Insulation options?
Zone 3 remodel (gutted)
Please help decide how to finish off the insulation on the inside.
from the outside
zip r6 sheathing
1″ xps
block wall
essentially r11 on the outside of the block.
I have started to put 1″ xps on the interior of the block wall and 1″ furring strips… I am realizing that this is taking up 2″ of space… and I have the ability instead (and probably a lot easier) to just frame a 2×4 wall on the inside instead.
I am spray foaming the roof deck with 8″ of open cell.
So, on the inside of the block wall… is there a better option than 1″ of continuos rigid on the inside?
would it be better to spray 3 1/2″ (or less) of open cell in a stud wall?
Am I losing the value of thermal mass here?
Thank you
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
If you're going to spray it, use a 2x3 stud wall offset from the blocks, this will allow the spray foam to make direct contact behind the studs. Or offset the 2x4 wall for more foam potential. Is space a concern?
Why are you using ZIP-R and 1" XPS? One or the other should be enough in Zone 3.
And have you considered 1 inch XPS with strapping to attach the siding (which I assume you are using).
You also might want to check out Martin's article on thermal mass: https://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-thermal-mass
As long as at least half the R-value is on the exterior there will be some mass benefit, so go ahead and continue sheathing the inside if you like, but use EPS instead of XPS.
Using 1" EPS instead of XPS on the interior (or exterior) is both cheaper and a lot greener, unless the XPS is reclaimed/re-used goods. In only a few decades the XPS will have the same performance as EPS, as it loses it's climate damaging HFC blowing agents.
Even without foam on the interior your stackup beats the R8 code min for mass walls, but that's not to say it wouldn't do better thermally with Ryan's 3.5" of o.c. foam with 1" of thermal break between the 2x3 stud edges and the CMU. Depending on the moisture content at the footing there is some potential for moisture issues with open cell foam on the wall though- it's definitely safer to use EPS or XPS, both of which are closed cell and vapor retardent.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
On the roof deck 8" of open cell would be only R30-R32 (0.5lb vs. 0.7lb density), and below the code-min R38, and it would also have insufficient vapor retardency to protect the roof deck without at least the code-prescribed R5 on the exterior:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
If you are re-roofing you could put 2" of EPS or 1.5" of polyiso above the deck and meet code with R30-ish foam underneath. If you're not re-roofing, bump the interior foam to 10" and use 0.7lb density (R4/inch) open cell foam, and install an air-tight smart vapor retarder like 2-mil nylon on the interior side to protect the roof deck from wintertime moisture drives.
Dana,
My roof is crazy... So this is a remodel that I gutted, But I am not re-roofing. The roof deck is constructed as follows )from inside to outside. Thick metal decking supported by metal web joists, followed by 4" of lightweight concrete, followed by some sort of pebbles.. They then converted to shingles by putting 2x6 on edge 2 foot on center, plywood and roof shingles. So I wouldn't even know how to calculate the rvalue of that! But I am sure it could be considerable.. So I think I should be ok with 8"?
So, to be clear, you have a vented 2x6 rafter type assembly on top of a 4" concrete roof, and you're applying the the 8" foam to under side of the the concrete?
Is there any fiber insulation between the 2x6 framing?
The pebbles are probably ballast for a membrane roof on top of the concrete(?)
Dean,
It sounds like any option that has already been discussed will work on your walls.
If you are insulating your concrete roofing deck from below, then the concrete will be cold in winter and hot in summer. Think about where that concrete bears on the perimeter walls, because there are potential thermal bridges wherever that concrete is supported by an exterior wall. The more interior wall insulation, the better (as far as the thermal bridge is concerned).
I agree with Dana that 8 inches of open-cell spray foam isn't enough R-value for a roof. You want to aim for at least R-38 -- and of course, more is always better.
Dana,
yes 8" under the concrete. The 2x6 roof on top of the concrete is really unvented no fiberglass and no way to get it there.... crazy... The pebbles was some crazy concoction...
The R value of the concrete/pebbles + cavity air + roof deck is no more than R2 combined, so it's not really adding much of anything for thermal performance- you're really at R30-ish center cavity, at most R32, which is substantially shy of code min.
The fact that the first condensing surface is concrete rather than wood with a vent space above (even if not fully vented) means you don't really need to add the R5 exterior side insulation for dew point control at the concrete layer. Concrete is tolerant of high moisture content, should those conditions occur in winter. An interior side vapor retarder (but not true vapor barrier) may or may not be necessary to protect the wooden roof deck above, depending on how much outdoor air can get into the 2x6 cavities.
There is really no membrane between the pebbles and the concrete? Concrete isn't very waterproof on it's own, and pebbles are a common ballast to hold down non-adhered membranes on low slope roofs.
i will check some of the sections we had to cut through if there is a membrane...