Exterior Insulation with recycled polyiso?
user-996031
| Posted in Green Building Techniques on
I’m considering the ins and outs of using recycled polyiso for some or all of the 4 inches of exterior insulation on a residence in western MA. The recycled polyiso is roofing insulation with a felt or paper facing. I’d appreciate thoughts on taping the seams and holes (from prior mounting); that is, how important is this if I place the air barrier at the sheathing (with house wrap between the sheathing and foam board). The foam will be attached with 1×3 strapping (rain screen) and long screws followed by cladding attached to the strapping. Any thoughts, cautions, experiences etc. are welcomed.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Ray,
What will be your air barrier material? I don't recommend using housewrap as an air barrier. If your existing sheathing is plywood or OSB, that can be your air barrier if you tape the seams.
One problem with recycled roofing polyiso is that thicknesses can vary considerably. However, if the recycled polyiso is cheap enough, you can sort the insulation by thickness, and choose similar thickness panels for each side of the house. You may have to reject a few panels for being too thick or too thin, or you may have to do some shimming or shaving.
It's always a good idea to tape defects and seams with a compatible tape.
Ray,
I would definitely place your air barrier at the sheathing layer under the exterior foam and not rely on the sheet foam to add much of an air barrier at all. My recommendation would be to use tapes or sheet materials to create your air barrier on the sheathing and test it with a blower door prior to installing any of the foam insulation. Make sure to have a target ACH50 (something ambitious as well, 1.0 - 1.5 ACH50 is very possible).
As well, try very hard to have all penetrations (dryer vents, boiler venting, ventilation, etc) planned for, installed, taped and sealed before adding foam. Let's just say I know from personal experience how much harder it is to do the in the opposite order...
We've seen what Martin described on a recent project. We had sheets of salvaged poly-iso vary by up to 1/4" over a 48" sheet, which meant the strapping need to be shimmed before siding, and was slow and tedious.
In fact, if I was doing it over, I would probably specify salvaged EPS foam instead and use thicker layers to make up for the lost R-value. Salvaged foam can be VERY affordable, we've skinned a house 4-6" thick for only $2,500 in foam.
Thanks for your reply, Martin. Yes, the air barrier is the plywood with taped seams. Sorry I wasn't clear. The house wrap may be unnecessary but I see it as functioning as a drainage plane for the sheathing. Also, true that some foam is tapered for roof pitch but I would buy pallets of foam all the same dimension. What I don't know is how well the facing on this kind of foam board will hold tape. Perhaps that's a question for the manufacturer. Also, I wonder what benefits or issues might creep in if I use a layer of recycled foam and one of new foil faced foam to achieve the desired 4" of foam.
And make sure to have a healthy fastener budget. SIP screws aren't cheap: http://www.mcfeelys.com/product/TMR-4080/14quot-x-8quot-Extra-Long-Wafer-Head-Timber-Screws--Spider-Drive
Ray, we're both talking about foam sold as flat, not tapered roof insulation. I don't know if the sheets compressed over time on the old roof (unlikely), or if it is just manufactured to lower tolerance because no one cares if roof poly-iso is truly flat.
We've seen the same thing with new Dow Tuff-R poly-iso, but you can reject non-flat new stock, it's much harder to reject salvaged stock that you just paid freight to ship...
Jesse, I hadn't realized the possibility of the subtle variations you describe. It is hard to see when you are just looking at huge stacks of foam board. Compression was my first thought but then you describe similar issues with new Dow product...Interesting. When you used the salvaged polyiso on the project you described above did you have issues taping the seams? Thanks a lot for sharing your experiences.
I wonder if roof insulation screws are useful for this application.
http://www.bestmaterials.com/detail.aspx?ID=9221
They are my new favorite fastener, strong, tough, cheap. I made up staging planks from 2x4's winding them straight thru the wide side into the next without breaking them. 6 inches of solid wood. Dual head[#3 square+ phillips] means they don't strip the head.
Keith, it's what I used on my house. 6" to secure the stick frame to TF, 8" to secure 1x strapping through 5.5" of polyiso and 12" and 14" to secure dimensional 2x strapping through 8" of xps roof insulation.
I used Venture sheathing tape on some felt faced polyiso I had to use when we ran out of reclaimed xps.
Ray,
If you get some foam samples and are really concerned with whether a tape will stick or not, send me a note through http://www.smallplanetworkshop.com and i'll send you a few feet of SIGA Wigluv 60. It'll should stick to it fine, but you got me curious...