Questions about insulating attic and reclaimed insulation
Hello
I recently had an energy audit on my house and increasing the insulation in the attic was a top recommendation (Currently there is about 2 inches of cellulose plus a layer of batts, but they’re disturbed and a mess all over due to previous owner’s wiring work).
My plan has been to remove the cellulose and batts, clean it up, fix the attic ventilation (currently no soffit vents, only gable, and it gets very hot), air seal everything with foam (there are lots of gaps and holes, and lots of insulation dust filtering down) then put back down enough insulation to get me up to the recommended R48-R60.
Doing that with blow in fiberglass (what I originally planned) is about $1200 for the batts alone (about 900 sq feet). That doesn’t include rebates of ~$400
Someone on this site mentioned to check into reclaimed or factory second insulation locally – on local classifieds (https://classifieds.ksl.com/listing/50898559) I found reclaimed polyiso for $10-$20/sheet (2″, apx R-14) If I can get it at the lower price that may be much cheaper, although I’d have to do a lot of cutting and sealing to put it into joists and fit it into the attic (not a problem, I have a few hard working teenagers eager to put to work 😆)
So my questions are:
1) Is there any concerns/risks with putting polyiso roofing insulation in the attic floor (maybe off gassing, etc?) Or since the roof space should be vented is it less of a concern?
2) Is there concerns with layering multiple layers of polyiso? I’m also slightly concerned about the loss of insulation at lower temperatures (I’m in Utah, so it will get down to freezing, though not usually much below, in the winter).
TIA – I Appreciate the posts on this site, have been super helpful.
Here’s a pic of my current attic for reference – as you can see, the insulation is all over and not very deep.
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Replies
>"1) Is there any concerns/risks with putting polyiso roofing insulation in the attic floor (maybe off gassing, etc?) Or since the roof space should be vented is it less of a concern?"
RECLAIMED foam would usually have long-since outgassed anything that might have been happening early in it's service life.
>"2) Is there concerns with layering multiple layers of polyiso? I’m also slightly concerned about the loss of insulation at lower temperatures (I’m in Utah, so it will get down to freezing, though not usually much below, in the winter). "
If it were getting down to -10F or colder in the attic it would require some derating for temperature, but if it's barely below freezing on the cold side of the foam. 70F on the warm side the average temp through the foam layer would be 50F, a tempat which it's performance will even be HIGHER than it is at a 75F average through the foam (the test conditions for labeling purposes.) See Figure 2:
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_09b4/0901b803809b49c0.pdf?filepath=styrofoam/pdfs/noreg/179-00263.pdf&fromPage=GetDoc
For used foam don't believe the R14 labeled performance though- it can lose some performance with age. Derating to R5/inch for age would be conservative, not that the difference between R5/inch and R5.6/inch matters much when it's performance is being robbed by thermally bridging by joists or rafters.
The bigger issue is the air-tightness, since a cut'n'cobble can develop air leaks at the edges over time- more than might be obvious. Continuous layers (or at least one continuous layer) over the tops of the joists with taped seams will do a lot better than a cut'n'cobble job, assuming you can even get 4x8 sheets in the attic without much trouble. (Ripping it in to 2' wide lengths is sometimes necessary to fit through attic hatches.)
I appreciate this reply. I would need to rip it into smaller sections to fit into the attic, but was planning on spray foaming all the gaps with expanding foam, so hopefully that is less of an issue. That said, I can definitely focus on instead cutting with more exactness and tapping it into the bays. Is there a better option than great stuff to use here?
I was planning on putting a secondary layer across after the first layer within the stud bay, so it would be across the tops of the joists. I would still have to fit around the roof trusses, but I would hope that would help mitigate some of the issue because there is now multiple layers running different directions as continuously as possible. I was also planning on air sealing before putting the foam down as well.
Cut n cobble is not a term I had heard before, thanks for sharing that.
>"I would need to rip it into smaller sections to fit into the attic, but was planning on spray foaming all the gaps with expanding foam, so hopefully that is less of an issue."
Expanding foam doesn't adhere very well to polyiso (it sticks better to wood), and isn't flexible enough to stay tight as seasonal humidity changes cause the joists to expand and flex. In very short sections like the ends of joist bays at band joists can foam can stay pretty tight, but I wouldn't count on it when filling in long sections of joist bay. Having a redundant air sealing strategy is still a good idea.
"Cut n cobble is not a term I had heard before, thanks for sharing that."
The former editor and founder of this website, Martin Holladay seems to think I coined the term (over a decade ago, on a different forum). I'm not so convinced that he is correct on that, but it seems to have entered the green building vernacular.
Nothing wrong with your approach, but I think you're under-appreciating the pain of install and the.... quality variation that you are going to get with an all teenager workforce.
I agree with Dana that a continuous taped layer (and in a perfect world, offset seams and multiple taped layers) is the right way to go, and I think you'll be unpleasantly surprised by the cost of the tape.
Reading through several of the postings with that term, it appears the primary (largest) concern is that because it restricts moisture transmission, it can result in some moisture problems if applied to areas where moisture is a concern (such as exterior walls or the underside of the room), particularly if there is inadequate (or no) venting. Since I am planning on fully venting my attic for airflow, and only applying this to the floor (not the roof), I didn't consider that a concern - do you concur?
I would just use blown cellulose here (or blown fiberglass if you’d rather use that material) and skip the polyiso completely. Cutting foam into an attic like that will be miserable work, and you’ll end up with worse performance than the blown insulation.
You don’t need any batts if you use blown insulation. Blown insulation, especially blown cellulose, should usually be the cheapest way to insulate an attic and you can easily do it yourself (I recommend a tyvek suit while you’re working, and you ABSOLUTELY need a mask!!!). Blown insulation will also usually perform best in this application too.
Bill
>"I would just use blown cellulose here (or blown fiberglass if you’d rather use that material) and skip the polyiso completely."
Indeed there appears to be adequate depth for an all-cellulose solution here. Using cut'n'cobbled foam foam at the baffles for maintaining clearance to the roof deck would mean a higher-R on the tapered ends of the cellulose layer. At 1.5x the R value of cellulose, a continuous layer on the underside of the top chords with a bit of cut'n'cobble between keeps average R-value high enough to be worth installing R60 cellulose in the middle. R60 in the middle is pretty meaningless if the outer edges are R20 and lower due to clearance constraints.
Agreed. Maybe use a thick piece of polyiso under those top chords with the far edge cut at an angle to fit against the top plate out near the eave. A little blast from a 2 part spray foam kit would both air seal the joint and glue the piece in place.
A note for the OP too: prior to beginning your new installation, tack those electrical cables up under the top chords or high along those vertical members to keep them out of the way. Those wires will trip you up and snag the hose while you’re working, which is mr murphy’s Favorite way to get you to put your foot through your ceiling somewhere. I like the plastic NM cable clamps with a small nail on either end for places like this since they’re easier to drive with a small hammer in a confined space.
Bill
Not sure if this is entirely on topic, but recalled seeing this video that might be worth considering:
https://www.youtube.com/watch?v=F7D4Q3mjgdQ
discussing, "conditioned attic," benefits over blown in insulation on the floor of the attic. This might be more relevant to Southern climates.