Using rigid insulation for roof slope


Old 08-01-02, 02:47 PM
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Using rigid insulation for roof slope

I have a 1924 Arts and Craft Bungalow in Minneapolis. It has a standard knee wall attic space construction using older 2X4s (almost 4" deep).

I would like to use rigid foam board to insulate the slope of the roof, so I can maximize R value without loosing too much head room.

Any recommendations about what brand to use, and what thickness I'll need to achieve?
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Old 08-01-02, 04:23 PM
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Foam woul be a monster to put in. Also you may get condensation problems on the sheathing.

Before I recommend anything I need to know if the room/s is drywalled or not and if it is how much access you have to the cavities.
Old 08-01-02, 07:56 PM
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It's down to the bone

Tonight I actually finished removing all of the cheap fiber board that was used for wall. The whole thing is down to the studs and subfloor.

I really don't mind the trouble of putting in styro or iso board if it will make the house more energy efficient (especially in the winter.

I'll be putting in a ridge vent and eave vents as well...

Old 08-02-02, 06:26 AM
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I take it that it's easy to get to the rafters. That being the case I would recommend using a radiant barrier(RB) insulation. It is more efficient .95% vs about 20% for the foam. Also it's much easier to install and it can eliminate another problem that is inherent in this type of construction. It also costs less.

The problem is heat transfer through the rafters. Since you have direct conductivity of high temperatures thru the rafters the foam will be unable to contol this energy summer or winter. I know this because I have a foam panelized home that I am in the process of insulating from the inside with RB in a manner similar to what I am out lining as follow.

Install a Type 3 (two layer RB) to the surface of rhe rafters as you would a vapor barrier. The second layer will pop open on the inside of the cavity. Install 7/8" steel furring strips(HD) across the rafters. Glue/screw the drywall to the furring strips. Installing the RB to the bottom of the rafters makes the rafater a low emisivity surface instead of a high one thereby reducing the energy thru the rater a cavity by a considerable amount. This is the only method know I of to be effective in this type of construction.

If you want a sample of the material e-mail me your address at [email protected]

Thank you for considering my opinion.
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