Insulating porch on piers
Hi Everyone,
Zone 5. I have a roughly 26×13 summer porch built on piers. 6×6 posts every 8.5 or so feet down to sono tubes, 2×10 band and floor joists, 2×8 roof rafters. Porch is on north side of home, about 2-3’ off the ground, which slopes gently downhill away from house.
Porch currently has glass sliders, wood deck flooring, 2 skylights, and is totally uninsulated. Roof is unvented.
I plan to convert to a 4 season room with 14 windows (ugh) and would love feedback on my insulation plan. I want to make the room as tight and energy efficient as possible within reason and reasonable budget.
Ceiling/under roof:
Ceiling is currently cathedral, about 10’ at peak. Given 2×8 rafters, am I correct that my best option to reach R-49 is closed cell spray foam underneath the roof sheathing (which is 1×10 pine board)? From what I’ve read, flash and batt has drawbacks? Alternatively, I suppose I could frame out a 7.5’ ceiling, and insulate the “attic” floor, but imagine that would require me to add roof venting to meet code? What would be optimal? Also, should I ditch the skylights while I’m doing this? From an energy efficiency perspective, they’re horrible, correct?
Floor:
Option 1: 1-2” dow thermax across bottom of floor joists, airsealed, covered in ¼ plywood. R-30 unfaced batts between the joists. Subfloor/floor. Is the thermax the only vapor barrier needed?
Option2: Closed cell spray foam on underside of subfloor…seems expensive to hit my r-value?
Open to other suggestions. Cut-n-cobble followed by closed cell spray foam etc.
Walls:
2×6 framing to match existing 6×6 posts.
From exterior to interior: 1” xps, zip sheating, r-23 fiberglass or rockwool, drywall.
Adding the exterior XPS and sheathing will cost me 1.5” of roof overhang, and leave me with about 6” of overhang…is that manageable? From what I’ve read, using the XPS on the interior is tricky?
If I use fiberglass batts, faced or unfaced? No interior vapor barrier correct? Any issues with this setup, or recommended enhancements?
Thank you!!
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Replies
See this article for information on insulating a cathedral ceiling: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
Filling the 2x8 with closed cell probably won't get you to R-49, but maybe Dana will offer his input. (It will likely be cheaper to combine foam with air permeable insulation, however.)
You don't need a vapor barrier on the floor, but you do need an air barrier. Note the details in this article for using a combination of air permeable and rigid foam insulation: http://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor
In Zone 5, you need 27% of the r-value to come from the rigid foam. The ratio is the same for 2x4 and 2x6 walls. You would give up some r-value with a 2x4 wall, but it might be a better option for your project. See this article for additional information: https://www.greenbuildingadvisor.com/articles/dept/musings/combining-exterior-rigid-foam-fluffy-insulation
Also consider buying reclaimed rigid foam at 1/3 to 1/2 the cost of new material. Check your local Craigslist and the internet for deals.
Hypothetically, wouldn't 7" of closed cell SF juuuuust get me to r-49? Without furring out the roof rafters, do I have any other options?
I assume the thermax across the bottom of my floor joists would be the air barrier, provided I seal with tape and spray foam. I'm curious if the Thermax X-Armor with the thick foil facer (4mm I think?) would dissuade rodents, or whether I still need to encase in ply? Probably still the ply but curious what others think.
I will plan to use reclaimed materials for the rigid, for sure.
On a retrofit, will I really get called on R6 of rigid + R23 of fibrous on the wall? That's very close to 27% but a tad under...