Rainscreen design advice
Bbrown3381
| Posted in GBA Pro Help on
Did a search on this and came up with nothing… The question centers on a rainscreen design for verticle cypress siding on rigid foam over CMU. Zone 2. Block house. One story.
Thinking 1″ rigid foam then horizontal furring topped with a core vent type product then siding.
Interested if this is proper design, if core vent is over or under furring strip and how far apart to place furring strips. Install attaching furring strips over the foam to CMU via tapcons would be a heck of a lot easier with the vent on top of the strips.
Thanks for the time to answer.
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Replies
Robert,
First of all, if you are going to all this trouble, I strongly urge you to increase the R-value of your wall by installing 2 inches of rigid foam instead of 1 inch. In later years, you won't regret the upgrade if you take my advice.
Second, you could get away without the Cor-a-Vent type product (I assume you are talking about a plastic mesh product) if you want. Very little water gets past the siding, and the water that does get past the siding will dry readily via evaporation, even without drainage past the horizontal nailers.
Third, if you want to install a Cor-a-Vent type product, you can.
Your horizontal furring strips should be 24 inches on center.
Thank you
If using just furring strips should they be PT
Robert,
No. A rainscreen cavity is generally very dry, so there is no reason to use PT lumber.
Robert,
For highly permeable sidings like wood, in climates without a lot of rain, and for walls with good overhangs, horizontal furring is fine. You get one of the three attributes of a rain screen - the capillary break. If you want the other two - the ability to drain any bulk water that gets in, and ventilation of the cavity, you can do two things: Either place a layer of furring vertically before the horizontal, or stagger your horizontal furring so that there are gaps allowing water to drain and air to circulate. For even more drying potential you could vent the top of the cavity too. How far you go depends on how much drying you need.
Board & batten siding is more susceptible to wind driven rain penetration than horizontal board siding types. Zone-2 in hurricane areas is very different from calm zone-2 high-desert areas.
In zone 2 it only takes an inch of exterior EPS (R4) to meet code min, but the cost of upgrading that to 1" of foil faced polyiso (R6) is negligible, as is the cost of going with 2" of EPS. If you use foil-faced foam, a foil facing the rainscreen gap adds a measurable performance improvement, and makes air sealing the seams dead-easy with a high quality temperature rated foil tape (Nashua 324a or similar). The labor & furring cost to install 1.5-2" foam is really the same as with 1" foam. There's a very modest uptick in fastener costs for the longer TapCons. At typical scrap rates and material pricing the installed cost of another R4 would be on the order of 35-40 cents per square foot.
If this is a retrofit rather than new construction the windows are by definition "innie" mounts, unless you are changing out the windows too. If that's the case, a spray-applied weather-resistant barrier directly on the CMU is easier and better than most other solutions, since the existing flashing is designed to direct bulk moisture only out to the exterior of the blocks.
If you are concerned about bulk water behind vertical siding - or cedar shingles, both of which are applied over horizontal strapping - you can use a drain wrap (Raindrop or similar) against the sheathing and behind the strapping which will allow the water to drain. I doubt it is necessary, but this is an inexpensive, quick solution.