WRB Sealing The Envelope
Hello, All.
Newbie and aspiring #Green#LEED#Passive self builder here.
So I am just starting to take my first steps towards self building my 1st home. This has been sometime in the making and I have had many many hours to venture down the rabbit holes of green building research. Framing Starting in a month or so.
I am looking for some input on peel and stick WRB’s over zip sheathing with a combination wood / metal rainscreen siding system.
Location 1 hour east of Dallas Tx. Climate Zone 8, average annual temperature 65f, mean summer temp 85f. Average rainfall 39″.
Here are the details of the build. Foot print, 72′ W x 38′ D slab on grade. This includes an attached garage 26′ W x 36′ D 936 sq. ft. With 1′ eaves front and back. House 1380 sflq. ft. 30′ D x 46′ W, with 4′ D front and back eaves over patio / walk way.
External walls will be 2x6x10 16″ OC. with 6/12 sissor trusses 24″ OC. over house (2×6 top chords) with attached trusses over garage also (2×6 top chords). I plan on using spray foam insulation throughout, walls being open cell, rafters will 1st be insulated with 1″ of closed cell then finishing off with open cell.
My question is regarding whether or not I should use a peel and stick WRB over zip system. I feel strongly about using a peel and stick WRB on the roof atleast and am having difficulties deciding on which product to use. The roof will be Black metal lifted by a cross batten system using firing straps. This will create a continuous vented cavity from the base of rainscreen siding up through to the vented ridge cap with attional air flow pulling from large up to 4′ fully vented eaves.
I would like to use a radient / foil faced peel and stick WRB like “MFM Peel & Seal”. Are there any reasons not to use a foil faced WRB directly on the roof deck? Any proponents for using the peel and stick WRB over the entire structure not just roof? Cost is a big factor here which is why I am considering only doing the roof. Also, while not worth a tone, Zip systems warranty sounds like it would cover the roof I’d “ice dammed” but not if the wall sheathing is covered by peel and stick WRB.
The other potential WRB’S are “Stormguard”, “MFM Ultra HT”, “Grace Products” or “Sharksking Ultra SA”. By cost “Stormguard” is the cheapest and readily available nearby.
Looking forward to your input.
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Replies
> My question is regarding whether or not I should use a peel and stick WRB over zip system.
Simple question: why? If you're going to put a peel and stick WRB over sheathing, don't use Zip.
> I plan on using spray foam insulation throughout, walls being open cell, rafters will 1st be insulated with 1″ of closed cell then finishing off with open cell.
> The roof will be Black metal lifted by a cross batten system using firing straps. This will create a continuous vented cavity from the base of rainscreen siding up through to the vented ridge cap with attional air flow pulling from large up to 4′ fully vented eaves.
It seems you've described a vented roof assembly. There is zero reason to install closed cell in this assembly, especially at such a thin layer. You'd be paying double the labor (since the crew would have to cover the house twice) for nominal benefit.
Thanks, Patrick.
I guess my thought process on using a peel & stick WRB over zip is redundant.
If I were to switch out the zip, for standard osb roof decking, or the whole home, do you reccomended any one peel & stick WRB over another? Any opinion on the MFM aluminum faced?
With the spray foam, my thoughts were if I ever did have a leak, the closed cell will act a waterproof layer and keep moisture out of the open cell. You brought up some great points and have changed my mind however. That 1" of closed cell under 3200 sq. ft. of roof decking would have been equivalent to 1/3 the total insulation cost.
There is nothing green about using foam insulation; it's a problem-solver when nothing else will work, but if you're building new there are many better options. Foam insulation is environmentally damaging, expensive and although rare, there are many cases where it did not cure properly which turns into a nightmare situation.
Open cell foam is better for the environment than closed-cell foam, but open-cell doesn't do anything that a batt or blown insulation with a separate air control layer can't do just as well. All foam glues whatever it touches together, which you might think is a good thing but almost all homes will eventually experience renovations and having to deal with foam makes that much more challenging.
If you have a roof leak, you want to learn about it as quickly as possible; you don't want to hide the leak until it has caused extensive damage. On that front, closed cell is the worst thing you could use; open cell is a little better and a fluffy insulation such as fiberglass or cellulose would be best.
Radiant barriers only work if there is an air space on one or both sides. When they do work, they can seem effective when compared to having little or no insulation. But once you add a reasonable amount of insulation, the radiant barrier's contribution is minimal, if it's even working at all. A little dust on the shiny surface slows or stops the radiant effect.
If it's a vented roof, the only reasons to NOT use a peel-and-stick over the entire roof are cost and environmental impact. If it's an unvented roof, it's best to not trap the sheathing between two impermeable layers, though it's probably ok as long as the sheathing was dry when covered and leaks never develop.
I recommend reading these articles:
https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
https://www.greenbuildingadvisor.com/article/martins-ten-rules-of-roof-design
And the related links. If you don't have a subscription, considering what you want to do and the questions you are asking, I think you would find a subscription worthwhile.
As a shameless plug, I also think you'd benefit from reading the book I co-wrote: https://bookshop.org/p/books/pretty-good-house-michael-maines/17201583?ean=9781641551656
I also recommend these two books:
https://www.energyvanguard.com/book-house-needs-breathe-or-does-it/
https://www.tauntonstore.com/musings-of-an-energy-nerd