What’s the best way to reroof and add insulation to this older house?
The house 60yo+ has a low single pitch roof (2/12). Roof sheathing is 1x6T&G. I plan to remove the tar built up roof, add 2″ foam rigid foam insulation, and cover with roofing paper and metal roofing (standing rib).
Problem is this. Roof is built on 1×6 rafters with a few inches of sawdust insulation, and is vented at each end. Ceiling below is dry wall with an oil based paint (probably several layers).
I’d propose to remove the sawdust insulation from the soffit ends and blow in loose fiberglass to fill the space between the rafters, then close the ends to seal the space.
I’d appreciate any advice to make this a good job. The house is in Lewiston, Idaho 83501.
Thanks
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Lewiston is Climate Zone 5, so you want at least R-49. You would need to have 41 percent of the insulation on the exterior (R-20). Perhaps nailbase would be the way to go.
On the interior you would need to install the air permeable insulation in direct contact with the sheathing, but you do not have enough space for the remaining R-29. Maybe a few other members will comment and add their thoughts. Would you consider removing the ceiling?
M,
The best approach is probably to install all of the insulation above the roof sheathing, using either (a) several layers of rigid foam, or (b) nailbase, or (c) SIPs, and then to seal up any ventilation channels.
For more information on these approaches, see How to Install Rigid Foam On Top of Roof Sheathing.
If you can find a way to remove the existing sawdust from the rafter bays (are you sure the rafters are 1x6s, not 2x6s?), and if you can fill the rafter bays with dense-packed cellulose, and you can seal up the ventilation openings, you would end up with about R-20 of insulation between the rafters. That would allow you to add about R-15 of rigid foam above the roof sheathing while still maintaining the necessary foam-to-fluffy ratio. With this approach, you would end up with a roof assembly with an R-value of about R-35 -- less than minimum code requirements, but obviously better than what you have now.
Thanks for the replies. It's 2x6 construction. This helps a bunch. If I remove the soffits I have access to the spaces between the rafters. I'm thinking a long tube and a large shop vac should work. There may be some obstructions blocking the vacuum tube but I think they can be worked around.
You can get to code-min performance on a U-factor basis with 5.5" of cellulose between the rafters (~R20) and as little as 3" of continuous roofing polyiso (R17-ish) above the roof deck, but 3.5" (R20-ishwould be more assured. The insulation may only be R37-R40 at center cavity, but the R thermal break over the rafters, the R-value of the roof deck itself, and the thermal performance of the multiple air films will bring the whole assembly U-factor down to the U0.026 code-max range.
There are multiple vendors of reclaimed roofing polyiso on the Lewiston craigslist, but most are going to be an all-day round trip.
https://lewiston.craigslist.org/search/sss?query=rigid+insulation
Thanks again,
Is there any limitation on the season of replacing the roof? We're in a relatively temperate, dry climate, but I plan to do this this winter. I'm thinking about thermal expansion, moisture accumulation. My plan was to use metal roofing over the new foam insulation, without additional decking.
@MSATTERW.
You're going to have to install underlayment and decking ,or possibly furring strips, screwed through the foam into the rafters.
M,
In many climates, roofers work year-round. That said, roofers always pay attention to weather reports, since high wind or heavy rain can wreak havoc quickly.
If you are experienced roofer, you know how much of a roof can be opened up at once, based on how quickly you can make it weathertight again. You also know when you need a big tarp, and how to secure it if you need it.
If you aren't an experienced roofer, you can easily get into trouble if you haven't considered these issues.
If you're working on your own roof, and you're not a roofer, talk to your insurance agent about coverage.