housewrap, rain screen, and flashing
Time to spec out how I want everything done. The general plan is strip off all old siding and casing. Typar everything, 1/4″ Homeslicker rain screen mat over that, with 3/8″ CedarVentPlus channels at the top and bottom edges to help with nailing and keeping out bugs, then pre-primed red cedar R&R shingles. I recently added semi-dense pack cellulose (I wanted dense pack, but they had clogging problems at 3.0 so I have a feeling they might have dialed it down a bit) in the 2×4 walls (1920’s construction).
My concern is that without spray foam in the bays, I could trap vapor in them, and being in the Boston area, that could condense in the winter. I therefore want a really good air barrier (to reduce movement of moist air into the bays), but with high vapor permeability (to leak moisture out if trapped). Sheathing is original plank, with large gaps between planks (say, 1/4″). I’m hoping the rain screen will prevent reduce vapor penetrating from the outside then condensing on the wrong side of the wrap. By the way, I don’t trust adhesives (including on tape) to ensure the air barrier for the life of the shingles.
Windows are double-paned inserts, so I have empty weight pockets. To keep water out and to ensure a good air barrier, what should I specify to the siding sub?
I’ll start the discussion with my best guess.
1. Insulate/seal weight bays.
2. Typar to edge of window openings, sheets to span between floors without a seam, overlapping to drain to the exterior. Question: how should Typar be attached to minimize breaks in the air barrier?
3. Seal Typar to sheathing with adhesive (PL polyurethane window, door & siding adhesive?) around window penetrations except top edge of windows for flashing, along top & bottom plates (to avoid vertical bridging in the joist bays) and along wall lines between rooms (to avoid horizontal bridging).
4. Tape seams with Typar tape.
5. Cap flashing over head casing inserted under Typar.
6. 5/4″ fir casing, which is also the exterior stop to the double-hung window inserts. Is there a side flashing I should use?
7. 1/4″ exterior OSB around casing and exterior edges to act as a nailing support.
8. CedarVentPlus battens/channels at bottom edge of starting course and first nailing line of shingles, at top near roofline for venting, and also as a mechanical seal at Typar seams between floors to keep them tight against each other after the adhesive fails.
9. Homeslicker rain screen mats. How should they be attached to preserve Typar air barrier?
10. Cedar shingles, SS nails, two per piece 3/4″ from side edges, one more in center if over 10″ wide. Or staples to reduce penetrations to the Typar?
Some more detailed questions:
Should the Typar bridge over the window weight pockets, or wrap into the bays and be stapled, then separately ensure the bays are sealed? There’s only 1x on the window frame with the stop attached. I thought about pushing the Typar into the pockets a little and sealing the edge to the jamb with adhesive plus stapled strapping to mechanically hold the wrap to the frame. Seems like I’d just annoy the sub if I asked for that, though. Maybe push it all the way into the pockets up to the inside casing and back out, then insulate on the outside of the wrap, with some foam board just under the casing for water-proofing?
Should I space the shingles out, or have them pushed tight? Seems like a loose fit to 1/16″ gap might be good, though I guess the recommendation is for larger gaps. The siding will only be redone on the south and west walls which get a fair amount of sun (with evidence in their condition compared to the north wall; east was all redone), and will have the rain screen to dry quicker. They usually stack the shingles tight around here.
Caulk along the edge of each shingle course next to casing? Forget caulk? Does Vicor apply when new windows aren’t being installed?
What do I do at the bottom corners of the windows, both at the bottom of the casing where it meets the sill, and at the bottom corners of the sill?
He wants to powerwash before painting. Thoughts? I said a low-power scrub-down might be okay, but I’m worried about forcing water into/behind old shingles.
OK, lot’s to chew on, but I figured I needed to get the specification into the contract or there’s no incentive to get the right thing done, or correct it if they don’t.
thanks,
—mike…
Madison Renovations
Cambridge, Mass.
Replies
Use 30lb asphalt paper under cedar shingles. It is better than Typar or Tyvek for cedar, and recommended by the cedar bureau.
Paul
>Use 30lb asphalt paper under cedar shingles. It is better than Typar or Tyvek for cedar, and recommended by the cedar bureau.
I will be putting a full rain screen air-gap between the wrap and the shingles, so there will be no contact, and the back of the shingles can breathe. Tar paper won't give me a continuous air barrier (interior to exterior), which is my main reason for selecting the wrap. Because of the rain screen, any water barrier would work to keep rain out. I'm trying to keep moist interior air from getting into the wall cavity.
---mike...
MadRen,
I do not have any constructive advice for you.. but I am a little bit leary that housewrap will provide a completely reliable air barrier.
I'm sure that it will reduce air movement .. but it sounds like you are looking for a complete air barrier....
Here is a link to a project in Mass that used housewrap and rigid foam and yet still turns out to be rather leaky.
The homeowner thinks that the icicle in the photo was from a plumbing leak...but I still wonder.
http://superinsulating.blogspot.com/
Icicle photo
http://4.bp.blogspot.com/_krzeS_nEpwk/SccUu4q3xMI/AAAAAAAAAxs/zihmaPHR5F4/s1600-h/813+Arlington+Mar+14.jpg
and here is a discussion at GBA
http://www.greenbuildingadvisor.com/homes/old-house-gets-superinsulation-retrofit
Edited 6/8/2009 3:28 pm by homedesign
>I am a little bit leary that housewrap will provide a completely reliable air barrier.
I don't expect a complete air barrier, but I'm trying to reduce infiltration as much as possible within the constraints of not gutting the bays and filling with foam. So I stuffed in cellulose to help restrict air movement within the cavity, then the wrap to try to keep let the cellulose do it's thing. I'm more concerned with condensation in the cavity than overall leakage. I was just afraid the insulation will slow down air movement enough to allow condensation, whereas when it was leaky, there was so much movement that the cavity could dry out.
By the way, I did have a thermal scan to check for insulation; mostly okay.
---mike...