Ted's Energy Tips

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.

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.