Need to repair Homasote roof; should I add insulation on top?
[Retry at posting]
In my house outside of Philadelphia (~5500-6000 HDD+CDD), I have Homasote roofs in the bedroom wing (~22×26) and great room (~40x~20), on exposed 4×12’s, 4′ OC. Most of them are in fine condition (build ~1973ish), except one plane of the bedroom wing, where there were 2 layers of roof and a leak developed near the peak; one ‘bay’ of the roof is now sagging and needs to be replaced. Luckily that section isn’t visible from the inside.
The roofers are quite confused by a roof with no plywood, and how to repair the section. Also… R5 cathedral ceilings isn’t great, even with no thermal bridging. (Snow does stay on the roof a long time, I’ll note, on the north side.) They want cut out the homasote from that bay and other damage as needed, and put 3/4 plywood, ~3/4″ of polyiso, and the 3/4 plywood flush with the homasote surface. That would be no more insulation than is there now, at best, and I’d be concerned about condensation and drying. Alternatively, they could get new Homasote and restore the R5 roof deck – but they seem to be having trouble sourcing it.
Since that plane needs to be re-roofed and repaired (there are a few other spots that appear to be compromised, perhaps), what are other options?
Is it ever possible that it would make fiscal sense to put foam/nailbase/whatever over the roof? Heating is high-efficiency modern heatpump (SEER 16ish, heating (HSPF?) around 8-9 I think), $0.14/KWHr. I think upping to R15 (1.5-2″ polyiso say) for that one plane maybe 15×26 would cut heating/cooling costs by at most $200/year, is that correct? If so, it’s a long payback… and really I’d probably need to redo both planes to avoid weirdness at the peak by adding insulation on one side and not the other, which means re-roofing that side too (14-ish year old roof on that side in good condition).
Thoughts/suggestions? Need to decide ASAP whether to do this; waiting on a quote. Thanks!
Replies
The house is almost 50 years old, the "ok" part of the roof is almost 15 years old.
You may want to consider fixing this part and bringing it up to code.
https://paenergycode.com/envelope/default.html
You need R-49 in the ceiling.
This section is 50 years old, the house is 90. Barring damage, the shingles should be good for another 20-30 years on the 'good' section. The payback for R30 would be in the many decades (ignoring interest) I suspect.
I'm waiting on costs to add insulation on top; adding on the bottom would a) lose the beams, and b) be far more expensive I'm sure due to finishes, having the bring in scaffolding, etc.
It's an old, leaky house, mostly one story, with ~80 windows - quite a few large floor-to-ceiling ones. To a large extent, if I'm going to invest in energy efficiency, it makes sense to choose the biggest bang for the buck (more air sealing, for example). Bumping R5 to something a bit higher, especially in a section that needs repair and re-roofing, may make sense. When we moved in, we found the sunroom had no insulation (2x6 ceiling with shingles nailed to the top); we added 4" of polyiso foam to the ceiling there between the beams with drywall underneath - we had to do work there anyways due to replacing skylights, but it's expensive to do that sort of thing, and that was easier than this area.
The delta savings per year for R49 vs R15 for the entire roof (~800 sq ft) would be circa $60/yr. Going from R5 to R15 would be perhaps $180/yr; which was the reason for the question. R5 to R49 would be about $240/yr, but if it costs $6000 to do (insulation + decking + additional 400 sq ft to re-shingle + edging boards), then the payback ignoring interest is on the order of 24 years - and I suspect it'd be considerably more than $6000 for that change. Even just insulating the one plane that needs repair and re-roofing would be hard to justify cost-wise, that would be ~$90/year for R5 to R15, and then you have something odd going on at the roof peak due to thickness difference.
You have a good point in one way, though - if it makes sense to do any additional insulation, you've paid for most of that; the delta is just how thick the insulation is (at least up to the point where screws get a lot more expensive -- maybe 6" screws?) At some point each added say R6.5 of a 1" polyiso board is more expensive than the savings over a reasonable payback period, and that's the limit. So if it makes sense at all (and it may not, $-wise), then one can add insulation until the costs start to push the payback out.
Update: apparently there already is 1-2" (I haven't seen it yet myself) of some type of foam insulation on top. So at least total R10, R15 if 2" (a bit more if it's polyiso, if they had that in ~1970).
The question remains similar - is it worth trying to add more (especially if it requires re-roofing the other side)
Trying to justify a payback period in order to the right job is going to be difficult. Especially with energy costs as cheap as they are now. There are a few other things to consider. 1)Energy costs in the future are almost certainly going to be much higher than they are now
2) Code. Your old place is not compliant to current codes nor is it required to be. But new codes in the future will require older houses to up their game.
3) Doing the right insulation now will provide better comfort, avoid drafts, cold spots, ice dams etc. You cant put a price on that.
One other thing to consider with sub-code insulation levels is that you can end up with condensation inside the structural members or insulation.
This may be related to why your roof failed in the first place. (may not.. )