# Attic knee wall and floor joist insulation



## Windows on Wash (Aug 30, 2011)

Why perforated foam?

Why not just attached foil faced foam (if code allowed) to the kneewall? Be sure to cut back some of that subfloor and ensure that you have that joist area sealed off and insulated as well. 

You need to blocker off the floor joist cavity on both sides to be most effective.


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## daniel23111 (Nov 3, 2015)

Windows on Wash said:


> Why perforated foam?
> 
> Why not just attached foil faced foam (if code allowed) to the kneewall? Be sure to cut back some of that subfloor and ensure that you have that joist area sealed off and insulated as well.
> 
> You need to blocker off the floor joist cavity on both sides to be most effective.


Thank you for the reply.

I am thinking about perforated foam because i already have a vapor barrier in the batt insulation. I believe perforated would let moisture escape and not be trapped between the foam and the batt insulation. I am no expert in this however.

Foil faced foam is around the same price and buying the foam and foil separately. I think separately would be easier as the foam is fanfold 4' x 50' rather than 4' x 8' sheets. Maneuvering 4' x 8' sheets in to the space would be a challenge.


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## Windows on Wash (Aug 30, 2011)

Perforations in the foam make little to no difference in the perm rating. If you have a poly facing on the foam, the perm rating is what the perm rating is. 

Unlikely to get moisture trapped in a vertical wall that isn't adjacent to a bathroom.


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## daniel23111 (Nov 3, 2015)

So do you think i should use any type of rigid foam on the walls or just a foil?


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## Windows on Wash (Aug 30, 2011)

The foil is a class I vapor retarder as well (i.e. same as foil faced foam). 

I would just pull the vapor barrier off the FG and just insulated it and using the 1 step (i.e. foam board) approach if you want to. 

You don't require a vapor barrier in this climate either way.


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## daniel23111 (Nov 3, 2015)

Thanks. So your suggestions pulling the paper off the already installed fiberglass batts? Or am I misunderstanding?


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## Gary in WA (Mar 11, 2009)

IMHO, leave the paper facing alone. Add some double poly-faced perforated fanfold if you want though the perforated RB is stopping wind-washing also AND acting as a RB, if proper install. 

Gary
PS. perforated poly-faced has a perm rating of 1.7 for 1/4" fanfold, google it. They also perforate RB for moisture movement (14perm) through it as well... for those unaware.

Install an access in the other side to block joist space under wall- any attic air will fall to floor and create convective loops in the heated joists from room below. Detrimental.


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## Windows on Wash (Aug 30, 2011)

Gary in WA said:


> PS. perforated poly-faced has a perm rating of 1.7 for 1/4" fanfold, google it. They also perforate RB for moisture movement (14perm) through it as well... for those unaware.


Those of us that do siding every week are "aware". :wink2:

My point was that the perm rate will be the perm rate. Micro perforations should make little difference in the perm rate. If the wall is airtight, which it should be by spraying any gaps or cracks at the backside of the drywall, the air movement through the fan fold should be non-existent. 

From this article: http://buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

_Okay, so we don’t need the sand layer to handle the “curl” thing. But what about using the sand layer to protect the polyethylene? Hah. The polyethylene does not need protecting. You can poke holes in it, you can puncture it, you can tear it, you can leave gaps in it, and pretty much have your way with it as long as it is in direct contact with the concrete. Huh? But, but, vapor barriers have to be continuous and free from any holes. Actually, no. Air barriers need to be continuous and free from holes, but vapor barriers do not need to be. Lots of vapor moves by air movement, not a heck of a lot of vapor moves by vapor diffusion. The concrete slab is the air barrier, and the ripped and torn and punctured polyethylene sheet is the vapor barrier. It’s that Fick’s Law thing. Diffusion is a direct function of surface area—if I get 95 percent of the surface covered I am pretty much 95 percent effective—and the parts that are left I have filled with concrete which is also pretty good as a vapor barrier. I could wear golf shoes and march around the plastic vapor barrier and not do much damage. But put that sand layer in there and you are doomed._

I think the whole vapor barrier consideration here is largely moot. Moisture isn't going to be moving through a sealed and airtight vertical wall. 

Without knowing the exact radiant barrier specified, I can't comment on the vapor permeance. I do know that many of the ones that are used around here are straight up reinforced metal foils. If that is the case, they are a class I. 

I would still do rigid foam to the outside wall. The thermal break will make the studs warmer and the whole wall more efficient.


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## daniel23111 (Nov 3, 2015)

Gary in WA said:


> IMHO, leave the paper facing alone. Add some double poly-faced perforated fanfold if you want though the perforated RB is stopping wind-washing also AND acting as a RB, if proper install.
> 
> Gary
> PS. perforated poly-faced has a perm rating of 1.7 for 1/4" fanfold, google it. They also perforate RB for moisture movement (14perm) through it as well... for those unaware.
> ...


Thank you. In regards to the floor joist blocking, are you saying just doing one side would be "detrimental"? Would doing one side help at all?


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## Windows on Wash (Aug 30, 2011)

Doing one side is better than doing no sides.


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## daniel23111 (Nov 3, 2015)

Thank you for all of the feedback. I am relieved that installing an air block under the knee wall floor joists on one side will provide some benefit.

Now i just need to determine how I want to handle the wall situation.


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