Housewraps and rainscreens
Hello,
I am looking for opinions on the usage of housewraps and/or rain screens on walls and roofs with rigid poly-iso exterior insulation. A little background first. I am building a small home, off-grid, home in Eastern Washington. We are climate zone 5b and very dry (only about 14″ of rain annually and very little humidity). The structure is a 40’x40′ post-frame with 9′ side walls and attic-room trusses. The trusses are 2′ OC and will set on top of 5.5″x12″ beams that span between 3-ply posts which are 10′ OC. The walls and roof will be sheathed in 7/16″ Zip to provide shear (per structural plans). Roofing will be a clip style standing seam metal and siding will be exposed fastener metal panels. The attic room will be used for living space and is 20′ wide and I am going to run ductwork and utilities in the truss areas outside of the attic room, so I want to keep the entire area within the conditioned envelope, so the insulation will be at the roof line. The combination of 2′ OC trusses and the insulation at the roof line, makes installing a vapor barrier on the interior side extremely difficult. This lead me to two options: 1) closed cell spray foam on the underside of the Zip sheathing 2) continuous insulation on top of the roof sheathing (min R-20 per IRC-806). I am going with option 2 and installing 3.5″ of poly-iso (R-22) on top of the Zip with R-23 or R-30 mineral wool strapped to the underside between the trusses. The poly-iso will go down in two layers. A continuous 2″ layer followed by 2×4 furring strips spaced for standing seam roofing clips (~3′) with 1.5″ poly iso between furring strips. I am going with a similar assembly for the walls, except only 1.5″ of poly-iso (I can get a good vapor barrier on the inside of the walls). This brings me to my questions: Do I need any additional protection (self-adhered ice/water barrier) between the zip and the poly-iso? How about a rainscreen product like HydraGap or Cedar breather? Should I use a rainscreen on the outside of the poly-iso under the metal roofing or siding? The metal cladding company I am using said that in a dry climate like ours they do not require any rain screen behind the metal panels as the panel profile creates enough gaps for air to move and dry things out. However, this isn’t the case between the poly-iso and the Zip sheating.
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Replies
I'm giving this a bump as I'm curious to know the answers myself.
You mentioned having both a vapor barrier inside the walls and polyiso outside. I'm concerned this might prevent the wall from drying out in either direction. Perhaps a smart vapor retarder would be a better choice.
I'm also a bit confused about the actual drainage plane on your roof. If water were to get behind the metal it would be running down a polyiso surface that is interrupted by horizontal wood battens every 3'. That sounds questionable. If the entire surface were covered with a peel and stick type membrane that would provide drainage continuity, with only the metal roof fasteners themselves interrupting it.
The idea of a drainage layer between the sheathing and insulation, or between the 2 layers of insulation, sounds problematic because there wouldn't actually be any airspace for the water to drain.
But I'm outside my comfort zone here and would welcome someone with more expertise.
You don't want vapor barriers on both sides of those walls. If you're planning on using foil faced polyiso on the exterior of those walls, then that foil facer is a vapor barrier. You do NOT want an interior side vapor barrier here. If you want to try to minimize vapor ingress into the wall assembly, use a smart vapor RETARDER such as Certainteed's MemBrain or Intello. Vapor RETARDERS slow vapor migration but still allow for some drying, making them safer here.
I don't really see an issue with that roof assembly aside from the Zip. I don't think Zip gains you much here when you're putting polyiso over it. I'd use regular CDX plywood here instead.
Bill
I shouldn’t have said vapor barrier. I meant retarder. I will use something like Membrain/Intellio/majrex. My main reason for using Zip is to get a better panel quality than standard commodity OSB and to have use it has my air barrier. But I hadn’t thought about CDX. Would taped CDX offer the same air barrier qualities?
Matt Risinger installed the polyiso directly against the Zip on his own personal home he constructed, for what it’s worth.
Matt’s use of poly-iso was an influence in my decision. I like the idea of another layer of sheathing on top of the insulation, but also seems like way overkill for my situation. I think am going with Matt’s monopoly house strategy with the sheathing and continuous insulation. The use of 3.5” of insulation was on purpose and just happens to meet the code requirement for CI outside of the sheathing.
He did use another layer of sheathing on the roof, but not on the walls. His wall assembly is Zip -> Polyiso -> Rainscreen 1x4 furring strips -> Hardie lap siding
User ...917,
Walls:
You have a few options for an effective WRB. The taped foam, a house-wrap over the foam, or one behind. If you go behind, I would suggest a dimpled house-wrap to provide a drainage plane. That will also reduce the necessity of using a rain-screen behind the siding, making it more likely the contours of the metal siding will be sufficient to act in that capacity.
Roof:
There is no benefit to providing a gap between the metal panels and the foam / strapping layer below. Just use a good impermeable sheet underlayment, and make sure the sheathing is detailed as an air-barrier.
One caution. These hybrid permeable / impermeable roofs can have moisture problems without an additional warm side air-barrier. That's usually the drywall on cathedral ceilings, but with attic trusses it is very hard to achieve. Unfortunately I don't have an answer as to how you deal with that when the underside of the batts are interrupted by truss webs.
Our climate is extremely dry and I plan on ventilating the attic areas with the exposed insulation to prevent moisture buildup. For example in our current house in was only 17% RH at the end of May on an 80 degree day.
“home in Eastern Washington. We are climate zone 5b and very dry (only about 14″ of rain annually and very little humidity). “
I am not telling you to ignore the rules but if you chose to given how dry your climate is it would almost be impossible to get your building wet enough for long enough for anything to grow mold and rot.
I think you should avoid the use of spray foam. It is the most expensive, least green and riskiest way to buy an R of insulation.
I do not see the appeal for Zip roof sheeting. Its only real advantage is that your roof will be water tight sooner and less likely to get wind damaged to the roofing felt before the roof can get installed.
I think spending time money and effort to build a rain screen on a home in the desert is silly. If you lived on the other side of the mountains and it rained 300 day a year then the rain screen would be important.
I don’t like house wraps they are almost always poorly installed and then left exposed long enough that the wind rip it full of holes. I am a fan of the regular Zip wall sheeting It provides a solid air and water barrier. Note avoided the Zip+R it weakens the walls.
Don’t spend time money and effort on water vapor in your climate you don’t have enough to be a problem do the code min for vapor barrier and you will be fine.
Seems like the post frame is redundant as you are building you are building walls around them to support the Zip on 24 inch center. The 24 inch on center walls will easily carry the load. If you need some decorative beams the 24 inch walls can hold them up also.
Walta
Thanks for the reply. This is what my gut was telling me, but I haven’t seen many (any) homes built around here as tight as I plan on building. I didn’t want to run into unforeseen issues because I wrote something off due to the dry climate. What I’m gathering is to deal with the bulk water somewhere and then let give at least some pathway for the assembly to dry both directions and I should be fine.