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Air Sealing House

Brad_C88 | Posted in General Questions on

Hello,

I will be building a new 2 story, 3500 sq ft house in Climate Zone 4a (Northern VA). There will be a basement that I plan to remain unfinished for quite some time. The house will have 2×6 exterior walls and I will be using Zip Sheathing. The insulation in the attic will consist of Closed Cell foam sprayed against the roof decking to achieve the code required R-38 (so about 7 inches). The exterior of the house will have vinyl siding. I do not have very deep pockets for the perfect insulation details so I am trying to achieve the best bang for my buck.

2 questions:

1.) I will be doing exterior insulation of the basement, slab in the basement, and the garage slab. I want to use 3″ of Rockwool to achieve the R-10 which is what code requires for basements. My question is about the details of the Rockwool on the foundation walls and the dimple mat. Which goes on first? Also, should I dampproof the exterior walls before applying the dimple mat and Rockwool?
I plan to use flashing to transition from the wall sheathing to protect to the Rockwool. I know it will be a larger piece of flashing but I cannot afford to cover the entire exterior of the house in insulation.

2.) Next question, I am looking at different options for the best way to air seal my house and like most of you, I have gone down a rabbit hole of what to do and what not to do. Looking for advice, should I consider spray foaming the entire interior of my home with 2″ of CC foam to air seal, or caulk the sheathing to the 2x6s on the interior of the house to control airflow, or consider Aerobarrier? 

I have read through Joseph Lstiburek’s book on building in a Mixed-Humid Climate and from what I understand I want to avoid having both an exterior and interior air barrier system which would lead to moisture problems in the stud cavity.

Let me know if any more information is required
 
Appreciate the advice

Brad

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Hi Brad,

    Caulk and tape are best for air sealing. I would try to minimize the closed cell and opt for HFO foam where foam is the best choice.

    If you want a conditioned attic, I would consider the flash and batt approach. I’d move the basement foam to the interior and use reclaimed rigid foam. In the walls, I would use an air permeable insulation and use any saving to install a ventilation system.

    1. Brad_C88 | | #3

      Steve,

      From what I have read, exterior basement installation is the best bet since it will warm the foundation while preventing condensation on the interior walls. Also, when the time comes to finish the basement it will be a simpler install since I do not have to worry about a vapor barrier. Correct me if I'm wrong please

      Brad

      1. Steve Knapp CZ 3A Georgia | | #6

        Here's what Martin says, "I used to believe that the best location for basement wall insulation was on the exterior. In recent years, however, I’ve decided that interior basement insulation makes a lot of sense."

        Really, it can go on either side. But I think the consensus is that there are more issues with exterior insulation (especially protecting the above-grade portion).

        1. Eric Whetzel | | #11

          We went with 5" of Rockwool on the exterior of our foundation: https://kimchiandkraut.net/2017/01/02/foundation-for-passive-house/

          To Steve's point, we went with metal coil stock flashing around most of our house, with some stucco around two basement windows in order to protect the Rockwool.

          Here's a ProTradeCraft video where OA Design + Build used panels of pvc from Azek to cover the insulation at grade: https://youtu.be/opmk9NoN9xE

          We really enjoy the look of exposed concrete walls in our basement:
          https://kimchiandkraut.net/2019/03/11/dressing-up-the-basement-steel-beam-and-concrete-walls/

          Damp proofing and then the Rockwool should be sufficient. You shouldn't need the dimple mat --- at least that's what Rockwool told me back in 2016 when I asked. Couldn't hurt to contact them to see what they recommend today.

          We used mostly the European-style tapes from http://www.foursevenfive.com for air sealing.

          I also used the Prosoco R-guard series of products: Joint & Seam, Fast Flash, and Air Dam. They worked incredibly well too.

          In general, I found the tapes easier to use for the seams in the Zip or plywood, while the liquid membranes seemed ideal for around windows and doors:
          https://kimchiandkraut.net/2018/02/11/wrb-for-passive-house/

          I agree with Bill --- I'd use the Aerobarrier only as a 'last step', if at all, rather than as your main approach for air sealing. It's a newer product, so you may not want to depend on it exclusively for all of the air sealing.

  2. Expert Member
    Zephyr7 | | #2

    1- I prefer rigid foam here, I don't trust mineral wool to stay good in terms of insulating value underground.

    2- Don't use spray foam in your walls just for air sealing, it's really overkill for that. I would got with exterior rigid foam (polyiso here), taped and sealed to the exterior. That doubles as an air barrier, and it also helps with thermal bridging as well as adding R value to the wall assembly. Be sure to use at least the minimum amount required for your climate zone, more is better as long as you stay under the amount that makes trip difficult (typically more than 2 to 3 inches becomes "thick" as far as trim is concerned). Use batts in the walls, I like mineral wool here.

    Use polyurethane caulk for maximum durability when sealing these things. Tape is good for seams like those between sheets of polyiso, just make sure the tape will adhere well to whatever you're sticking it to. Some materials, like polyethylene, are notoriously difficult to keep things stuck to. Canned foam is good for larger holes such as around pipe and wire penetrations.

    I think of aerobarrier as a "last step" to be used if you want to get the last little bit of sealing done after you've already done all the big stuff using other methods. I wouldn't consider it as a primary method of air sealing.

    Bill

  3. Walter Ahlgrim | | #4

    Why the conditioned attic?

    Will the attic be full of HVAC equipment?

    Walt

    1. Brad_C88 | | #5

      Yeah, HVAC will be in the attic and all the ductwork for the second floor.

      Brad

  4. Wooba Goobaa | | #7

    It is my understanding that properly installed / taped ZIP is your air barrier.

  5. Walter Ahlgrim | | #8

    In my opinion Spray foam is a very ungreen product and using it is an act of desperation best reserved for the few places where no good alternative is possible, take the time and make a better plan. Putting the insulation on the roof increases the surface area losing heat by 50% using spray foam quadruples the cost per R of insulation compared to cellulous. R38 ceiling is just plain sad.

    Seems to me you need a new and better plan one with flat ceiling covered with R60 cellulous with all the equipment and ductwork in the conditioned space.

    When you build a house many times you will be presented with stupid options that make construction easy but you will regret later. Summon up the courage to say no to stupid.

    Walta

  6. Eagleeyeshawk | | #9

    I always enjoy Walter telling someone else to say no to stupid besides me!

  7. Jared Zoller | | #10

    Check out matt risinger’s “monopoly house”. In one of his most recent videos he uses hunter panels cool-vent panels on the roof. I’m personally a big fan of this approach. Definitely having the right blow grade rigid insulation is important, lean on the manufacturers for their guidance. Not all are created equal. I’d stay away from spray foam...if you use zip system sheathing and obsess over the details...spray foam is a waste of money. I’m a big fan of rockwool although I wouldn’t use it for the below grade exterior insulation. I like rigid exterior for below grade since it protects the basement damproofing/waterproofing during backfilling, its a task that can get crossed off the list early in the building process (it always ends up being a last minute thing for me when it’s installed on the interior side), it won’t get in the way of the MEP trades.

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