Foil-faced polyiso on exterior of wall
Joanie21
| Posted in General Questions on
Climate zone 4, Westchester Co, NY. House is framed plywood, wrapped with Typar drainage, innie windows installed. GC about to install 2” foil faced rigid foam all over exterior. Plan is cellulose inside 2×6. Rainscreen over foam for wood shiplap siding
Dumb question. Which direction should that foil on the foam be?
Also, GC bet resistant to doing two 1” layers staggered claiming too windy location & twice the price for labor.
Thx
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Usually the foil facing is on BOTH sides of the foil-faced rigid foam. I’m assuming this is the usual foil-faded polyiso. Normally which side faces which direction doesn’t really matter, but if there is a difference it should be printed on the insulation panels which side should face which direction.
It makes sense that two layers would be more labor to install. Make sure they tape the single layer seams properly and you’ll be ok.
Bill
Joanie,
I've never looked into the question of directionality when it comes to to installing sheets of rigid foam, and you learn something new every day, but I'm pretty sure the answer is to install them in which ever direction makes the most efficient use of the material.
Staggering two sheets of foam can provide a better air seal, but your builder is right that it is more work and installing exterior foam is a labor intensive project. If the foam is your primary air barrier, then it's probably a good idea to stagger the seams.
Here's an article on installing exterior rigid foam insulation:
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-sheathing
Joanie,
If you're using a brand of rigid foam with aluminum foil on just one side, the foil faces the rainscreen gap. If you want any thermal benefit from a radiant barrier, it has to face an air space. Otherwise, the radiant barrier provides no thermal benefit.
Wow! I totally misread this question and thought you were asking if the sheets should be installed vertically or horizontally, which I thought was an interesting question, but you never know what someone's experience is. Listen to Bill and Martin on that.
Time to get some coffee...
The only polyiso I know of that has a difference in sides is Dow Thermax where one side is the fire rated side. There may well be other variations like Martin hints at, Ive just not used any other kind myself. The regular stuff I’ve always seen has either felt on both sides, or foil on both sides.
Bill
Gtk thx
Thank you all! Buying my GC a GBA membership for Christmas.
Still vascillating about ERVs. Anybody? Dedicated system 3000 sq ft 3 BR 3.5baths. Mini splits are my HVAC. Why is Zehnder $10k total with ducting & boosters and UltimateAir 200DX-UL $3K unit only. will all the ducting & switches be $7k by the time I’m done? I’ll look at other brands (and smaller systems because I think ASHRAE standard overventilate). Advise me here or find the OG thread from yesterday’s Q&A
Joanie,
I would look at which ERVs with ECM motors you can get installed in your area. Generally the ones with hexagonal cores or dual cores are the most efficient. ECM motors are worth it as the cost of power on something that runs 24/7 adds up quickly.
Couple of options:
Vanee G2400EE
BROAN ERV200 ECM (looks like a rebaged Vanee unit)
Airflow AIR100HEXF
Rervosmatic RERVD 100ES
With innie windows over foam the window flashing is usually lapped to drain onto the WRB between the foam & sheathing. It's better in those stackups to use a crinkle type WRB to allow for drain space and a modest amount of capillary break. With smooth housewraps it may be better to extend the window flashing out to the rainscreen channel.