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Bonus room ceiling and kneewall insulation

New construction zone 4 on the line with zone 5 (47460)

Bonus room over garage with radiant floor heat in garage floor

2x12 bottom cord in the floor and 2x8 rafters.

I plan to pre-hang drywall on the knee wall in the bonus room and spray 3.5 inches of open cell foam in between those studs. Then I plan to install 1.5 inch fiberglass faced PolyIso rigid foam on the attic side of the knee wall and tape the seams.
In the ceiling I plan to install the polyiso foam on top of the raft bottom cord (in the small atitic ceiling) then spray the open cell foam against that rigid foam from below.

What to do about the area between the upper attic and the lower attic? This is a vented attic space so accuvent baffles are what I plan to use continuously from soffit to ridge vent.
2" closed cell against accuvent tunnels for venting?
unfaced ruxol against the accuvent tennels?

Please help me understand the best options.

Asked by Anthony Hughes
Posted Aug 18, 2014 12:47 PM ET
Edited Aug 19, 2014 8:48 AM ET


12 Answers

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Add rigid foam before drywall to walls and ceiling. The rest is fine.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 18, 2014 1:12 PM ET


Not sure how sandwiching the cavity insulation in between 2 layers of rigid foam would be best practice?

Answered by Anthony Hughes
Posted Aug 18, 2014 1:24 PM ET


It sounds like the part of your building which you don't know how to insulate is a vented cathedral ceiling. If that is what you are trying to describe, here is an article that tells you how to insulate these assemblies: How to Build an Insulated Cathedral Ceiling.

If you plan to insulate the kneewalls instead of the sloping ceiling, make sure that you install blocking between the floor joists under the kneewall bottom plate, as well as blocking between the rafters above the kneewall top plate. These pieces of blocking must be installed in an airtight manner. More information here: Two Ways to Insulate Attic Kneewalls.

Answered by Martin Holladay
Posted Aug 18, 2014 2:04 PM ET


I have reviewed that article extensively and that's why I listed specific details about my design so I could get you to inform me about my approach with the materials I am specifying.

Answered by Anthony Hughes
Posted Aug 18, 2014 4:23 PM ET


I'm not sure it's the "best option," as you put it. But in your climate zone, you can use AccuVent baffles, 2 inches of closed-cell spray foam, and Roxul under the sprayed foam if you want. Remember that you need a total R-value of at least R-38.

Answered by Martin Holladay
Posted Aug 18, 2014 8:21 PM ET


Do you have a better design/stack up that you think would work better Martin? I am realy just spinning my wheels here with this decision.

Answered by Anthony Hughes
Posted Aug 19, 2014 8:16 AM ET


There are lots of variables.

1. Will you be doing the insulating work, or do you plan to hire a contractor?

2. What types of insulation do contractors in your area prefer to use? In some areas of the U.S., cellulose insulation is common, while in others, cellulose is almost unknown.

3. If you are hiring an insulation contractor, what does the contractor recommend -- and why?

4. What R-value are you aiming for?

5. Of the following three factors, which are the most important to you: low cost, high performance, and environmental responsibility?

Answered by Martin Holladay
Posted Aug 19, 2014 8:51 AM ET


1. I plan to use a contractor for some of the work (spray foam, wet spray cellulose) but I will be doing any batts or rigid board.

2. Batts and Spray foam dominate my area. Wet Sprayed cellulose is an option with one contractor and dense packed cellulose is not existant

3. I am not having any luck with knowledgable contractors in terms of what to use for High R value or what to use in conjunction with my extewrior foam. They have all suggested spray foam in exterior walls, as well as against the roof deck. There are only 5 contractors in the area. All are in the business of selling foam

4. I am aiming for the highest R-value possible to make my bonus room comfortable. While we are on this topic, I am aiming for the whole house to have the highest comfort level possible.

5. Low Cost, High Performance

Martin, I have a bunch of 1.5" fiberglass faced polyiso that I used on the outside of my sheathing on the mian part of the house. I would like to incorporate that foam into my knee wall and bonus room attic insulation plan. I also have a ventilated roof deck in the bonus room so those two factors are what is causing me so many problems. I don't want to spray foam the roof deck (to risky) and I would like to minimize the spray foam to create a moisture barrier if I can accompluish that with my rigid foam i have. I have a drawing I did that shows some ideas I have for the insulation plan.

Attic Insulation.jpg
Answered by Anthony Hughes
Posted Aug 19, 2014 9:37 AM ET


It sounds like it's time to get some bids from your contractors for an R-38 cathedral ceiling. Once you have the bids in hand, you should be able to make a decision.

If you are uncertain about the advantages of different approaches, you should re-read my article, How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted Aug 19, 2014 10:15 AM ET


Martin, I have done that and they all suggest a spray foam approach, hense the reson I am trying to design my own.
As I stated in my previous post, I have reviewed your article thuroughly and still am not able to make a decision based off what I have to use and what I want to do. That's why I attached the drawing. For you to critique it.

Answered by Anthony Hughes
Posted Aug 19, 2014 10:27 AM ET


If you can manage to get R-25 worth of Roxul batts under your proposed 2 inches of closed-cell spray foam, your plan will work.

Answered by Martin Holladay
Posted Aug 19, 2014 10:51 AM ET


Put rigid foam on walls, then spray walls and cathedral together from inside room.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 19, 2014 12:28 PM ET

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