Spray foam gap filler with styrofoam sheet board

[Viper98912]

Hello GJ,

Pretty sure I know the answer to this one, but decided to ask anyway.

Our master closet is long and sits somewhat in the roof, kinda like a bonus room. It's always the hottest and coldest room in the house, which I originally attributed to the fact that the backside of the 3 walls (that are in the roof) are only covered with styrofoam sheet. So the wall thickness from the inside to the outside is drywall, R13 batt, and then the styrofoam sheet (which I think is only about 1/2" or 1"). Since the styrofoam sheet is thin, it slightly bows out due to the batt, and the edges of the sheet don't really sit "tightly" anywhere. I'm pretty sure the heat, cold, and windy drafts are coming in through the roof eaves (as is normal) and going through all of the cracks between the styrofoam sheets. I pulled out a piece of drywall today while working on something, and noticed I could see a sliver of light from the inside of the closet, out into the roof (there's a decorative window in the roof in that area). So this sliver of light confirms that there's gaps between the styrofoam.

I'm thinking of grabbing a can of great stuff and filling all of the cracks between the sheets to help keep some of the drafts out. (What I'd really like to do is install some tongue and groove boards and get them setup to create a real sheathing on the backside, but unfortunately access is very limited in the roof area with only a 2x2' ish door to squeeze through, and with the roof in your face).

So my question is, does the great stuff can of spray foam have any adverse effects on the styrofoam board? I don't think it does, but sometimes things in spray cans can eat away at styrofoam within seconds, and I'd hate to make that mistake and have a new/larger problem at hand.

 

[jack stand]

First I'd get up into that little miserable space with some Tyvek and tyvek tape and property seal your closet wall that  is an exterior wall. Bring along "cap nails" long enough to secure the existing foam sheathing and the tyvek (or Typar). Keeping the airflow out of the f/g in the wall cavities will go a long way with this problem and maybe avoid any major drywall/interior repairs.

 

[Viper98912]

Thanks for the additional tip jackstand

 

[egdede]

I think you are fine. Might want t use the door/window formula so as to not pop any existing styrofoam up.

 

[captaindiode]

Do you have forced air HVAC? If so, adding a supply duct to the closet will help. You should still air seal first.

 

[larry4406]

First I'd get up into that little miserable space with some Tyvek and tyvek tape and property seal your closet wall that  is an exterior wall. Bring along "cap nails" long enough to secure the existing foam sheathing and the tyvek (or Typar). Keeping the airflow out of the f/g in the wall cavities will go a long way with this problem and maybe avoid any major drywall/interior repairs.

On our new home construction where we have rooms in attics, we air seal the "knee walls" using Thermo Ply Green (non-structural). This is nailed tight to the uprights on the attic side (where your styrofoam sheets are). We ensure this is installed tight and all gaps foam/caulk sealed.

For your retrofit application, I think Jack Stand's approach is sound.

Also, check for gaps at the floor level. Its not uncommon for blocking to missed between the attic trusses at floor level. This creates a gap that is not sealed off by the drywall.

 

[RoadBeater]

The spray foam would work well. In my attic, I put up simliar foam boards, and sealed the joints with metal tape, then greatstuffed the joints at the top and bottom. It did make a significant difference.

 

[Zeke]

Good thought on foam compatibility. I'm sure if you read the can info it will say.

 

[Viper98912]

Hello everyone, ran a can today and had no issues. Spread it thin so it'd just sit lightly inside the gaps. I still have more to do but hopefully it starts to make a difference (especially on windy days).

As for the forced air HVAC, there's actually two vents in there (off of the same feed, so it's more to spread it out than actual dual flow) but they're the farthest away from the unit so it's not the most efficient.

Thanks for the replies everyone