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Building envelope

ksteltenpohl | Posted in Energy Efficiency and Durability on


We live in southeastern Wisconsin and are currently a year into a complete gut/remodel of a home built in 1963. We have the interior complete and are living in the home (family of 4).
We are about to have the siding installed (LP Smartside lap siding) and have a few questions/concerns.

– 2×4 wall construction (16″ centers)
– interior spray foam closed cell urethane on all walls with no interior vapor barrier
– attic insulated with loose fill blown in fiberglass insulation R-50 with interior vapor barrier
– existing sheathing is 3/4″ black “horse hair”?, Tyvek housewrap and then an additional layer of 3/4″ pink foam sheathing. (This has a layer of film on it – seams taped)
– On the new additions there is 1/2″ OSB sheathing , Tyvek house wrap & an additional layer of 1″ pink foam sheathing (this does not have film on it – seams taped)
– Furnace is forced air, oversized bath fans in all 3 bathrooms and a commercial grade exhaust fan in kitchen.
– Windows are new Marvin Integrity casements with standard Low E/Argon gas dual pane windows
– Indoor humidity levels range from 35-45%

We have experienced some condensation problems on bottom 1/2″ – 1″ of glass when outside temp is below 15 degrees F and indoor humidity levels are above 40%.
I’m thinking some of moisture in house is still from all the tile work and plaster coat we did so my thought was to install an in-line dehumidifier next year if we are still having issues with condensation.

Should I install an air gap on the exterior of the foam sheathing prior to installing siding? If yes, will Greengaurd Raindrop provide an adequate airgap behind siding if installed over foam sheathing? Or will it compress too much to allow air flow/drainage? Our local building codes do not require a drainage plain/air gap behind siding and most builders in the area do not install more than a “wrinkled” house wrap if they do at all.

Should I have greater concern for condensation issues we are occationally experiencing now?

Thank you for your help with these questions!

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  1. GBA Editor
    Martin Holladay | | #1

    One of the unusual aspects to your wall assembly is that you have chosen to make a foam sandwich -- with relatively vapor-impermeable foam on both sides of your wall sheathing. That's not ideal, because it means that your wall sheathing won't be able to dry very quickly if it ever gets wet. But as long as the sheathing was dry on the day the spray foam was installed, and as long as your exterior wall flashing is impeccable, you will probably be OK. Just don't use this system again on your next house.

    It's usually not a good idea to install "oversized" bath exhaust fans or a commercial sized range hood fans. For more information on potential problems due to oversized exhaust fans, see Makeup Air for Range Hoods.

    Does your house have a mechanical ventilation system? Or are you just depending on intermittent use of your exhaust fans for ventilation? You may want to read this article: Designing a Good Ventilation System.

    I strongly urge you to include a ventilated gap between the back of your siding and the rigid foam layer. For more information, see All About Rainscreens.

  2. ksteltenpohl | | #2

    Thank you for your reply Martin.
    My reasoning for installing the foam sheathing was to cut down on thermal bridging and increase R-Value of walls since they were only 2x4 construction. I was concerned about the "foam sandwhich" effect when I discussed originally installing foam sheathing with a reputable local energy consultant. He claimed as long as I had the breathable Tyvek house wrap behind the foam, the wall assembly can still wick moisture to the outside and it would drain behind the foam on top of the tyvek. I did get the Tyvek on immediately when I removed the existing siding so the sheathing didn't get wet prior to installing spray foam. I will be very careful to flash everything properly and will use the greengaurd rainscreen house wrap over the foam. Is there a different type of foam sheathing I should have used that would help with thermal bridging but also allow for the wall assembly to dry better?

    The bath fans are oversized by a little, not too much but the kitchen fan was recommended to me by heating contractor based on quite operation since fan is on outside of house but he did size it to my 1st floor living area so I'm not sure if it is "oversized". I do not have a mechanical ventilation system outside of the fresh air intake & filter the forced air furnace has.

    Thank you again for your help,


  3. GBA Editor
    Martin Holladay | | #3

    Q. "Is there a different type of foam sheathing I should have used that would help with thermal bridging but also allow for the wall assembly to dry better?"

    A. Using foam sheathing on the exterior of your wall assembly to address thermal bridging is fine -- the thicker the rigid foam, the better. The error you made was installing closed-cell spray foam between your studs. That's what's limiting drying of your sheathing.

    It would have been better to install dense-packed cellulose or open-cell spray foam between your studs. Either of these options would have allowed your wall assembly to dry to the interior.

  4. ksteltenpohl | | #4

    Since the closed cell spray foam is already installed, should I remove the XPS foam on the outside of the sheathing?

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