house. Anything cold/uninsulated that’s exposed will be prone to condensation.
      Regarding 2 – if you go this route, you want to provide some airflow through this space or you can get moisture buildup. Typically, with forced air heating, you’d put a small supply and return on opposite sides of the attic. Treat it like the interior of your home. They recommend the same treatment to crawl spaces that get sealed up and insulated.

      Be sure to check with your Devon’s building department to see what they require for fire-proofing in the attic. The foil faced insulation board should be enough of a barrier, but they might be sticklers and force you to put sheetrock over it. I wouldn’t want you to do anything that gets your home flagged by an inspector when you try to sell it.

  6. Pingback: Insulating Attics and Cathedral Ceilings | Ted's Energy Tips

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