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Insulating A Bonus Room Over Conditioned Space With Foam.

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Old 04-06-2012, 08:10 AM   #1
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Insulating A Bonus Room Over Conditioned Space With Foam.

Insulation experts,

I live in Alabama (it gets hot and humid in the summer) and I am finishing out a bonus room in a ventilated attic space. The bonus room sits over conditioned space below. I want to make sure the room is insulated properly.

When I began the project, the room's sub-floor was already down with R-24 batt underneath. Also, 2x4 knee walls were framed on the long side of the room as well as around two dormers. The roof rafters are 2x6.

I called a local insulation company to recommend how best to insulate the room. The rep looked at it and recommended closed-cell foam. He said I should install rigid foam board behind the stud walls and he would foam the walls and then apply foam directly to the roof decking (including spraying over ridge/mushroom vents), creating a totally sealed, "ice chest" up here.

I liked the insulation company's suggestion, it wouldn't require too much work to install the foam board backers; but, then I really got to thinking about it and had some issues. First, I have two dormers in the room, if I spray foam seal the dormer walls, do I need to be concerned with ventilation in the sealed area behind the walls (remember the soffits are vented)? Second concern is spraying the main knee wall. When I seal that wall, it will create a sizeable unconditioned attic space behind the knee wall. Here again, what about ventilation?

I'm thinking to do this proper, I need to install ventilation baffles in each rafter bay from soffit to roof peak. Next, frame a small area at the roof peak's that will create a small void to allow ventilation on one side and foam can be applied on the other (the baffles will terminate in this void area). If I do this, I can have ventilation for the unconditioned part of the attic and have a sealed "ice-chest" bonus room.

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Old 06-23-2012, 06:40 PM   #2
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Re: Insulating A Bonus Room Over Conditioned Space With Foam.

You could spray foam all your roof slopes to R-31 and have no venting(hot roof). Then you would not have to vent any area. This works in our area and we have engineer stamps to approve it.
I'm from Nova Scotia so building codes may be different. Always check with your local building inspector before work begins.


Last edited by Maritime Insul; 06-23-2012 at 06:45 PM.
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