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Help with knee wall insulation project

Hello, I am looking for some help with an insulation project for a knee wall in my old Bungalow style home. I am in Michigan so the winters can be very cold and summers hot/humid. I am currently insulating the knee walls in the upstairs area where our bedroom is. The bedroom gets extremely hot in the summer due to the knee walls, but somehow stays relatively comfortable in the winter.

What was in there before was some old tar paper type insulation with a very thin layer of insulation sandwiched in the middle of the tar paper (looks like brown cellulose?). This was on the back of the knee walls, while the floor of the knee wall space is adequately covered with blown cellulose. I have taken out all of the old tar paper and replaced that with faced r-13 fiberglass (the walls are 2x4 studs) with paper facing the living space. I am now in the process of blocking the floor joist cavities under the knee wall with 1" foil faced foam board and air sealing with spray foam. I have air sealed all electrical outlets with spray foam as well.

My questions are:

Will the r-13 and air sealing make a difference? I know more R value is ideal but I am working with 2x4 studs, and the main limiting factor: BUDGET. I am in between jobs and simply could not afford anything thicker (I priced everything).

Will I need to add house wrap or some form of sheathing over the back of the insulation? Can this be done in the future as more funds become available?

Are there any other cheap additions to what I am doing to make the space more comfortable?

Like I said, budget is my main concern and I'm just looking to improve the comfort level at the moment. Any suggestions are greatly appreciated.

Asked by Matt Warner
Posted Sat, 08/02/2014 - 23:08
Edited Sun, 08/03/2014 - 06:23

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5 Answers

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1.
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Matt,
R-13 batts, installed without an attic-side air barrier, are insufficient. I understand that your budget is tight, so doing this work correctly may be beyond your financial abilities right now.

But your wall needs more R-value, and it also needs an attic-side air barrier. One possibility is to install a continuous layer of rigid foam on the attic side of your kneewall.

An even better approach is to install your insulation along the sloping roof line, rather than trying to insulate your kneewalls and the floor of the triangular attic behind the kneewalls.

For more information on these issues, and for advice on how to proceed, see Two ways to insulate attic kneewalls.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 08/03/2014 - 06:21

2.
Helpful? 0

Matt,
I found a very full truckload (though a small Toyota truck) of reclaimed polyiso sheets in various thicknesses for $80 on craiglist. We've used it on all sorts of projects including encapsulating the ducts in the open crawlspace of the house we are building. I bet you can find something similar fairly quickly. That might help with the cost issues.

Answered by Lucy Foxworth
Posted Sun, 08/03/2014 - 10:35
Edited Sun, 08/03/2014 - 10:37.

3.
Helpful? 0

Thank you both for the replies. I had originally considered doing the roofline, but for some reason a half wall was built in the knee wall space, making it nearly impossible to access the entire roof deck. Plus it would have been a lot more insulation to purchase.

I had also originally purchased the 1" foam boards to use over the old insulation that was there but I abandoned that plan, figuring it would not be sufficient, and the 4x8 sheets would not fit through the access doors.

I actually still have all of the foam board and was considering keeping it and using it over the r-13 if needed, as you suggested. If I do that will it create a moisture problem in the fiberglass since it is faced? The foam boards are the 1" R TUF boards from Lowes with the foil facing (I believe Polystyrene? It's the white foam). I went with these since they were the cheapest.

If I do back the fiberglass with the rigid foam, what do I do about the rafter bays where the knee wall meets the roof deck? Do I block those as I did underneath with the foam? I have soffit vents venting the roof. How do you approach those areas? There is currently the old tar paper insulation stuffed into the bays along the flat floor of the small attic above the knee wall and I was debating on pulling that and stuffing the r-13 in there, but was worried about blocking the roof venting. I assume adding baffles is the answer, but does that still work if it is blocked with the rigid foam?

Thanks again.

Answered by Matt Warner
Posted Sun, 08/03/2014 - 12:05

4.
Helpful? 0

Matt,
Read the article I linked to. It discusses the questions you are raising.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 08/03/2014 - 16:48

5.
Helpful? 0

Matt,
Q. "What do I do about the rafter bays where the knee wall meets the roof deck? Do I block those as I did underneath with the foam?"

A. As the article explains, the answer to this question is yes.

Q. "I actually still have all of the foam board and was considering keeping it and using it over the R-13 if needed, as you suggested. If I do that will it create a moisture problem in the fiberglass since it is faced?"

A. I assume that you are talking about kraft-faced fiberglass batts. The kraft facing is a "smart" vapor retarder, and won't cause any problems.

The foam is unlikely to cause any problems, especially if it is thick enough. If you have the patience and enough foam, you should follow the recommendations for minimum foam thickness found in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

If you can't quite install enough rigid foam to meet the minimum thickness levels recommended in that article, don't worry. Since the space behind the kneewall will never be as cold as the outdoors, you'll probably be fine.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 08/04/2014 - 05:22

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