High Performance Wall Assembly on a Budget
Hello!
I’m in the planning process of re-siding and sheathing my 1970’s contemporary home in Southwest VA (CZ 4A). Currently, the house has 2×4 framing at 16″ OC with 1/2″ foam board, felt paper and cedar siding as the sheathing. Most of the siding is original. I figured if I go through the hassle of replacing the siding, I should take the next step in adding sheathing and improving the wall assembly performance. Upon initial research I wanted to add Zip System sheathing or even Zip R sheathing, install a rainscreen system and use Hardie fiber cement siding. However, it has come to my attention that I’m indeed not made of money therefore cannot do such an expensive wall assembly. This lead me to think about what I could do realistically, with a smaller budget and easily accessible materials. This is what I’ve come up with and I’m curious as to how others will think this will perform in my climate zone. My goals are to create a durable, resilient, energy efficient assembly that promotes a healthier indoor leaving space and requires less maintenance.
– R15 kraft faced fiberglass insulation
– 7/16″ OSB taped at the seams
– Tyvek housewrap WRB taped at seams
– 1″ R-Tech Polyshield EPS rigid foam (R4.35) taped at the seams
– Ripped 11/32″ plywood fastened as the rainscreen system with Coravent at the bottom and top of the assembly
– LP smartside lapped siding or eastern white pine (stained on both sides)
Note – the interior is 1/2″ GWB
Do we think this would be a well-performing assembly? Is it risky? I’m curious to hear other people’s thoughts. Again, I’m looking to do this on a modest budget and get easily accessible materials. I appreciate any input folks might have!
– James
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Replies
I would use ripped strips of 3/4” plywood for your rainscreen. 11/32” isn’t really thick enough.
I would try to find some reclaimed polyiso for the exterior rigid foam. 2” reclaimed polyiso is pretty common, and will probably end up costing close to the same as that new EPS you were planning on using, but the thicker polyiso will have far better performance over all seasons.
Don’t forget a bunch of tubes or polyurethane caulk and canned foam toe air sealing. It’s worth it to get a gun for the canned foam for a project of this size too — the gun quickly pays for itself by letting you start and stop with the cans as needed instead of having to finish an entire can every time.
Bill
Hi James,
From my limited experience that wall seems like it’ll perform fine. The only questions I suppose I’d have would be if it’d be worth using polyiso instead of eps, and what you’ll do about the window transitions?
On the first, EPS is rated at about 3.6 r per inch, and polyiso at around 5 (iirc). In a non-marine cz4 any thickness will do, so would it be worth the additional cost for the extra 1.4–2 r? My guess is no, and that for twice the cost (at least according to Home Depot shelf prices) it’s not going to matter and that you be better to spend those monies elsewhere.
The other question is about the windows and if you’ll be reusing them, or if you’ll be investing in new windows?
I guess I also wonder if it’s worth taping the tyvek seams beyond where it’s required. If your sheathing is taped, and your foam, it seems a little redundant. I don’t think there’s any harm, but I’m not sure it’s doing anything that those other components of the assembly aren’t already handling well.
Polyiso is usually rated about R6/inch, and a bit more in thicker thicknesses (R13 for 2”, for example). R5/inch is sometimes used to “derate” polyiso in extreme low temperatures. EPS is usually around R4/inch.
I’d agree with you about polyiso probably not being worth the price premium over EPS here at new prices, but with reclaimed prices polyiso is a nice upgrade.
Bill
Hey dude, Im in the process of doing my second retrofit with this method. In Atlanta GA (zone 3) so my numbers will probably be a little different.
I like your system and I think it will work. I would like to add some tips
your OSB and your foam are being used as an air barrier. you can save cash by using only the foam. The cheapest tapes that I found were 3M all weather flashing tape, 3m construction tape, and WeatherMate Construction tape. 3M all weather Flashing tape is my favorite, but WeatherMate Construction tape is the best value (IMO). You can order roles of tape off of amazon and see which ones suit you the best.
If your house is doing OK structurally, i would probably leave off the osb. (since it would only be adding shear strength. I would remove the siding and and felt paper, then add the 1in EPS (EPS is cheap which is nice). buck out 1in around windows and add a 1in shelf along the bottom for your foam to rest on. then tape your seams.
this should get you airtight.
11/32 plywood is good. you just need a little air-gap. the Siding-plywood-foam. should add up to about 2in, which is good because you can use regular 2.5 in siding nails to attach your siding (shoot through all the layers into the studs.)
If you're worried about the siding pulling off the house, you can use regular screws.
going thicker with foam will always save more energy. Its not too hard to find used polyiso down here in in ATL, so you could probably find someone offering that up there. (Used polyIso is going for $15-20 for 2in 4x8 sheets) going thick with your insulation on the outside (even if you find a good deal on polyIso) gets expensive. mainly from having to get it attached to the house.
if you do 2in of insulation, you'll probably need to go to 3/4in rainscreen strip, (pressure treated decking ripped to 1.5in is good for this) and your fasteners will probably need to be screws, and that gets expensive.
long screws are expensive, but you can save some money if you buy them in bulk. For my 900 house is spent almost $1k in rainscreen screws compared to $150 in siding nails.
If you are looking for a good deal on screws, FastenMaster on Amazon is a good place. Im using their #14 6in screws to attach the rainscreen over 2.75in of used polyiso.
regular bug screen is great for sealing up the tops and bottoms of your rainscreen.
Dont worry about taping the seams of Tyvek. I dont think it has an airtightness raiting, for that your have to go the the Tyvek Comercial wrap. (at least last time i checked)
Best of Luck