Insulation, Radiant and Vapor Barriers - Rigid foam in the attic?

My problem: I live in a cape cod house in upstate NY (downwind of the Great Lakes, that is) and I get enormous ice dams forming on the roof, especially in the valleys formed by the one large gable. I have a continuous ridge vent and soffit vents that were installed at the time of the last re-roofing (2001), but still tons of ice form near the eaves and there's much snow-melt higher up.

The house was built in 1952, and it does have some insulation, both original and added on later. I suspect the problem is in the knee walls that enclose the upstairs bedroom and loft/hallway. When I look in the attic, the mineral wool batts are only about 3" thick, at the most and they are between the studs, not over them. There seems to be plenty of additional loose insulation on the attic floor in the area surrounding the walkable floor area.

What I'm wondering: Rigid foam would seem to be the ideal method of covering the studs on the knee wall. It would help prevent the thermal breaks, as I guess they're called. It would seem easier for a DIY job than putting up additional batts.

What I don't understand:
(1) Manufacturer's sites all seem to recommend rigid foam for other applications -- not attic knee walls
(2) The diagrams they show for knee walls don't show any covering of the studs
(3) They always say rigid foam needs to be covered with gypsum board for fire-safety reasons. Would this hold true even in an unheated attic that's not used as living space?

Sorry for the long post and all the questions, but any insight would be appreciated. Thanks.

This is a good question and I'm glad you asked it. If you already have insulation, even 3" of insulation, then the heat that is getting into your attic is not through conduction. Adding insulation may help, but not because of any R-Value. More likely, you have a large volume of hot air coming through the second story bedrooms & hallways. You need to go into the attic and air seal all the walls: caulk & tape, use a foam sealant, etc. Then if you still have problems, look to adding insulation or foam boarding.

In attics, the code has reduced fire protection requirements for most foam boards. However, fire protection is still required. Some products have done special testing to qualify for further exemption based on their exceptional fire performance, but for that you should ask the manufacturer.

Regardless, foam board will be a drastic change in the temperature profile through the walls, especially considering how much hot air must be coming hrough them. So just slapping up foam board could cause a condensation problem on the "warm"-side face of the board.

Wow...that was fast! Thanks.


You need to go into the attic and air seal all the walls: caulk & tape, use a foam sealant, etc. Then if you still have problems, look to adding insulation or foam boarding.

I presume you mean around electrical outlet boxes in the knee walls, the warm air register and cool air return housings that are visible from the attic and any other openings in the insulated knee walls, correct? And I suppose an upgrade of the door gaskets and insulation used on the back of the access doors wouldn't hurt.

So just slapping up foam board could cause a condensation problem on the "warm"-side face of the board.

This would be true even with rigid foam that has a vapor barrier?

Thanks again. :)

Seal any and all penetrations, and all edges/ corners of the gypsum board.

Rigid foam board will act as a condensation plane especially if there is a vapor barrier facing. It will happen as long as there is insulation between the rigid board & the gypsum board.