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Community and Q&A

Bad allergies and building a new house in Seattle

WGHess | Posted in Green Building Techniques on

I’m building a new house in Seattle . The foundations is complete and the framing is underway.

I have bad allergies to mold among other things and was unable to live in a house that I remodeled two years ago (also in Seattle). (scroll to the bottom and read up) So this time I am starting from scratch.

My new house has the following envelope/heating features and I’m looking for recommendations/comments on details that will prevent mold and ensure high indoor air quality.

Basement  (slab and walls have already been built)
Followed the waterproofing recommendation in Hammer and Hand Guide
Using 1″ EPS foam behind studs (1″ gap between concrete and studs) and Mineral wool batts between studs

Walls (being constructed now)
From outside to in: cement siding, 3/4″ rain screen, 1.5″ exterior rockwool, VaproWrap SA, sheathing, mineral wool batts, gypsum board, latex paint

Ventilated roof with asphalt shingles over roofing felt
(Any recommendation on type of roofing felt?)
Insulation is being added to ceiling (type of insulation is TBD, any recommendation?)

Air and Heating system
Hydronic infloor heat in 4″ basement slab and 1.5″ gypcrete at main/upper
Lifebreath HRV for whole house continuous ventilation
AC for top floor only (Mitsubishi air handler in basement)
Requiring minimum ACH of 3
Airsealing with Aerobarrier (any thoughts on this?)

Framing during wet time
contract requires that the wood moisture content be below 18% before insulation and drywall. I’m also requiring that all wood piles be tarped and that the subfloor be swept and any standing water be removed daily. Unbelievably, the last 2 weeks have been perfectly dry in Seattle which is a rarity for October. If anyone has any tips for keeping mold at bay during the ‘open time’ I would be appreciative.


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  1. GBA Editor
    Martin Holladay | | #1

    Mold occurs in a damp environment. Once your house is complete and you move in, buy several hygrometers, and keep an eye on your indoor relative humidity. If your environment is dry, you won't get mold.

    High indoor humidity levels are associated with mold. So is bad housekeeping. Keep a clean house, use your bathroom exhaust fan, mop up spills, and you should be fine.

    1. WGHess | | #5

      The insulation contractor is providing different options and I'm curious which option you recommend and why. (Seattle, Marine Zone 4A)

      Exterior walls:
      A - Original plan is to do 1.5" of EXTERIOR rockwool and 5.5" of interior rock wool. ($$$$)
      B- 5.5" of rockwool ($)
      C - 2" of closed cell foam and 3" of rockwool ($$)
      D- 5" of closed cell foam ($$$)

      Which option is the best from a moisture/condensation/mold control standpoint?


      1. BrianPontolilo | | #6

        Hi Willard,

        I think your original plan makes sense, but there are some caveats. Here's an article that you will find helpful if you haven't already seen it:

  2. Jon_R | | #2

    > l prevent mold and ensure high indoor air quality.

    Consider a dehumidifier and slight positive building pressurization (so you can control the source and filter incoming air).

  3. Andrew_C | | #3

    That house sounds like it starts with better specs than many. Hammer and Hand have a lot of good ideas that they’ve shared. Hope you have a builder that can execute the details to your satisfaction.

    Various ideas wrt indoor air quality and moisture:

    Read BSI-017_Star_Trek_IAQ_Solving Indoor Air Quality Problems at BSC’s website for a few laughs and some good ideas (summary on page #1). Read BSI-070 for a few more laughs. [Lstiburek dug up this quote: “If there is a pile of manure in a space, do not try to remove the odor by ventilation. Remove the pile of manure.”
    Max von Pettenkofer, 18581]
    FYI, if you search for BSC documents with “mold” you’ll get more than a few. Or, search BSI and “indoor air quality”.

    New houses tend to have a lot of moisture, partly from wood materials, and also from concrete. It takes a while for the house to dry down to stable moisture levels. During this period (6-12 months, perhaps?), I would plan on running a dehumidifier, probably located in the basement if the house has one. In humid climates with mild shoulder seasons, you may have to run a dehumidifier even after the house has stabilized, so get a good one that is reasonably quiet.

    If you have a sump pump instead of drain-to-daylight for your basement, make sure you get a sealed top for the sump basin. This may also help with your radon abatement and allow you to just use a passive system instead of having to add a fan.

    If you have allergies to “other things” besides mold, and even if you don’t, try to avoid introducing pollutants; ventilation systems are important, but dilution is not the solution, or at least not the best part of the solution. Cabinets, countertops, carpets (just say no), and a plethora of other things that you put into your house can contribute to reduced indoor air quality. Do some research.

    Those are just thoughts. I wish my house had your specs.

  4. Expert Member
    Akos | | #4


    Pointers with my experience of trying to keep ragweed at bay. Add an additional external filter box to your HRV. Merv 13 is not enough, spend the money on a large in-line HEPA filter.

    During ragweed season, I turn the ERV to max and close off the exhaust damper to pressurize the house. This makes the biggest difference in indoor particulate count as well as symptoms.

    Good Luck

    P.S. I found getting gypcrte here quite expensive (need a company that has the equipment to pump it). Ended up putting down 1.25" of sandmix cement instead, the weight difference is not that much, easier to fix if you need to break up a section.

  5. Expert Member
    BILL WICHERS | | #7

    Don’t use any carpet. Carpet can hold on to allergens and house dust mites.

    I agree with the need for a better than normal air filter too. My wife suffers from chronic sinus issues so I’ve taken precautions to keep airborne irritants down. A good air filter helps a lot. I’d stay away from any flex duct too since it’s difficult to clean anything that accumulates in the ridges.

    I’ve had good luck with “better than felt” synthetic roofing felt, but I don’t have enough experience with it to really know if it’s better over the long term.

    Keeping the house a tiny bit pressurized with you HRV will ensure that nothing sneaks in through any leaks. All your fresh air will come in through whatever filter you use this way.


  6. andy_ | | #8

    "Airsealing with Aerobarrier (any thoughts on this?)"

    Have you already found an Aerobarrier contractor in Seattle? I'd be curious to hear more about it once you get to this stage. It seems like a good product and I'm considering it too in case I don't hit my air sealing target by more conventional means.

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