Reuse SIP polystyrene cores
I am building my home and ordered SIPs for the roof and walls. This was 3 years ago! We fell short on funds and the home remained untouched during this time, until we were able to rebuild our bank account. During this time the panels suffered moisture damage (replaced the tarps that covered them 3 times even!) and we were not able to use them. We continued with stick framing instead. We were left with all these panels laying around, so we stripped the rotten OSB leaving just the polystyrene cores. All the moisture was removed and they were put in storage. I want to reuse these in the walls and roof (insulation) but not sure how to go about it. The roof is 14″ IBeam wood rafters (cathedral ceiling) and the walls typical 2×6 construction. I cut the polystyrene to fit in the walls and roof but need to know if I need to vapor wrap anywhere, glue or spray foam, or can these just sit alone in the walls and roof? I will have a 3″ air space for ventilation in the roof. Because polystyrene balls are light, they tend to release and fly off the panels when dry. I want to contain this somehow so I don’t have these balls falling out of the soffits. Can this be avoided by putting some sort of plastic, tar paper or other on top of the panels in the cathedral ceiling? The roof is already on so I will be installing these from the bottom up. Any other recommendations would be greatly appreciated. We want to reuse these and not see any of it go to landfills.
Thank you.
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Replies
Neo,
The best way to install rigid polystyrene insulation is in a continuous layer on the exterior side of your wall and roof sheathing. It makes no sense to cut it into narrow strips and insert it between your framing members.
If it's not too late, rethink your plan.
Thanks Martin for the quick response. The polystyrene was supposed to be (and engineered to be) the insulation inside the walls and roof (structural insulated panels). I would think that since they were designed that way, I should be able to follow that original concept and modify it so I could still have polystyrene in the walls.
Neo,
The difference is that you now have stick framing -- in other words, a thermal bridge every 16 inches or 24 inches. The whole point of SIPs is to minimize these bridges and to limit seams to only one every 48 inches.
People use polystyrene to insulate buildings all the time; plenty of builders know how to use it. But they don't cut it up into narrow strips. They install it as a continuous layer on the exterior side of their wall or roof sheathing.
Could I cut them so they leave a small gap around the studs and rafters then spray foam around them, thus eliminating the thermal bridge? What other use would I have for these panels? They cost us $20k.
Neo,
Q. "Could I cut them so they leave a small gap around the studs and rafters then spray foam around them, thus eliminating the thermal bridge?"
A. Yes. Many homeowners have used that method. However, that won't eliminate the thermal bridge, since you'll still have studs that are more conductive than the spaces between the studs.
Q. "What other use would I have for these panels? They cost us $20k."
A. You could install them on the exterior side of your wall or roof sheathing.
"A. Yes. Many homeowners have used that method. However, that won't eliminate the thermal bridge, since you'll still have studs that are more conductive than the spaces between the studs."
So, if I were to go this route what else would benefit this system i.e vapor barrier? Would the idea be similar to spray polyurethane (no v.b. needed)?
I have no other place to use these short of just trying to sell them since the house is almost complete.
Does anyone know what they did with the old rigid insulation on the Building Science offices when they replaced it last year?
Neo,
Building codes do not require the use of a vapor barrier; however, in some climates they require the use of a vapor retarder.
Your polystyrene meets the definition of a vapor retarder, so no further membrane is required by the building code.
However, you always need to pay attention to airtightness, no matter what strategy you use to handle vapor diffusion. Seal all penetrations and seams carefully to limit air leakage.
Thanks Martin for all the advice. I understand this is not the ideal situation but I was going to do my best to make them work.
You can cut the foam board into strips to insulate your framing cavities if you want to. Normally you would cut it 1/2" to 3/4" narrower than the space it's going into, and completely foam the gaps at the framing on both edges with canned foam. If the foam is already breaking down and losing material from its surfaces, then I suppose (if I were going to use it) I would find a way to enclose it completely instead of leaving a surface exposed in the roof.