How to properly use the insulation when I already have house wrap installed.
I have a house framed, taped, and wrapped with Tyvek Commercial D at 8500 ft in Colorado zone 5a. I am about to install the windows. My problem is that I have a load of recovered (previously used) 2.75″ polyiso that is fiberglass faced. The plan was to install the flanged Alpen windows integrated into the house wrap and then sheet the exterior with the polyiso, and add 1×4’s (they were cheaper than 1×3″ for some reason). for rainscreen and install siding.
However, I just read that the backing on the polyiso does not tape well and that the polyiso will absorb water if installed this way. Installing 1″ of EPS over the polyiso is one way but it is way over my budget.
It is a no-no to add a second layer of house wrap over the polyiso as that would put all the insulation between two layers of house wrap.
Does anyone have a good way to make this work properly without breaking my budget?
Also, can someone explain to me the difference between a second layer of house wrap vs something like faced EPS that has been properly taped? It seems (at least to my common sense) that faced EPS that is properly taped is much the same thing (vapor/moisture wise) as another layer of properly installed house wrap.
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mhenson,
"It is a no-no to add a second layer of house wrap over the polyiso as that would put all the insulation between two layers of house wrap."
I don''t see why that is a problem as long as the house wrap is vapour-open, which Tyvek Commercial is.
Edit: As Bob says, I'm not sure the second layer is necessary though.
polyiso willl get saturated in a puddle, but on the wall it drains so won't be a problem. And since you already have a weather barrier, the lack of tape is not a big deal.
BOB IRVING
Below is the thread that caused my concern, can you validate or invalidate the conclusions?
https://www.greenbuildingadvisor.com/question/using-reclaimed-fiberglass-faced-polyiso
As Malcolm stated, housewrap is vapor open, so you can layer it without worry. You're probably thinking of a VAPOR BARRIER, like polyethylene sheet, which can trap moisture. Tyvek is not a vapor barrier, it's an air barrier, and it's intentionally vapor open to avoid exactly the kind of problem you're worried about.
You can improve tapeability (handy to make up a word every now and then) of fiber faced polyiso by putting a coat of primer on the areas you want to tape prior to taping. Use a primer that can bond to whatever the facer is, and then the tape can stick to the primer. A trim roller is great for this "priming for tape" step, and one heavy coat is usually fine. You don't need to be an artist here, you just want to give the tape something to adhere to.
Bill
What is your plan for jamb extensions around your window to transition to your siding which is raised from the wall by your exterior insulation? I would consider ThermalBuck since you are using flanged windows.
ThermalBuck is great except for the price, my insulation is 2.75 inches thick and I am doing innie windows. Building the jamb extensions from wood makes more sense to me.
Thank you for the good advice, I will layer Tyvek over the insulation and probably use the 1x4's to hold it in place. Staple caps won't hold and 3.5 inches of wood every 24 inches behind the siding should be plenty to protect the insulation.