Replies

1. Expert Member
   Michael Maines | Feb 03, 2018 12:49pm | #1

   Ytsehoos, I applaud your effort to get to 1.0 ACH50. It's not that hard to do, if you have a good plan and execute it well. I designed the 2016 ProHome and have designed quite a few other "Pretty Good" houses and renovations. A few comments:

   Zip sheathing makes a good air barrier. I don't always recommend it, though; it depends on the overall wall assembly. If you are planning on continuous exterior mineral wool, as we used for the ProHome, in the appropriate thickness, the sheathing will stay dry and taped Zip sheathing is a great way to create the air barrier.

   Peel and stick WRBs are the new kids on the block, and a great idea but they are expensive. There is usually another way to get similar results at a lower cost, but sometimes they can be the best choice.

   Rain screen furring does not have to be treated, because it dries before any damage can occur. We used 1x4s on the ProHome but I would probably recommend using 2x4s, and checking with your siding manufacturer before committing to a detail.

   The Membrain on the interior of the walls wasn't really necessary; it was the result of sponsorship, which I tried to be clear about in my write-ups. It doesn't hurt anything, it's just not necessary in the walls, if you have painted drywall and your exterior insulation is the right thickness (or, more technically, in the right ratio compared to the framing cavity insulation.)

   We used a vented attic, with an interior air barrier of Membrain at the ceiling. It's not the easiest product to use for an air barrier; it's my third choice, behind Pro Clima Intello Plus and Siga Majrex--again, the result of sponsorship. Drywall or Zip sheathing can also be used as the air barrier, if properly detailed. Membrain can work; it's just fragile for the application as an air barrier, but its variable permeance characteristics are very good when a vapor retarder is necessary.

2. 
   Ytsehoos | Feb 03, 2018 01:27pm | #2

   Awesome! Thank you very much for the info.

   To help with additional info, I'm in climate zone 4A (Chattanooga) and I'm planning on between 1 to 2 inches of rock wool outside of 2x6 with R-19 batts. I believe that I've read here that I'm pretty good with nearly any depth I choose in my zone.

   Hopefully, we can get a basement for mechanicals, it will depend on how rocky the location turns out to be. I know a vented attic with loose fill is the most efficient route for a well insulated attic. Spray foam is expensive and you increase the surface area by 30% or more. I was planning to go with traditional roofing material.

   My house design isn't the best for super efficiency. So, I'll have some first level ceilings to seal as well as second floor ceilings and some attic facing walls. Would I be ok with acoustical sealant and sealed boxes or would you recommend the Intello Plus in those areas? If so, what sealant should I use with it?

   Here's the house plan;
   https://www.architecturaldesigns.com/house-plans/country-home-plan-with-marvelous-porches-4122wm

   I'll be building the garage at a later date so the house is currently the only issue. Planning on taking the ceiling all the was across the first level to buy back another space upstairs with a dormer facing the back.

3. Expert Member
   Michael Maines | Feb 03, 2018 03:12pm | #3

   Ytsehoos, you're welcome, and you're correct--Hamilton County is zone 4A so no interior vapor retarders are needed, which means that any thickness of exterior insulation meets code. The energy code (IECC) calls for a minimum of R-20 cavity insulation or R-13 cavity plus R-5 exterior in your zone. If you have R-20ish in your wall cavity, as you should with 2x6 walls, then either one inch (R-4) or two inches (R-8) of mineral wool should work just fine. If you're making the extra effort to install exterior insulation, you might as well maximize labor costs and use the thicker product.

   Your design looks like it has a saltbox roof, so it will take a bit of thought to define your continuous air barrier, which is best kept tight to the thermal barrier as well. In your case, that would mean insulating the slopes on the back side of your house. You don't have to use spray foam to achieve your required R-49; you can furr the rafters down with 2x4s on plywood gussets to create a vented cavity as deep as you want, for any insulation product, as long as it's covered with an approved ignition barrier (or thermal barrier in some cases). If you do this it will make air-sealing easier as well.

   If you decide to keep the thermal and air barrier tight against the living space, you will need to deal with multiple ceiling-to-wall-to-roof transitions. It's possible, and may save some money on insulation, but it will cost more in labor (and higher risk of mistakes).

   Acoustical sealant is usually nasty stuff, though effective. I recommend Pro Clima Contega HF or high-quality, zero-VOC tapes instead.

4. 
   Ytsehoos | Feb 03, 2018 09:32pm | #4

   Ok, so after researching some more, your suggestion would be;

   A) Immediately under the sheathing place a vent baffle like AccuVent (making sure to be airtight), fur down 2x4s with OSB/ply to a desired depth, fill with insulation, provide 2nd airtight barrier and be sure there is a ignition barrier between that and the living space. But I need to take care to air seal both top and bottom sides of the insulation.

   B) Shoot about 2-3" of CC spray foam (R15 for CZ 4A) then I could add other insulation, all immediately under the sheathing.

   One of these definitely sounds easier than the other! Ugh, I wish I had a simple roof plan...

5. 
   Ytsehoos | Feb 04, 2018 08:30am | #5

   Am I also on track to think that any of the gusseted and fill assemblies must lead to a ridge vent? And unless the dormers are vented, I'll have to spray those?