Insulating knee wall space with shallow angle


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Old 02-16-09, 09:32 AM
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Insulating knee wall space with shallow angle

I have a knee wall space 14 feet long and 3.5 feet high at the entry point that sits above a bathroom and two closets. It has a shallow 14 degree angle ceiling. We had thermographs made of the two side walls from the adjacent bedrooms, and these showed cold. Inside the knee wall space the ceiling was insulated with fiberglass batts, but in one rafter it was never completed, so it is laying on the floor. The house has cathedral ceilings so on frosty mornings you can see the frost melt on the roof where there is no insulation. There are soffit and ridge vents, but where I can see the ceiling I do not see any of these foam or cardboard ventilation guides.

I want to re-insulate the knee wall space, and I want the whole space to be part of the conditioned space of the house. I had one company come out that uses blown cellulose, and they wanted to blow cellulose on the floor of the space, and then blow wet pack cellulose on the two walls which would mean they are blowing against the back of drywall - which I would never allow. This would also leave the space unconditioned. I could try putting fiberglass batts back up which would be hard now in such a tight space. Why could I not use two inch thick XPS cut to the width of the rafters maybe stacked two deep to make it R20? This would be like a rigid board that could be stuffed down the rafter space. If I leave a little space from the ceiling, then this would provide for. some venting above the XPS
 
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Old 02-16-09, 10:52 AM
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Here's a link to get you started. You will need to do a lot of reading and planning to get it right, as knee walls, attics, and cathedral ceilings are some of the most difficult areas to insulate.

Welcome To Home Energy Magazine Online

Good luck
Bud
 
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Old 02-17-09, 07:49 AM
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One update since original post

I did more research since the original post. There is a company (see Clip Vent™ | Insulated Roof Venting System by TMF Building Products, LLC - New Hampshire.) that makes clever clips that attach to one inch XPS so that you provide a two inch vent space at the top of the rafter for air flow. Then you can use whatever insulation you want to fill out the rafter space. There was an article on this in the Journal of Light Technology which is referenced on the clip vent web site.

There is a lot of good literature on cathedral ceilings and knee wall space insulation. It is much easier if the insulation is done right when the home is built compared to the corrections needed after the home is constructed.
 
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Old 09-07-09, 03:55 PM
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Originally Posted by Richard904
I did more research since the original post. There is a company (see Clip Vent™ | Insulated Roof Venting System by TMF Building Products, LLC - New Hampshire.) that makes clever clips that attach to one inch XPS so that you provide a two inch vent space at the top of the rafter for air flow. Then you can use whatever insulation you want to fill out the rafter space. There was an article on this in the Journal of Light Technology which is referenced on the clip vent web site.

There is a lot of good literature on cathedral ceilings and knee wall space insulation. It is much easier if the insulation is done right when the home is built compared to the corrections needed after the home is constructed.
That link for TMF Building Products, LLC - New Hampshire no longer works and I can't find any link to them using Google.
I used to work for a company that made the Clip-Vent channels for TMF Building Products. Before I left that job, TMF Building Products was being sued for patent infringement on the Clip-Vent system.
 
 

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