Vertical Rain screen wall in climate zone 4C
I am an architect designing a house on Salt Spring Island which is in Climate Zone 8. The code mandates rain screen and the client desires vertical red cedar ship lap boards. The house has very high ceilings at 14′ and 12′ which requires a fire break at 10′. My questions are to do with how to best design the wall to mitigate humidity inside the house and create the most energy efficient house possible for the owner.
My current wall build up is:
1/2″ Wallboard with latex paint
2×6 stud wall construction with R21 Owens Corning fiberglas insulation
1/2″ OSB sheathing (also to create the required shear walls)
2″ rigid insulation taped acting as a Class II vapor barrier.
7/16″ Cor-a-vent attached at 24 inch centers horizonatlly as a nailer for the vertical boards
3/4″ vertical siding.
Total roughly 9 3/4″
My concerns are with the rain screen strategy using the cor-a-vent. It seems a strong enough product to use but certainly doesn’t comply with recommended 1 1/4″ to 1 1/2″ nail depth for the siding. Also will 2″ rigid insulation be enough in climate zone 8 to bring the dew point outside of the structure? I have read in some blogs that it should be more like R15 vs the R10 which I have.
Also I am not including an interior poly on the studs as I am assuming the wallboard/primer and latex should suffice.
I would very much appreciate any comments/suggestions to the above. Thank you!
Marcus
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Replies
Does Canada use a different system than the US? Salt Spring Island looks to be a zone 4C... There's no way it's zone 8 (Fairbanks).
Why not just go with a battens/counter battens system, as is done in Central Europe?
https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing Yes, R15 for 2x6 walls. Why fiberglass insulation?
Hi Mike, yes maybe it does, as I am in Boston in 5c, I looked up a climate map for Canada which zoned it 8, but I agree it can't be. 4C makes more sense in which case I could go down to 1.5 inches of rigid outside the structure.
Marcus
Hi John,
I used the fiberglas insulation as the spray foam is apparently "unknown" on the island and would be hard to come by, so I went with the R21 fiberglas which is readily available.
Salt Spring is within kayaking distance of here... zone 4 marine.
Marcus, I would either use battens / counter battens as is commonly done here in Vancouver Island, or a proprietary product such as Home Slicker. I have never seen Cor-a-vent used in rain screens except as an insect screen at top or bottom of the cavity.
David, Most of the kayaks travelling from Salt Spring to your island don't carry building supplies :)
Marijuana is now legal here. Not so much reason to import via kayak anymore.
Hi all, thank you for your responses. I agree it is definitely 4C, which has answered my rigid insulation question.
@ Malcolm - If I use battens and counter battens, is there an issue with water build up on the counter battens? That is why I was looking at Cor-A-vent, however Cor-a-vent doesn't seem right to me. I agree with eh insect screen use of it without a doubt. On the island is the batten cross batten strategy made up of 1x4's or do you have to use a 2x4 as the cross batten to achieve the nailing depth required by code? Thank you!
As I understand it, to be fully code legal in Canada with only a class-III interior vapor retarder requires the sheathing not drop below 4C at the 99% outside design temperature with a 20C interior temp. (This is a much more severe constraint than IRC-based US codes.) With R21 cavity and ~R10 sheathing you're probably not there yet. Your 99% design temp is probably about -8C. (see http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf ) You would likely need to bump that to 3" of polyiso to get there. (Look up the real design temp and code spec, but I'm pretty sure even R15 /3" XPS might be cutting it close, but it's a slam-dunk with iso.)
The ratio of the foam-R to cavity-R has to exceed the ratio of the delta-T between the design temp and +4C, and the the delta between 4C & 20C. With 16 degrees delta on R21 you have R1.3 per degree delta. If the design temp is -8C, that's a 12C below the 4C mark, so the sheathing R needs to be greater than (12 x 1.3 =) ~R16, not R15, but it's close enough that a degree or two warmer makes a difference.
I'm sure there's a clearer way to explain the crude dew point calc, but R10 surely isn't going to cut it unless codes have been relaxed to something along the lines of the IRC. (If the code has been relaxed and you can use R7.5 on the exterior, use 2" EPS or 1.5" iso rather than XPS, since they use blowing agents with less than 1% of the greenhouse gas potential as those used for XPS.)
Rock wool batts are usually a better fire barrier than high-density fiberglass, and run about the same price. R23s are pretty air-retardent too.
Air tightness is critical aspect of getting the full performance out of high-R walls, and utilizing the ruggedness of the sheathing & framing to detail as part of the primary air-barrier is usually good option. There needs to be a bead of caulk (or construction adhesive between any stud plates and the bottom-plate to subfloor, best done as it goes up. The sheathing can be caulked to the studs inside the stud bays before the fiber insulation goes in. Treat band joists to subfloors & foundation sills similarly, but use spray foam to seal the foundation sill to the concrete. Spec an acoustic sealant type caulk for long term flexibility- a lot of caulks won't cut it over the long term.
Hi Dana,
Thank you! This is great. As I read the tables my 99% would be -3C (26F). I am using the Victoria, BC number as it is right across the bay from my site.
So if I use your equation, I am 7C below the 4C mark which would be 7 x 1.3 = R9.1. I will look into the EPS and ISO instead of the XPS and appreciate the Rockwool suggestion. Let me know if I am reading those tables correctly. If I change to the Rockwool then the calc. would be 7 x 1.43 = R10.
Thanks again, very helpful. Marcus
Marcus,
Both sets of battens are the standard 37mm x 10mm furring commonly sold for rain screen in lumberyards on the coast. The recommended spacing for the vertical furring is 200mm.
For insect protection at the bottom of the cavity I would suggest you use a perforated L flashing designed for this purpose and available at Slegg Lumber on Saltspring.
The best resource for detailing rainscreen and other building envelope situations I have found is the BC government publication "Building Envelope Guide For Houses" put out by the Homeowner Protection Branch. Well worth a look.
Thank you Malcolm, I'll check it out.
Marcus
If R10 makes it GREAT! But it might be on the hairy-edge:
The wind shadow effect of the mountains might still give you a lower design temp than Victoria though, since their temps are better moderated by the waters in the strait of Juan de Fuca than in the leeward bays & islands closer to the Strait of Georgia. Design temps can differ over surprisingly short distances in that area sometimes, especially heading just a little bit west rather than a little bit north. Compare the 99% design temp differences listed between Nanaimo and Victoria!
Given that SSI is roughly halfway between those cities it's better to split that difference and call it -5C, but it might actually be a degree or so cooler for micro-climate reasons. There may be some weatherstation data out there to dial it in when designing close to the edge, but bumping it to R12 (2" iso) should give you some wiggle-room with the inspector.
I have relatives both on Whidbey Island, and in Port Orchard WA, and have noted some of the micro-climate quirks over short distances in those waterways-it's not always obvious.
But specifying 2" polyiso (or 2.5-3" of EPS) is still a greener option than 2" of XPS due to the large (>100:1) difference in greenhouse gas potential between HFC134a vs. pentane blowing agents. The cost differences are "in the noise", but the difference in lifecycle environmental impact is significant. (In Europe XPS is blown with CO2, which is even more benign than pentane, but the R/inch is closer to that of EPS than the N.American HFC-blown XPS.)
Hi Dana,
Thank you! I like the 2" polyiso option over the XPS or EPS. It still allows me a possible nail purchase into the studs from the outlying horizontal member for the rainscreen. I agree with you regarding microclimates around there, very changeable. Very similar to out here on Cape Cod where the weather changes every 20 minutes.
Marcus, a couple of other things. You don't see OSB being speced for either wall or roof sheathing very often here on the coast. Most projects use 1/2" plywood, which holds up better in this moist climate.
The other is that on Saltspring, like the rest of the temperate rainforest, there are Carpenter Ants, and they love foam. An infestation can severely compromise the exterior wall insulation. It happened to Building Science's own office, which Joe Lstiburek incorrectly dismissed as being due to moisture infiltration. Foam closely mimics the dead wood they prefer to nest in and the ants will tunnel through any exposed foam. Unfortunately insect screens are not an adequate defence and either using treated foam or treating the structure with a borate spray such as Boracare is highly advisable.
According to ASHRAE, SaltSpring Island would be climate zone 5.
http://www.greenzone.com/general.php?section_url=12
Further descriptions at this link
http://www.greenzone.com/general.php?section_url=27
Hi Malcolm, Thank you very much for that. Very good advice. Carpenter ants are big out here on the Cape too and devour houses. I will be sure to detail the foam enclosure correctly and spray the house.
Hi Aaron, Thank you for these links. I had not seen these before. It looks like we are on the border between zones 4 and 5 and when I delve in to the detail a little further, it looks like Salt Spring is still in the Zone 4C. The HDD at 18C ranges from 2500 - 3100 which fits with the 4C description in these charts. Thank you for the links!
Marcus
Marcus, here is another take on the subject from the GBA archives:
https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/what-s-best-approach-rainscreen
Hi Malcolm,
This is great. Thank you. I had started going down the road of Homeslicker. I am intrigued by the Hydrogap product as well. As it is now, I have adjusted my wall to be:
1/2" wallboard with primer with paint
no poly
2x6 framing with R-24 Rockwool insulation
1/2" plywood sheathing
1.5" polyiso rigid insulation
10mm Homeslicker10 with Typar backing
horizontal 2x4 furring strips cut on the top with a 15 degree angle toward the homeslicker for drainage and fastened through to the studs with 50d nails
vertical shiplap boards fastened to the 2x4's with 6d nails
All of this minimizes the depth of the wall and meets code for cladding and furring strip fastening to the structural frame. I think I'm getting there...Marcus
Why 2x4s? Can't you use 1x4 or even 1x3? And, ripping all of it with a bevel seems unnecessary.
Hi David,
The code requires siding to be attached to wood by at least 1.25 inches, that's why I went with the 2x4. The angled top of the 2x4 to allow greater drainage down through the Homeslicker.
I could probably use a 1x4 if the inspector reads the 1.25 inch requirement for the nail to include the nail length within the siding itself, as that would be 1.5 inches. It all depends on the opinion of the inspector. I can find out for Salt Spring.
Marcus,
Another link to rainscreen building code requirements:
http://joneakes.com/jons-fixit-database/2112-Rainscreen-Detailing-and-the-Canadian-Building-Code
Hi Malcolm,
This is also a great resource! Thank you. This answered my concern for the need for vertical strapping if I employed a proprietary drainage product such as the 10mm Homeslicker10 with the Typar backing. By using this I do not need the vertical strapping and go with my beveled horizontal strapping. All is coming together.
Marcus
Dear Marcus,
As an alternate rainscreen design you may wish to consider the Climate-Shield rain screen system. It uses the Cor-A-Vent as an insect barrier only, as you suggested, and creates an unimpeded 3/4" wall cavity and requires no furring strips in a vertical installation. More information here: http://www.mataverdedecking.com/architectural-specifications/
Best regards,
Chris