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Water and air control strategies for retrofit applications

Brian C | Posted in Green Building Techniques on

I have an old house with no insulation aside from 1/2″ of polyiso installed to the outside of old wood sheathing and siding and covered in vinyl. Based on previous posts and research I have decided I want to add exterior foam insulation while residing. I am trying to make a plan to get the greatest “bang for the buck” and I am encountering varying advice. Here are my observations:

– building science corp. published a deep energy retrofit builders guide based on the national grid pilot program. In this guide they use house wrap, either adhered or un-adhered as an air control layer and lapped and taped exterior foam as a water control layer.

– here on GBA comments by Marc Rosenbaum indicate that house wrap is not a good air barrier and foam is not a good water barrier (airtight wall and roof sheathing article)

– the article “where does the housewrap go” suggests that the housewrap should be aligned with the windows. BSC’s guide does not align the windows (outie) with the wrap (under foam).

– other and possibly biased sources also suggest that taped foam is not a durable wrb.


1. Is there a right way to do a retrofit with exterior foam or do a number of methods work? What is the best bang for the buck?

2. For a retrofit situation, what would be used for air and water barriers if housewrap and foam (respectively) were not.

3. What would be used as a fully adhered sheathing wrap for board sheathing? Liquid applied wouldn’t work because of the gaps. Would it be something like ice and water or enV-s?

4. BSC’s guide does not recommend folding the housewrap into the RO. Instead they use a transition membrane in the RO sealed to the housewrap with caulk. What is a transition membrane? Why not fold the housewrap?

5. Would a liquid applied flashing work as a sill flash with exterior foam?


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  1. GBA Editor
    Martin Holladay | | #1

    You're right: there is no single set of recommendations for performing a deep-energy retrofit or for attaching rigid foam on the exterior side of your walls.

    Some builders choose to install housewrap under the foam; others choose to install housewrap over the foam. Either approach can work just fine, as long as your workmanship is good. So relax a little bit.

    You've probably already seen it, but in case you haven't, I recommend this article: How to Install Rigid Foam Sheathing.

    Here are the important principles:

    1. Pay attention to airtightness. You seem to want to install a self-adhered membrane that will act as an air barrier as well as a water-resistive barrier (WRB). That's OK, but it will probably be expensive. If you really want to go down that route, you might consider one of the vapor-permeable self-adhered WRBs like VaproShield WrapShield SA or Henry Company Blueskin VP. For more information on these products, see New Green Building Products.

    If you choose a different method of achieving airtightness -- and there are many -- you'll save money compared to these expensive products.

    2. Know where your WRB is. You can use housewrap for your WRB, or rigid foam, or a liquid-applied membrane. Whatever product you use, you need to know the location of your WRB, because that is the layer that will be integrated with your window flashing system.

    3. Flash your window rough openings. My mantra is, "Flash the rough opening, not the window."

    4. Include a rainscreen gap behind the siding.

    If you follow these principles, you'll be fine.

  2. Brian C | | #2


    With the goal being best air, water, and thermal performance per dollar spent what would you use as your respective barriers and methodology if you were doing it?


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