Adding rigid insulation?
Hi, I’ve been reading through information for a few days now, and am honestly more confused now than when I started. I’m sure this question has been asked before but I couldn’t find anything specific to my area(zone). Here is my situation… Thanks in advance for all replies.
I am building a new house in 4a. Southern Illinois to be more specific. Its a basic ranch with attic, full unfinished basement. It has 2×6 walls. Right now we have homeguard housewrap (quasi-equivalent of tyvek?) Doors and flanged windows installed. No siding yet, will start as soon as rain quits and mud dries a little.(1) Here comes the question, I just recently thought about using 1″ xps on the exterior over the housewrap under vinyl siding, would this be ok? Thought this would be good to prevent thermal bridging, but it always seems to be under the housewrap.
More info: here is where I start getting confused when it come to the vapor barrier, wrb, vapor retarder, drying to inside/outside, etc… For the interior insulation, we were given the option of cellulose for R-21, Or a “hybrid wall” , which includes one coat of closed cell foam topped with cellulose for R27-30. (2) Would either of these work with my proposed exterior setup having rigid foam over housewrap?
To sum up, my proposed building envelope is vinyl siding, 1″ xps, housewrap, osb, interior insulation, drywall.
Again thank you for your time and help. Hope I’ve included enough info clearly enough. Anyone who replies be prepared for more questions, lol.
Mike Watts
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Replies
To add one thing, my biggest concern I guess is trapping any moisture anywhere and causing mold.
Mike,
Your instinct to reduce thermal bridging by installing a layer of exterior rigid foam is a good one. In your climate zone, 1 inch of rigid foam will work. It's thick enough to keep you out of trouble, although thicker foam would, of course, provide better thermal performance.
If you install exterior rigid foam -- and I think that you should -- then you shouldn't install any vapor-impermeable spray foam on the interior side of the wall sheathing. You want your wall sheathing to be able to dry inward if it ever gets wet. I urge you to install dense-packed cellulose between your studs, without the spray foam suggested for a "hybrid" wall. The cellulose won't inhibit inward drying the way that closed-cell spray foam would.
Ok Martin thank you. I could 2" of rigid insulation to beef it up a bit, with the dense packed cellulose. I guess then I would need vertical furring strips. Is xps the right choice, or does it only matter in regards to cost vs efficiency? Do the seams need to be taped, since I have housewrap underneath?
Thank you again
Mike,
Either polyisocyanurate or EPS is more environmentally friendly than XPS. Taping the rigid foam is a good idea but not strictly necessary. For more information, see How to Install Rigid Foam Sheathing.
Saving the foam budget for the exterior instead of where it's thermal performance is being robbed by thermally bridging framing is the right thing to do.
If the exterior foam is two layers with staggered seams you don't take much of a hit from the expansion/contraction opening of the seams but some. But as a single layer taping the seams is more important. An 1/8" wide 2" deep open seam with free air convection is almost as much thermal bridge as 2x lumber, but when you tape it and make it air tight the trapped air film has real R value.
In your climate 2" of 1lb density foil-faced polyisocynurate has the about same or better annual performance as 2" of 1.5lb XPS, and is easy to air-seal with foil tape. Polyiso is blown with pentane with a 100 year global warming potential about 7x CO2, compared to XPS which is blown primarily with HFC134a at about 1400x CO2. And XPS uses more polymer per R. In the event of fire polyisoyanurate chars in place, whereas polystyrene ( XPS and EPS) melt and drip flaming gobs of goo, and has a somewhat lower kindling temp. In very cold weather the polyiso takes a significant performance hit, but in your climate & stackup it's not enough to matter. At 2" on the outside of R20 ish cavity fill it performs almost as well as 2" XPS even during the winter months, and will outperform it during the shoulder seasons.
Over a handful of decades as it loses it's blowing agent doing it's damage, the performance of 1.5 XPS falls to about R4.2 per inch @ 75F average temp through the foam (ASTM C518 labeled performance). The fully derated R-value of the polyiso would be about R5.5/inch.
Closed cell spray polyureathane has similar blowing agent issues, since most of it is blown with HFC245fa at about 1000x CO2. Open cell foam uses about half the polymer per R (you'd get 4" of open cell or about R15 with the same amount of polymer as 1" of closed cell foam at R6.5-R7), and is blown with water, a VERY low impact material. It is by far the greenest type of foam insulation, but still has a heavier footprint than dense packed cellulose.
Thank you for all the help Martin and Dana. One more question though. Our house is a basic ranch but has a cathedral ceiling in the living room, kitchen area. Obviously we will need to insulate this gable end since it will be inside the building envelope, but on the other side of the house the gable end is for the attic. Surely we don't need to insulate this, so what would be the best way to go about making this side of the house plumb and even for siding installation?
Also, I forgot my other question. Do you need to cover the bottom some kind of flashing. So in other words, if you were next to the house laying on the ground, facing the sky, you would see 2" wide insulation and the furring strips. Does that part need to be covered all the way around?
Mike I think most would recommend flashing and some sort of bug screen for if you're using a rain screen. I would guess you're also in termite area in So. IL so the flashing will act as a warning if you see mud tunnels. I rented a brake and was able to cut and bend 150ft of flashing for this purpose in a few hours.
Mike,
For instructions on creating and installing metal flashing (dripedge) to protect the rigid foam at the base of the wall, see this GBA video: How to Install Rigid Foam Insulation Outside a House.
For information on installing insect screening at the base of a wall with vertical furring strips, see All About Rainscreens.
Mike,
Q. "Obviously we will need to insulate this gable end since it will be inside the building envelope, but on the other side of the house the gable end is for the attic. Surely we don't need to insulate this, so what would be the best way to go about making this side of the house plumb and even for siding installation?"
A. You have two choices: (a) Install shims or thick furring strips to create a deep rainscreen gap, so that the outer edges of the furring strips at the uninsulated gable are in the same plane and the furring strips on the insulated part of the wall, or (b) Just continue the rigid foam all the way up to the rakes because doing so is easier than fussing with shims.