Packaging styrofoam for rim joist insulation
I’m about to undertake a crawlspace encapsulation project. I’ve been dreading buying the huge amounts of plastic and polystyrene foam required because I know that it will all be around for centuries after I’m gone and my house is torn down. Then I happened to see an enormous pile of packaging styrofoam being thrown away at my office building. So I loaded up as much as I would need to insulate the rim joists.
I did some research. As far as I can tell, expanded polystyrene (EPS) is chemically the same whether it’s made for insulation or for packaging. The difference is the density. It looks like insulation density rated EPS ranges from 1.0 lbs/cf to considerably higher, with higher R-value associated with the higher density. I used a kitchen scale to measure the density of the stuff I got, and it weighed in at a measly 0.55 lbs/cf. Judging by the R-value ranges I was seeing, I estimate the R-value of this stuff to be approximately 3.0 to 3.2 per inch. So, if I install this stuff at 4 inches per joist bay, I can expect approximately R-12. Not perfect, but not bad either (especially for free with no environmental impact).
It does seem like the lower density foam is subject to more moisture intrusion than higher density foam, but as far as I can tell this is a feature of all EPS to varying degrees.
My question is, is it a terrible idea to use this stuff for some reason other than lower insulation performance? Moisture retention causing problems? Safety? As for safety, I have read that despite popular belief, burning EPS results in fumes not materially different from burning wood or paper, and the relatively low quantities I’m using makes me less concerned about fire (though I also don’t know about the flammability of this stuff).
Let me know your thoughts!
Replies
I would say, "yes, it's a terrible idea." That stuff is super flammable, try putting a match to a piece. Fire departments don't like it and I doubt your insurance company would either. I know the chances of fire are small but it's not worth the risk. Free stuff is never free unless your time has no value.
What if I added an ignition barrier, like 1/4 in drywall? Basically, from the outside to the inside: rim joist --> foam --> drywall cap, all with a spray foam seal.
1/4" drywall is not a fire barrier, I believe 5/8" Type X is the starting point for a fire barrier and the seams have to be taped and mudded. By the time you get done fitting pieces and finishing drywall, you could have had foam blown in and spend your time doing something more constructive.
IRC 316.5.4 permits foam in crawl spaces if it is protected by 3/8" drywall, even if the foam does not meet NFPA/UL standards. Also, this isn't packing peanuts, this is solid foam blocks as shown in the picture.
The point is not to save money and time, the point is to not create more polystyrene waste. All the crappy feelings I have every time I get a styrofoam coffee cup would be nothing compared to buying a cubic yard of the stuff.
Ah, my bad. When I see packing foam I automatically think peanuts although we do use some foam blocks.
While I do admire your desire to save resources I still think it's ill-advised. The foam, your labor, and other materials all have an energy cost. Having cut a lot of 3" styrofoam boards to fir in between rafters on a barn I can tell you it's a lot of work. They all had to be ripped to fit which made a huge mess and a lot still didn't fit. We went at it on evenings and weekends but probably put 30 or 40 man-hours into the job and it's still only 2/3's done. We've had a lot of issues with the boards falling out so have gone back and glued, nailed, foamed, etc., trying to keep them in place and we have not had to put drywall up.
Since you'd be using even smaller, lower density pieces the energy cost of your labor and drywall labor would be far more than any savings from not trashing the foam.
Here's an article from This Old House about 10 ways to reuse packing peanuts. You'll notice that none of those 10 include using it as insulation.
My first question is, what climate zone are you in? The reason this isn't a great idea for cold climates is the same reason fiberglass batts are a bad idea for rim joists. If your insulation is air permeable then warm, damp air from inside the house will move through the insulation and condense on the cold rim joist, where it can cause mold and rot. I'd be a lot less concerned about that happening in a heating climate, and mildly less concerned if you plan to cover the wall with taped drywall (depending on the assembly). But it's why building science types normally recommend using either the cut-and-cobble method of rigid foam sealed around the perimeter with closed-cell spray foam, or closed-cell spray foam alone at the rim.
Sorry, I should have mentioned I'm in Marine Zone 4 (PNW). The assembly will be in the open crawl space under the home, inside a (hopefully) encapsulated crawl with polyethylene and sealed vents. No drywall or other material over the EPS, but I do plan to spray foam around the edges of each EPS block.
It may be worth giving this a read - there's some good stuff in here about crawl spaces in the PNW: https://www.greenbuildingadvisor.com/article/building-an-unvented-crawl-space.
Thanks! I've been meaning to sign up for this.
The WA portion of the PNW requires R30 in the floor over a crawlspace. This applies to the rim joist portion as well. If there is any storage or mechanical in this now heated space, you will have to fire protect the foam.
Do it before July 1, 2020, or you will need to install R38 minimum with WA's new draconian energy code.
The floor insulation is not strictly true. Crawl space walls can be insulated with continuous insulation on the outside or inside in lieu of floor insulation. Where the crawl space walls are insulated, the R-value can be much lower than that needed for the floor - R10 if insulated on the outside of the foundation wall (insulation in contact with the wall), R-15 if insulated on the inside (also in contact) - and there are other options beyond that. But if you go the floor insulation route, yes, the current requirement is for R-30.