Air Sealing CDX + Tyvek details
Hi all,
Zone 5. Western MA.
1st (and hopefully last) time owner/builder. Gearing up to start installing standard Tyvek house wrap over CDX.
Ive already zip taped the 1/2″ CDX at the seams and liquid flashed the concrete to sheathing joint.
1.) As of now, the window and door openings do not have any tape or flashing where the sheathing terminates at all openings. Should I tape/liquid flash these spots before applying the tyvek, for air sealing purposes? Or does a standard tyvek protocol window flashing installation do a good enough job at air sealing those exposed sheathing spots at openings?
2.) The roof rafter birds mouth cuts go down from the top plate of the wall approx 3 1/2″, meaning the tyvek will not reach all the way up to the top plate and will leave 3 1/2″ of sheathing exposed under the eaves (which have 12″ overhangs). Is this typical? Also, where the top of the CDX terminates at the top plate is currently not taped or liquid flashed. Is that location important to tape/liquid flash?
3.) I’m pretty paranoid about meeting air tightness code = 3ACH 50. As I don’t have any air sealing experience to hark back on, and do not have the budget for cutting edge products and procedures. I was considering installing the Tyvek as a SECOND air barrier on top of the already sealed CDX = Continuous bead of caulk horizontally under the very bottom and top of the Tyvek. All Tyvek seams taped, fastened with cap staples. Is this overkill or a worth the extra effort? Any reason this would present problems? I know Tyvek is not the greatest air barrier, but Id imagine in conjunction with the sealed CDX, it would improve blower door results. (?)
(Over top of the tyvek will eventually be horizontal 3/8 or 1/2 furring strips with vertical pine shiplap siding. Exact details on how to potentially allow air flow behind furring strips TBD.)
Thanks in advance for any help
MD
Thanks!
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Any thoughts?
md1986,
1. I think the standard Tyvek details do a good job at openings - although for some reason they omit a head-flashing, which should definitely be included. https://www.youtube.com/watch?v=JhXx__AMzTo
2. It's good practice to staple a strip of house wrap to the top of the wall before framing the roof and soffit. However it's not essential.
If your sheathing is your primary air-barrier, then yes all the joints and perimeter need to be sealed, and it needs to be continuous with the ceiling air-barrier above.
3. The primary purpose of a WRB is to stop water infiltration from outside, so that is what you should detail for. Sealing the bottom of the house wrap risks trapping any water that gets behind it.
If you want a secondary air-barrier a more appropriate one is the drywall. That also has the benefit of stopping moist air from cycling into and out of the wall cavities, keeping the sheathing dryer.
Thanks for your time Malcolm,
Sounds good, I'll detail the WRB as only that, and not an air barrier.
I should have thought to put a piece of WRB over the top plate before I put the roof on. oh well!
In your case, the main air barrier is your sheathing, the fact that the housewrap is not fully air tight doesn't matter.
At the top of the wall, you want to carry the air barrier from the sheathing to your ceiling air barrier (typically 6mil poly under the ceiling drywall). The simplest is to flap a strip of house wrap or a wide piece of flashing tape over the top of the wall before the rafters are set. You then connect this to the ceiling poly. It sounds like your rafters are already set, you can still a decent job with sections of some wide flashing tape.
For the windows/doors same thing. The air barrier is the sheathing so this is what you need to connect to. What I like to do is stop the house wrap 1" away from the rough opening than flash the opening with flex flashing tape, cover this 1" of exposed sheathing to connect the air barrier than contiue it about another 1" over the house wrap.
3. "I’m pretty paranoid about meeting air tightness code = 3ACH 50"
You are well ahead of aproduction build, I don't think you'll have any issues of hitting this unless you forget to install a window before the blower door test. The wall air barrier is usually no the big air leak in most house. Typical areas to watch is wall/foundation joint (sounds like that is covered), Any canteleaver floor overhangs (this site has good details for this) and house/garage connection especially if you have a bonus room (again details on this site).
"do not have the budget for cutting edge products"
You can easily get a sub 1ach house nothing but some quality tape and elbow grease, there is not need for anything cutting edge here.
" I was considering installing the Tyvek as a SECOND air barrier on top of the already sealed CDX "
Doesn't hurt but wont get you much. I doubt it is worth the effort.
Vertical shiplap is not a great idea. Consider horizontal install or if it must be vertical go for something like reverse board and batten. This is similar look and the benefit is the battens act like rains screen furring so it saves you an install step.
Akos, Thanks for taking the time to help.
Yep, My rafters and roof are already on, its a gable roof, 10/12 pitch. House is a simple 24x32', no cantilevers, dormers, bump-outs, garage, etc. Just a rectangle with a triangle on top!
I'm doing a Vented attic with a continuous ridge vent. The 2nd floor walls on the eaves ends are 6' tall, and will slope up to 8' (at a 10/12 pitch). There will be soffit vents, leading to baffles, up into the attic, where the attic floor will be blown in cellulose. This small sloped section of ceiling is still throwing me for a loop, but I've read that if the baffle wraps down over the top plate, and the entire space above the top plate is insulated, I potentially wont have to insulate to the same level of a cathedral ceiling. The rafters are 2x12, so a good amount of space for insulation.
So I could just tape where the sheathing meets the top plate, in between the rafters? Does this need to be continuous all the way to the inside edge of the top plate, or is it just a matter of sealing the sheathing to the top plate?
Interesting you mention reverse board and batten, a seasoned builder I spoke to recently suggested the same thing. My initial concern with that is - The board and batten siding will only be nailed off into 1/2 CDX. That would be fine?
Thanks again!