PolyIso foam strips as rain screen furring?
I’ve been thinking about rain screen furring options.
In cases where we’re not using exterior insulation, it seems it would make sense to use 1/2″ x 2″ strips of foil faced PolyIso to create a rain screen gap over the sheathing. These are readily available in 4×8 sheets around $15 with a compressive strength of >16PSI
Here’s what I’m seeing as advantages over wood furring:
Unaffected by moisture.
No rotting / leaking through fasteners.
Not a food source for termites or ants.
Cheaper than lumber.
Easier / less messy to cut than plywood.
Disadvantages:
Requires 1/2″ longer fasteners.
What do you guys think about this? What do you feel is the best all-around choice of furring when exterior insulation isn’t used?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The fact that they can't receive screws for siding (or trim, gates, etc.) is a major disadvantage. Having to always hit the framing with a 2.5"+ screw is a hassle indeed, and something to consider.
I recently used poplar strips, which were very economical when purchased from a hardwood supplier. I bought 4/4 boards and ripped them down to 1x3 nominal.
In my case, not having a truck/van, I like to avoid sheet goods where I can. Buying boards of solid wood has the hidden value of being workable on a human scale — I don't have to rent any equipment to transport it, or even get a friend to help me unload those heavy sheets.
You don't need to hit the framing when fastening cladding if you have minimum 7/16" sheathing.
With 5/16" siding and 1/2" rigid foam strips, a 1 5/8" screw into 1/2" plywood sheathing is good.
Agree with Scott.
Also, these thin strips of foam will compress which will be a tremendous PIA.
Yes, foam is a food source for ants.
And no, foam is not easier to rip than plywood- it is far more difficult and much messier. If you feed the foam into your table saw a touch out of square (and you will), it’ll bind the blade and kick back. The volumes of microplastic dust ripping will create is one of the messiest things you can deal with on a jobsite.
Use solid material. For me, usually the best deal I can find at the time at a local mill on 1x3 or 1x4. No ripping.
See below re: compressive strength.
You don't use a saw to rip 1/2" thick PolyIso. A straight edge and utility knife is the way. Zero mess.
They can chew through it, but foam is NOT a food source for ants or termites.
I would advise against going this route for several reasons:
1- It's 16 PSI, but it's also dealing with relatively concentrated loads. Concentrated loads cancel out the "averaging" you usually get with foam. As an example, 10 PSI foam is good for 1,440 pounds per square foot, which is great under a slab! The slab helps to spread the load out over a large area of the foam. With a concentrated load, such as a 1.5" square between framing members, that same "10 PSI" foam is now only good for 22.5 pounds at the contact point -- not much. Furring strips are a bit more distributed than a single contact point, but they're still much more concentrated in terms of load bearing areas compared to something like an under slab situation.
2- Critters won't eat polyiso, but they are likely to burrow in it -- especially if it's moist, which may be the case near ground level where humidity levels can be higher.
3- Zero fastener holding strength. EVERYTHING has to go through the polyiso into the framing to have any strength, and the polyiso won't have nearly the shear resistance a piece of wood will have (i.e. the fasteners can easily migrate through the polyiso sideways under certain loading conditions).
I'd use plywood strips myself. If you want something more robust in terms of longevity, you could try PVC trimboard, but that stuff is expensive, and it doesn't have any real fastener holding power either.
Bill
1 - It's really not much a concentrated load here though. It's just siding.
Not like we're hanging a ledger off it.
With 2" wide strips, 16PSI compressive strength, and lap siding (least ideal), you'd still have a minimum of around 32lb compressive strength for a single siding nail through the plank.
On the other hand, with hardie panel siding (or any non-lap type) the load is spread across an area several times larger.
2 - As it is, rainscreen furring is already well above ground, but if we're comparing foam strips to wood, the termites will eat the wood as a food source. The foam would have the advantage here over untreated plywood or lumber, as it would be less likely to be chewed through.
3 - We're only talking about 1/2" thick PolyIso though.
Consider that the siding is pulled tight against the foam which is pulled tight to the wall. The combined physics here make it a total non-issue with 1/2" foam.
I definitely wouldn't recommend trying this with 10 PSI EPS.
16 PSI faced PolyIso in 2" strips seems like it would work well though, especially with non-lap siding.
It sounds like you're looking for people to support your use of polyiso here. I don't think it's a good product for this application, and would not recommend it's use here. I would not recommend it, would not use it myself for the reasons I mentioned, and I don't think it would satisfy manufacturer requirements for warranty purposes (which might not be an issue for you though. Hardie, for example, is notoriously picky about this sort of thing, others not so much).
You're considering using a far less structurally robust material here for a semi-structural purpose. I would strongly advise against the use of polyiso here.
Bill
I'm just looking at the physics here Bill, which do not fail here in the way you suspected.
Are you familiar with Cor-A-Vent's SturdiStrip rainscreen furring strips? They're just corrugated plastic strips 1.5" wide. There really isn't a big structural demand at play here.
On the other end of the scale, but relevant, you have builders running long screws through 4-6" of mineral wool. Comfortboard 80 has 3PSI comp strength and a density of 8pcf. The heavier mass and low compressive strength (less friction support) result in significantly more shear force vs the same assembly with rigid foam. It still works though with appropriate fasteners.
Alex,
FHB had an article on using foam-core strips as rain-screen, so although like other posters I'm instinctively not keen on the idea of using foam (or foam-core), I think you are right that it would work.
You’re not comparing apples to apples.
I do not think you’ll find anyone with even a nuanced level of experience or understanding to agree with you in this. Simply put, it’s a creative, but terrible idea.
Yes, ants eat foam- widely understood. They’ve eaten MY foam! You need to take great caution to prevent otherwise.
And again, the stuff will compress. When it does, getting your boards to plane out will be an awful experience. Especially toward the ends. I’m not drawing on physics or specific loads. Have built piles of houses and installed miles of ISO. Very familiar with the stuff. And no, I would not cut strips to install around an entire house with a knife. Easier though, I wouldn’t do this to begin with. Like I suggested, strapping from a mill and my material cost is similar per LF to your ISO strips. But I have zero labor involved.
One more note: 16g gun nails are improper for hanging almost any type of siding.
My concern is more with longevity. I'm sure what you're suggesting will seem fine initially, but I have serious doubts that it will last and stay as structurally sound as a wood block/board will over time.
Cor-A-Vent is usally used in long strips, where loads are more distributed. It's also pretty solid, less squishy than normal polyiso in my experience. I still wouldn't use small pieces of it for furring small points like you seem to be planning to do.
Ultimately it's your decision, but I warn you that what looks good on paper does not always turn out so well in practice.
Bill
re: "Cor-A-Vent is usally used in long strips, where loads are more distributed."
Cor-A-Vent's SturdiStrip rainscreen furring is only 1.5" wide vs the 2" wide strips of PolyIso we're discussing here.
Are you thinking of Cor-A-Vent SV as used in bug baffles along top and bottom?
At $2.50/LF, that would be quite expensive. They make a dedicated rainscreen furring strip called Cor-A-Vent Sturdi Strip. It's only a 3/8" drainage plane, so WRB wrinkles could block airflow, but a decent option for $0.50/LF.:
https://www.roofingdirect.com/shop/cor-a-vent-sturdi-strips-rainscreen-furring-strips/
At >16PSI, foil faced PolyIso has over 5x greater compressive strength than Rockwool Comfortboard 80.
Lapped plank siding may or may not cause a deflection issue due to the concentrated load at the edge of the plank. Anything fastened flat with load distribution up/down the strips would have more than enough support though. Nail drive depth or driver clutches would be easily adjusted to allow a smooth install.