Penetrations in deep exterior foam
In planning the construction of my house I may have some of those “obvious in hindsight” questions. When the exterior foam insulation begins to exceed the depth of a standard receptacle or lighting fixture box, how do you support them? Obviously you want to minimize the amount of insulation removed so providing solid blocking all the way back to the wall is out. I am picturing long screws through the back of the box and foam to solid blocking, with plastic spacing tubes around them to prevent crushing the foam due to fixture/receptacle movement. And for direct-vent gas heaters and water heaters will the double-wall exhaust flue be low enough in temperature to allow the foam to be brought all the way to the flue, or will it need to be wrapped in rockwool or other heat-resistant material at the point of contact? Typical installation ducuments are silent in this area (in fact they usually ignore the issues of insulation altogether).
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Replies
Nathan,
1. Your "long screws with plastic sleeves" trick should work -- or a small block of wood behind each electrical box.
2. I just called the technical help hotline at Empire Comfort Systems, a manufacturer of space heaters with through-the-wall venting. They advise installers to leave at least 3/4 inch clearance around their double-wall pipe, and say there shouldn't be "anything combustible," including rigid foam, within 3/4 inch of the outside of the pipe. I don't see why that gap couldn't be packed with mineral wool insulation.
That 3/4" Martin is quoting is specific to Empire. Other pipe systems and appliance manufacturers are going to have different requirements, so make sure you don't use 3/4" as a catch-all.
Perhaps I am overthinking the fire penetrations. A quick look at Roxul's website shows their Comfortboard IS is rated at around R4/inch. It would be simple to switch to this product in those areas of heat exposure and not loose much of the insulating value of the wall. Combined with their Comfortbatt for the in-wall insulation there would not be any combustible material in the area. Throw in some heat-rated caulking for air sealing and it would be an almost ideal installation. Thank you both for the time. I am probably spending way too much of my own time thinking about the details. However until I find that "perfect" building location time is all I have.