Insulating walk-in attics


A walk-in closet behind a knee-wall

Most walk-in attics are behind knee-walls – those little walls about 3′-5′ tall that intersect the sloping roof line. Usually the entire space would have been an attic, but they were reclaimed for living space and a wall was installed to make a bonus room. The resulting rooms are often neglected and poorly conceived and are the least comfortable areas of the house. In addition, their design leads to a variety of insulation and moisture problems that can be tough to rectify.

The photo above shows a typical one of these attic spaces. Usually the floors are covered with plywood so that boxes can be stored. But this photo gives a good view of some of the challenges with this type of construction. Consider that the floor that we see here is the ceiling of the rooms below and the wall to the right is the wall of a bonus room. How would you insulate this space?

Typical knee-wall insulation job

Check out the next photo showing a typical insulation job:

Ok, maybe this isn’t exactly typical, but it’s not unusual to see fiberglass batts shoved crudely into the knee wall, with nothing preventing them from slipping out and onto the floor. When that happens, you end up with a cold wall in the winter, and very hot wall in the summer.

Refer back to the top picture. What do you notice? See how the floor joists, running from left to right, go under the knee wall and there’s a big gap there. So you have to ask: “what is happening under that plywood floor?”

If you read my last post, you’d know that these attic spaces should be well ventilated with outside air in order to minimize the chance of moisture buildup in the attic space (assuming that you insulate the knee-wall). So this attic space should be cold. Now, if the insulation job looks like this, you’ve got problems.

Remember, the attic floor is the ceiling of the room below. Sometimes builders forget that and you end up with a situation like this where the insulation is all wrong. In this example, a large portion of the floor/ceiling is uninsulated, so whatever room is below it will be very uncomfortable for most of the year. During the winter, the cold ceiling will lead to strong convective downdrafts – this is a fancy way of saying that the cold ceiling cools the air which then drops down creating a breeze. So even though there’s no wind blowing in from the attic, it will feel like there is. And, during the summer, that ceiling will be heated to maybe 130 degrees, baking the people in the room below.

And that’s not all…

See the fiberglass batts going under the kneewall? What happens when the wind blows on the house from the right side of the picture? The wind blows into the attic pressurizing it. On the other side of the house, the wind sucks air from the attic. This creates a strong draft that goes right through the insulation, under the kneewall, and across the house. In the winter, this cools the ceiling below and the floor of the room above. It’s as if this insulation wasn’t there at all.

Some people (almost always contractors) accuse me of being too theoretical. They say ‘this is all fine in theory, but it never happens like that in practice. I’ve been building homes this way for 30 years!’ Well, my friend, if that’s true, then there’s 30 years of homes with uncomfortable owners and lots of wasted utility bills. Because here’s the proof:

Insulated attic floor but cold air blows under the kneewall

I’ve seen this in countless homes, and so has every other energy auditor and building scientist. In fact, the problem is so pervasive that, in most places, it is required to put an air tight seal in this space under the knee wall. So even if your contractor doesn’t believe me, they still are required to do it right.

Quick summary:

  • Block out the spaces under the knee wall so that the cold (or hot) air can’t move through this space.
  • Insulate the entire floor of the attic that is above any rooms below.
There are other details that you need to get right. By now, you should know that you have to air seal the knee wall before putting up insulation. So if there are electrical outlets, duct vents and so on in the wall, make sure they’re sealed so cold air can’t come in through them and warm, humid air from the house can’t escape out through them into the attic.

And one more thing – if you have a tub up there, don’t do what this builder did:

All that black insulation is soggy, moldy and dirty. There’s an opening to the tub for about the bottom 18″ of the wall. Out of sight, out of mind, they say. But it’s your house and you have to deal with a bath tub that’s 40F during the winter. If your builder wants to leave a big hole in the wall, hidden behind the bath tub, tell him to do it in his house and have his wife try to take a bath in a cold tub!

Tell your contractor to sit in this!

In fact, if you look at current building code, they are required to put an “air barrier” on that wall, and to insulate it properly. This builder did neither.

Doing it right

Energy efficient construction is about getting the details right. You can hide a lot of poor construction with trim, wallboard and cabinets but even though you can’t see it, it’s still a problem.

If it sounds like a lot of expensive detail work, think about it this way. You can get a cheap handyman to do it, but you’ll pay for it every day you live in your house. You pay in comfort and high energy bills. And then, when your roof rots out because they took other shortcuts, you’ll end up spending about ten times as much to fix the roof as you would have spent properly insulating that space.

In order to minimize the chance for installation error, the best solution for spaces like this is closed cell spray foam. Closed cell foam is rigid, highly insulating, air sealing and almost completely moisture impermeable. If you use spray foam, you don’t have to worry so much about all the little details, like caulking around every electrical outlet because the foam does the job for you.

Yes, foam is “more expensive” than fiberglass, but it actually does several jobs at once. When you compare the cost of a foam job to the cost of doing a fiberglass job correctly, you’ll probably find that foam is less expensive. And, in the long run, there’s no comparison. Foam wins hands-down.

Step 1: Remove existing insulation and fill in big holes

Often, those fiberglass batts or blown in insulation hides holes in the wall and floor. The first rule of proper insulation is to air seal because if you leave a hole, you’re leaving a place for air and moisture to go. That means drafts, mold and rot. Since the spray foam seals all the little holes, you’re looking for the big ones.

Opening to behind the bath tub

Let’s take another look at the wall with the bathtub.

The insulation contractor removed the existing fiberglass, then shoved some of it in the big gaps leading to the tub. They’re just going to spray the foam over this area, locking it all in, insulating and air sealing in one shot.

You could do this manually. You’d remove the fiberglass, and carefully fit rigid foam board into those spaces, then seal it in place, air tight. After that, you’d install insulation in the wall in a way that it was flush against the wall and free of air gaps. You’d also want to secure it in there so that it would be there for years to come.

After prepping the area, you would remove the floor boards so that you can get access to those gaps under the knee wall. Those would also be prepped for the foam job by filling in all of the gaps. Foamers will usually just ball up the fiberglass they removed and shove it into the holes to provide a “backer” to spray the foam against.

After this basic prep work, they’ll spray a coating of foam from the top of the wall down to the floor (which is the ceiling below).

Spray foamed knee wall

Notice a few things about this foam job.

First, the foam fully fills the wall stud area AND covers the studs.

Second, the foam crew didn’t remove the flooring, so there is no way that they actually filled in the gaps under the kneewall. This was intentional in this case because this particular space is over a garage, and they just finished opening the garage ceiling and insulated the spaces that mattered.

Why cover the studs?

And you thought insulating was easy!

Wood isn’t a great insulator. You get about R-1 for every inch of wood, so 2×4’s, which are actually 3.5″ wide, provide about R-3.5. That’s nothing when you’ve got an attic that’s probably seeing 130 degrees during the summer. It’s called “thermal bridging” – the low R-value sections of wall create a disproportionately large amount of energy loss.

Building code doesn’t require that you insulate these walls so well, but we know better. A normal wall just sees the outdoor temperature, maybe 90 degrees during the summer. But these knee walls take a beating, so every bit of R-value is needed.

As I was saying, you don’t get much R-value out of the studs, so you want to encapsulate them with foam too. This maximizes the R-value of the entire wall considerably. Consider this. Let’s say you have R-4 where the studs are and, in this case, about R-25 where the wall is. That means there’s about six times as much energy loss for for the wall stud compared to an equal area of the wall. Even though the studs are less than ten percent of the total wall area, you’re losing a lot of energy through them. So we cover them with a couple inches of foam. Now, instead of R-4, you get R-18 where the studs are. This gives a total wall R-value of about R-24. If you hadn’t covered the studs with foam, then the total wall R-value would be about 18. That’s a 33% improvement in the total R-value just by adding just 2″ of foam to the studs!

One last thing. Suppose you do what most people do – you just fill those cavities with fiberglass, stapled to the studs (which puts the vapor barrier on the wrong side and can lead to moisture problems). Assuming you do it perfectly, your whole wall R-value will be a measly R11. In fact, it’s worse, because of air leaks and other issues that compromise the fiberglass’s ability to insulate. They also don’t tell you that the R-value of fiberglass gets worse under extreme heat or cold, so you might lose another 10%-20% of the R-value.

With good insulation, life is good.

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25 thoughts on “Insulating walk-in attics

  1. Just had a bonus room sprayed with open cell foam. I had the garage ceiling sprayed (had the drywall removed), knee walls were not sprayed but the contractor took the foam from the ceiling to behind the knee wall at the soffit so knee wall is covered. I am surprised the bonus room is still hot. I notice one wall left in back of bonus room exposed to attic that is super hot. If the contractor did not spray that exposed wall in back of bonus room is this creating the heat? Contract had this wall in it but I can not visually see this wall without hiking thru my attic. I would think no way this was foamed since it is so hot.

    • A hot wall is like a space heater. So yes, any area not insulated can have profound effects on the temperature of the garage.
      You didn’t mention if the garage is now air conditioned. If it isn’t, then it will be hot regardless of insulation. Insulation changes the rate of temperature change

  2. Hello. I live in Columbus, NE . We have a walk-in attic that is an adjacent wall to our main Bedroom on the upstairs floor. The roof peaks at the top of our bedroom. The attic has a plywood floor with insulation that is sealed with plastic on the joining wall with the bedroom. The opposing slanted roof has very little insulation on it. Patches of fiberglass insulation. The joining wall in our bedroom is a closet. My question… Is it possible to open the closet into the walk-in attic and make the closet into a walk-in closet? Can we finish it out with drywall, carpet, etc? Then we want to drywall and finish out the other portion of the walk-in for an office space. Is this something that can be accomplished?

    • As long as you insulate consistently, along a continuous path around the walk-in closet area, you should be able to do this. Many walk-ins are done this way. This depends of course on the specifics of your space, but there’s no fundamental reason why you can’t do this.
      You will make your life easier if you insulate under the roof line with spray foam, turning the entire walk-in area into an insulated space.
      Since you live in a cold climate, you have to be very careful about air sealing, vapor barriers and ventilation. This is vastly easier to do when using spray foam, compared to fiberglass or other insulation of that sort because those require much more attention to detail. So, while the up front cost of foam is much higher, the long term benefits are worth it.
      I recommend finding a highly rated spray foam contractor (Angie’s List has been quite a good resource for finding contractors). Explain what you’d like to to and ask them for their recommendations.

      Be particular about who you hire for the job. There are many companies that have gotten into foam in the last 5 years since it’s so lucrative. Look for a contractor who has been in business doing spray foam as their primary business for at least 10 years. Do it right and you’ll love them as long as you’re in the house but pick the wrong person and you’ll be cursing them.

  3. I live in Cincinnati, Ohio, and my family is moving into an older house that has very little, if any, insulation over most of the rooms. Part of the walk in attic has boards under it but by far the majority is an open, bare floor & joists. The underside of the roof has fiberglass bats that are pretty dirty from what I can tell. I’m sure they have been up there for at least 20 years. I am having an energy audit shortly after we close and would like to insulate before we move in. Some of the options I am looking at are to seal open spaces in the attic floor and blow in cellulose at or above R-39 (I live in Ohio) or, if expenses allow, spray open celled foam over the attic floor and then add cellulose on top of that. At the same time I was just planning on leaving the other insulation up for the time being.

    A few things to consider:
    1) Price – I have about $4 – $5 thousand
    2) There is some uninsulated duct work in the attic.
    3) I am going to be replacing the roof with new shingles in about 3 years.
    4) Part of the roof over the attic will be painted with solar reflective paint just to see how well it works.
    5) There is a temperature controlled attic fan already in place.

    Any thoughts and opinions would be greatly appreciated!

    • Reminder – you never want to insulate both sides of a space like this. If you insulate the attic floor, you want to remove the insulation under the roof. You probably want to remove the old fiberglass there anyway.

      Spraying foam then topping it with cellulose should be ok and would allow you to do it more cost effectively than all foam. And you’re doing it right by putting the more effective vapor retarder (foam) facing the warm side (your living space) then topping it with cellulose which will let any moisture out. As long as the attic is then well ventilated, that should work well.

      Since you mention uninsulated ductwork, you would want to cover/bury it with cellulose to provide good insulation and minimize heat/cold loss. However, you should be careful in the summer where the cold ducts will tend to “sweat” from the humid outside air condensing on the cold ductwork. Depending on the level of humidity and amount the air conditioner runs, you could have considerable moisture buildup. I’d be concerned about that. One possibility is to have the spray foam company spray a few inches of foam over the ducts. That would be very effective at minimizing condensation issues.

      • Thank you for your response. Is open cell foam okay for the attic floor or should I go with closed cell? Which foam for the duct work as well?

        Part of the attic has plank nailed down on top of it so if I am understanding your article correctly, should I plug up the openings and then have it sealed with the foam? I am sure there are some places that should be sealed beneath those planks that will be out of reach. So is it still plug and foam or should I try to blow in cellulose?

        When I replace the roof in a few years, is it worth putting some foam boards on the roof deck since I am already going to be insulating the attic floor?

        Thanks for your advice. This is a great site!

      • Open cell would be fine there because it will allow any slow moisture buildup coming through your ceiling to diffuse up and out (into the attic space). As long as the attic itself remains vented, then the slow moisture movement will flush out. It’s fine for the ducts as well since the moisture movement is slow. Normally, we’d worry about condensation on the ducts but if you think about it, people usually use fiberglass with a plastic sheathing around it, tied by zip ties! A layer of foam, of any sort, will be vastly better than that.

        Regarding the floor, the key thing is to have a continuous, unbroken, layer of insulation. If you coat everything with the foam, that should seal it well.

        One thing you could do, If you make friends with an energy auditor with a thermal camera and a blower door, is have them run pre and post foam tests. This will let you see how much of a difference the foam made. Plus, if you have them run the blower in reverse (pressurizing the house), you’ll see any places in the attic where the foam isn’t fully sealed (using the thermal camera). Those areas can be marked and sealed manually if small, or by the foam company, if they missed large areas.

    • Todd, you should check with your local building officials to find out what their exact requirements are for covering spray foam. This varies from place to place. Some locations just require a fire retardant coating while others require something more fireproof like drywall.

  4. thanks Ted! You’ve been very helpful. I wish we could do the portable AC unit. We don’t have normal windows for input/output. They are pie shaped. It’s a unique house.

    What do you think about a duct booster fan on the supply that is going to the upstairs? I’ve heard good and bad. The good: It will help. The bad: it’s noisy and won’t last very long.

    • How about installing a couple small vents on the gable end of the house? They’d just look like dryer vents. One input, one output.

      Duct boosters are worse than worthless. You’re putting a small fan in line with the duct. If the massive blower in the furnace can’t move air through the duct, that little fan isn’t going to do anything useful.
      There’s something called “equivalent duct length” that’s computed by adding the length of all the straight duct segments plus a multiplier for each bend. For example, a 90 degree elbow might be the equivalent of 10 feet of duct. So what might seem like a 40′ fun from the basement to the attic more likely has air resistance equal to 100+ feet. Worse, those runs often have to run through walls, so they snake the duct all over the place, often up through narrow openings, around corners etc. You’d be better off running a direct chase straight down to the basement (which I’m sure you don’t want to do since that involves cutting through floors and ceilings etc.).
      There are ways around this but a duct booster isn’t one of them unless the thing is turbocharged! And even then, all the supply ducts on that line would have to be properly balanced in order to get air out at the end of the line.

      The sad news is that even if you had adequate airflow from one register, the heating/cooling conditions in an attic room are usually so tough that you’d need several good air vents in there to get an adequate supply for comfort. Take whatever you’ve got in the rooms below that attic room and multiply it by at least two to get an idea.

  5. Thanks for the quick response and advice. I’ve been told the hot and cold issues are due to the duct work and that there is no return for the upstairs. The hvac is in the basement. So the air travels from the basement to the 2nd floor. It’s then divided into three branches. 2 for the bedroom and the other for the bathroom. It’s a trickle of air coming out. In the summer we put fans in front of the bedroom registers right before we go to bed and the turn down the AC. Also have 2 ceiling fans going. The sun beats down on one side of the roof all day.

    I can definitely feel a difference as I walk up the stairs. The master is open to the stair way. There is no door. Just walk up into the master. The air seems to hover right at the break in the ceiling of the first floor for both AC and heat.

    Have had estimates for a mini split ductless system and for zoning. Neither are affordable. So I’m trying to do things within the budget. I was looking into insulation and found your site. It’s the first one I’ve been able to find with the same issues I’m having.

    Looked into spray foam and it was suggested to spray the underside of the roof deck to make the attic space conditioned. They also said they would put soffit channels in from the soffits to the roof vents with foam sprayed over those to keep air circulating.

    If you have a moment, any other advice would be much appreciated. I don’t want to take up all of your time.
    Thanks.

    • Glad you brought that up. Heating/cooling air distribution is a huge problem with those spaces since they’re often add-ons/afterthoughts AND they’re the hardest place in the house to condition since they have the most surface area exposed to the elements.
      Air returns are over-rated and make little difference in many cases. The only time they really have much of an impact is in tightly closed rooms – for example, a bedroom with wall-to-wall carpeting where the door closes tightly on the carpet. I know some people will disagree with this but in practice, I’ve not found returns to be very important. By far, the biggest impact is due to the long run from the furnace/AC and typically undersized supply duct. For what it’s worth, my older home has just a single central return and the flow is great from every register.
      As you noted, a mini-split is the perfect technical solution to this problem. It will deliver vastly more heating and cooling right to the place it’s needed. It’s also operated as its own zone, so you can leave it turned off most of the time if the space isn’t occupied. And there’s no duct losses. The downside, of course, is cost. A typical install will run about $3,000 for a good unit and quality installation.
      Here’s a secret that most people don’t know about – there are portable, ductless heat pumps that cost under $1000. You just have to route the air input/output through the window. They don’t have the efficiency of units like the Fujitsu or Mitsubishi, but for the cost, they’re hard to beat.
      The spray foam is a great idea and it will make your entire home more efficient if done properly. You’ll still likely need to work on the heating/cooling in that space but you will have the benefit of a more efficient home, so insulating with foam has added benefits that will reduce your utility bills. I also like it because it’s there for the life of the home so future homeowners will use less energy, so you’re doing something with long term benefits for all.

  6. I have this same issue. I have a story and a half home. The attic was turned into the master suite. This room is hot in the summer and cold in the winter. Knee walls and ceiling are insulated with fiberglass insulation. I noticed in the spray foam picture that the foam went from roof line to floor. Is there any concern for air circulation along the roof line from the knee wall up to the ridge? Thanks in advance for your advice.

    • For the foam, the ventilation becomes less of an issue than with fiberglass since the foam stops the moisture from getting into those cold cavities under the roof. With fiberglass, the moisture moves through then condenses on the roof which can be disastrous.
      In your home, have you determined what’s making the room cold? You might be able to find this out by going into the room on a cold night and feeling different surfaces – short wall, ceiling and floor. This will give you a clue as to the specific areas that might be trouble spots. Often, it’s the floor, which is the last thing most people expect.

      You can also buy infrared thermometers quite inexpensively. It’s a lot more accurate than feeling with your hands 🙂 Here’s a bunch on Amazon. They’re actually cheaper than I thought! Definitely worth $20. They can really help to quickly find problem spots.

  7. I am at odds with what to do with the mess left to me by a previous home owner/contractor. Basically the home has good soffit vetting all around, a powered gable vent and several other vents near the ridge. The issue is, when they partially finished the attic, they dense packed fiberglass in the cathedral ceiling and left the knee wall virtually uninsulated. So I warm air trapped behind the knee walls causing ice dams. At this point, I’m going to wait to fix it until I need to replace the brand new roof, but in the mean time, I plan to add fiberglass bats against the knee wall along with 2″ foam board. Then blow in cellulose across the floor. My question is, would it be worth it to shove 3/4″ conduit pipe from the upper portion of the attic behind the cathedral ceilings to the knee wall to get the warm air out, add vents, or do nothing until its time to make the repair?

  8. nice guidelines here. please, furnish me with pictures for 6 and 10 inches pipe insulation.

    thanks.
    warmly
    omoniyi friday

    • Why not explain bnrngiig in combustion air!! Basically you can foam your attic tight, and then you have to poke big holes in it to bring your combustion appliance fresh air, negating the whole purpose. Foam is a waste people, ignore all of these foam fools, it has been around for 40years and the only places you see it used is on youtube, and in custom jobs where the customer doesnt know any better.

      • There is some truth in what you say. Blindly foaming homes can cause problems. This is why it’s important to have a knowledgeable person/company work with you to ensure that you don’t create issues. In the United States, the BPI has been very aggressive in their training to ensure that contractors understand and test for combustion air problems, along with other issues that can arise from creating very tight homes.

        However, the answer isn’t just to poke holes through all that expensive insulation. The correct approach is to tighten the house and then selectively bring in air in a controlled manner to the locations where it’s needed. For example, the boiler room where the furnace is located.

        In addition, most modern, high-efficiency furnaces/boilers and water heaters are directly vented and bring in combustion air, without mixing that with the home’s breathing air. So the house can be sealed as tight as a plastic bag and these systems won’t care – it’s like they have their own straw so they can breath the air outside.

        Furthermore, the act of sealing the attic has additional benefits. Since the attic can no longer “suck” the air out of the living space, there is a reduced tendency for leaky windows and doors to waste energy. Because of this, combustion appliances often run *more* safely after sealing the attic because the home is no longer depressurized by the losses to the attic as long as there are other air sources available for the combustion appliance. Since building codes require vents in the utility rooms, or sealed combustion appliances, you can often seal the rest of the house as tight as you want without adversely affecting them.

  9. I agree completely with the points made about air-sealing and insulating, but would first consider solving the problem at the roof plane. My approach would be to insert 2″ rigid foam blocks at the eaves between the wall plates and the roof sheathing, then spray foam at least 2″ from the wall plates, over the foam blocks, and over the inside of the roof sheathing to form a continuous air/moisture/thermal barrier. Then I would complete the job with dense pack cellulose or high density FG batts. It looks like 2×10 rafters so that would yield about R40. If it was my own home, I’d consider building it down to allow more.

    If the existing roof from the kneewall to the ridge has a vent space, rigid board insulation between the rafters should be used to create a 1″ vent space from the eave to the existing vent area before adding the foam.

    There are a few reasons why I prefer this method:

    1) The insulated area is significantly less, so the UA is reduced as well as the cost.
    2) The attic space becomes conditioned space, so any ductwork, wiring, plumbing etc. that is existing or added in the future doesn’t require special attention.
    3) The potential for ice dams is further reduced.
    4) It’s generally easier than dealing with all the details in the floor and kneewalls.

    Oh yeah, the gable end walls will need to be air sealed with foam while you’re at it and treated with FG or cellulose to comparable R values.

    • I’m with you on this! There’s another article on doing exactly what you recommend.
      This post starts with the common method because most people will see that in their own homes so it’s less of a stretch to get them to at least do the easy/inexpensive correction than to do a full new insulation job. But if it were my own home, I’d go for conditioning the walk in space since in the long run, it’s much more effective and less prone to future problems.

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