Continuous ceiling air barrier with vented attic
dpilot83
| Posted in Green Building Techniques on
We are quite a ways into our build. We have:
ICF Basement
2×6 external walls
ZIP sheathing
Some windows are installed
Roof is on
This is my builders first time trying to build a really air tight house. The drive behind that effort is coming from me.
I felt we had two choices for getting a really tight air barrier transition from the wall to the ceiling:
1a – put walls (both external and internal) up
1b – drape pieces of air barrier over the walls (I was thinking Siga Majrex)
1c – install roof trusses and roof
1d – finish Siga Majrex installation
1e – install 2×4 strapping below the Siga Majrex to create a service cavity for electrical and so on
1f – install drywall to the 2×4 strapping
In example 1 there would be no penetrations through the Siga Majrex at all. Anywhere penetrations would be required through to the attic, we would install a piece of ZIP in that area and we would tape the Majrex to the ZIP and the penetrations would go through the ZIP. Then each penetration would be well sealed.
Option 2 was really the same as option 1 except everywhere there was going to be Siga Majrex, there was going to be ZIP instead.
Builder chose option 2. I was kinda happy about that because I thought ZIP would be more durable.
I was quite busy when he started putting trusses up though and he had done quite a bit before I noticed that he had not installed 2’ x 8’ sheets of ZIP on top of the wall assemblies before putting trusses on.
When I talked to him about it he said that would cause issues and he was just going to install ZIP on the interior before drywall and tape carefully around the exterior walls.
I was super busy at that time and I didn’t argue much.
Now I’m thinking about it though and I am wondering, isn’t that going to be really hard to get an equivalent air barrier? How will the ceiling air barrier transition across interior walls?
It also sounds like he’s planning on putting the drywall directly onto the ZIP instead of building a service cavity for electrical between the two. This seems like there will be many penetrations in the ZIP instead of a few. Makes me wonder what the purpose of the ZIP really is. Really could have done that with drywall basically.
Anyway, my biggest question is, how to we create a continuous air barrier especially across interior walls now that we’re so far into the project? No drywall is up yet. But the walls and roof trusses are installed.
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Replies
Airtight drywall approach is an option, but arguably not going to give you the level of air sealing you were hoping for. All of the lights and electrical/plumbing penetrations will be difficult to seal well. I agree ZIP seems pointless to do at this point. My try your best with drywall as air barrier and AeroSeal the rest? Is a conditioned attic too far out of the question? you said roof is on, does that mean finish cladding on the roof is already installed so that it is too late for exterior insulation up there?
Finish cladding is on the roof so external insulation is not a practical option at this point.
There are no passive house level builders within several hundred miles of me so I sought a builder who was:
1. Willing to stretch outside of his comfort level
2. Didn’t hire a bunch of contractors (I felt it would be hard enough for me to convince the builder to do things differently, and I thought it would impossible for that to happen and for the builder to successfully convey all of that to contractors)
I feel like I am stretching my builder’s comfort level in a lot of different directions with this build. He has been good to be willing to stretch but there is a limit to how much someone is able to stretch in a short period of time. I think if I tried to switch to a conditioned attic at this stage in the game that would likely push him beyond the point where we could maintain a positive working environment together.
He is a good builder. I would say much higher quality than 90% of houses built in the USA before I even started working with him. But it is also more traditional type building. Maybe 3 ACH 50 as a guess. No external insulation. Good craftsmanship but not the kind of craftsmanship that is what is required for a house with less than 1 ACH 50. This build with a 1 ACH 50 or less target as well as European style flangeless windows and external insulation has been a lot for him to digest and I’m thankful he has worked with me as well as he has.
dpilot83,
I don't see much point in the Zip now either.
I still think you can get very good air sealing on the ceiling. Caulk all penetrations and gasket the sides of the top-plates on all interior and exterior walls. Seal any fixtures from above.
It is very disappointing when you find out you are not getting what you wanted, expected and thought paid for.
The first question is was this detail clearly shown in the plans?
Assuming it was in the plans the builder was working from, you need to decide if this was an oversight or willful disobedience and how you plan on reacting. How you react is proportional to how likely something else will get overlooked.
At this point you are married to your contractor and you want, if at all possible avoid a break up while not becoming a door mat.
From an air sealing point of view if the goal is very tight but not extremely tight say less than 1.25 and more than .75 ACH50. I think that is still very achievable without zipping the ceiling by caulking every hole in the top plates before the drywall goes in and more caulk around the electrical boxes after the drywall ceiling is in.
Walta
Thanks.
I spent a lot of time looking for an architect that was somewhat local, would provide good plans and wasn’t obnoxiously expensive. I found someone about two hours away.
It was a less than satisfying experience. If I knew what I know now I would have tried to hire Steve Baczek (https://stevenbaczekarchitect.com/info/) even though he lives far from me. He’s given me some recent encouragement on this project even though he didn’t design the house and I’m not a client of his.
If anyone is reading this and wondering how important plans are when you are interested in high performance building techniques, they’re VERY IMPORTANT.
Having a high performance builder is nice but if you find someone willing to learn (like my builder) not required in my opinion.
If you’re like me and you have found a good builder who is willing to stretch to the next level but who hasn’t done this before, you NEED good plans. And we didn’t have them.
The plans need to come from someone who has a lot of experience in high performance construction. After talking to Steve and watching many of his videos over the last year I think all of this would have been way less stressful and had way more elegant solutions to many problems and would have had a similar cost to what I did (especially by the time we take the hard path towards fixing things) if I had paid him a bit more at the onset of the project.
So in answer to your question, no, this was not detailed in the plans. I told the builder I really cared about air tightness and I told him the hardest part was the wall to ceiling transition and the ceiling itself. I sent him some of Steve Baczek and Jake Bruton’s videos on how to do a vented attic with a really good air barrier and I emphasized the continuous ZIP and continuous Siga Majrex (both with shallow service cavities) options with him. But I never said, “you have to do it this way”.
To be quite honest, during that time I was sending him to a of stuff. I had been researching this stuff for a long time and he was getting a drink from the fire hose and I think it was just tough for him to keep up.
I talked with him today and he does have a plan to take air sealing seriously. I think he chose the hard way but he doesn’t seem oi be shirking the concept and I’m thankful he’s keeping after it. After that conversation I think we’re going to be OK.
I don't see a whole lot of point to a service cavity in the second floor ceiling. I assume you're not going to run HVAC ducts so basically it's going to be plumbing and electric. Plumbing is only going to be vents, they're going to be vertical so there's going to be a vertical penetration no matter what you do. So it's down to electric. Run all the wires above the zip, only penetrate where you need to and seal every penetration.
What will be nice about the zip is that the sealing fits in with the order of operations -- frame, wire, seal then drywall.
How many wires do you have in the ceiling anyway? The only thing that is truly required is smoke detectors. You can do a smoke detector in a pancake box, which just means a single penetration for the wires.
The zip is fine as is provided it is taped on the top plates of all the walls. You can then tape the exterior Zip to the top plates for air barrier continuity.
If the zip is not continuous over your interior walls, it needs to be taped to those top plates as well.
Depending on how many pot lights you have in the plans, putting that many holes through the zip won't be fun plus will a potential air leak. I would avoid them all together or install a small service cavity (2x on flat) to leave rooms for wires and slim LEDs plus driver. Installing the service cavity will be less work than trying to cut all those holes and making sure they are sealed.
I've gone away from pot lights to flush LED's. You can mount them in a pancake box. You wouldn't have to penetrate the zip except for the wire. You do have to keep an eye on box capacity though.
You don't need a service cavity if you have an accessible vented attic above.
My home blew a 1.1 with just gyp board on the ceiling under a vented attic. But I was fanatic about air sealing. Every electrical box in the ceiling either has putty pads or spray foam, on the ceiling side. Every other penetration (bath exhaust, makeup air, ERV, etc.) has spray foam all around the penetration. It is sealed tight.
Take a look at Scott True's youtube videos and determine if you could avail yourself of his soffit detail and do a conditioned attic - that would alleviate part of your issue and might even be more cost effective than the zip on the underside of the ceiling.