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Moisture issue after energy retrofit and installation of radon fan

benjaminf | Posted in General Questions on

Last fall I worked with an allied partner from my energy provider and completed an energy retrofit on my home and added a radon mitigation system. The home is a two-story in climate zone 6a, roughly 30×30, 2×4 construction with fiberglass batts and poly, exterior sheathing is a combination stucco and asphalt impregnated board with painted LP siding.

In the retrofit I did extensive air sealing prior to adding insulation. With the exception of exterior sill plates, all of the attic penetration were air sealed with a combination of foam board and spray foam. Failing attic ductwork was replaced with new R-8, joints/boots sealed with mastic, and buried under the insulation. Cellulose was blown in to take the attic insulation up to R50, and the attic hatch was insulated and weatherstripped.

The house sits on 3 and 1/2 foot crawl space. Air sealing was completed in the crawl by spray foam on the rim joist. 2″ foam board was added behind the crawlspace vapor barrier. After the insulation was added I took the time to trim and reattach the barrier to the foam board. An exhaust only ventilation system was added via a Panasonic bath fan and was commissioned by the auditor to meet the required 53CFM output at standby. At the conclusion of the air sealing the air leakage at 50 pascals dropped from 950CFM to 700CFM.

At the suggestion of the energy auditor, I monitored the crawlspace humidity and radon levels for several weeks and added an Aprilaire crawlspace dehumidifier and sub membrane radon mitigation system. The crawl space stays consistently around 74F and 45%RH and radon under <0.03 pCI/L on the lower level of the home.

This summer I began noticing signs of moisture around two of the interior receptacle boxes on the  East side of the house. The interior walls are plastered drywall and you can see moisture discoloration around the box – top, bottom, and sides. There are no signs of moisture anywhere else on the wall except for around the receptacle and plate cover. The exterior siding on both sites do not appear to be damaged or defective and are sheltered from the rain by the soffit. Both of the receptacles are noticably drafty in the winter and presumably sites of air leakage.  I checked around one of the receptacles with a wireless scope and found that there is a patch of insulation missing from around the receptacle and based off of the temperature of the other one, I am assuming a similar situation.

I am uncertain how to proceed with diagnosing the moisture issue. I’ve temporarily disabled the exhaust ventilation to see if the issue persists and it does. The issue seems to be the worst when the morning sun shines directly on the wall. Any thoughts on the potential cause or how to diagnose the source of the moisture?

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  1. Jon_R | | #1

    You can research "inward vapor drive". Practically, you should insulate and air seal the area better. And in summer, use balanced or "intake only" (vs "exhaust only") ventilation. Pulling humid outside air into a wall and cooling it isn't wise. Especially so with stucco, interior poly and a radon (exhaust) fan.

    Try going to 55% RH in the crawlspace to save some energy.

  2. GBA Editor
    Martin Holladay | | #2

    Here is a link to a relevant article: "When Sunshine Drives Moisture Into Walls."

    The moisture you are noticing might be due to inward solar vapor drive, or air leakage associated with depressurization, or both.

    1. benjaminf | | #3

      Thanks, Jon and Malcom. I'm going to try a few days with the radon fan off and see what happens. The building detail described in the "When Sunshine Drives Into the Walls" article is the same building detail I'm working with. After 10 years in the house, this is the first summer with the moisture issue, so I'm hoping its a depressurization issue. If so, I may try moving the attachment point on the crawl space barrier from the wall to the concrete footings to get a better seal on the soil.

      Martin, am I understanding it correctly that the two potential retrofit fixes for solar vapor drive are rigid foam in the interior under the drywall and/or a new properly designed siding detail?

      1. charlie_sullivan | | #5

        If I understand right, you have two fans, one for radon sucking from underneath the crawl floor membrane, and another is a bath fan for whole house ventilation at 53 CFM. The one that might be causing the problem is the bath fan, not the radon fan. I would leave the radon fan on and experiment with turning the bath fan off.

  3. GBA Editor
    Martin Holladay | | #4

    Q. "Am I understanding it correctly that the two potential retrofit fixes for solar vapor drive are rigid foam in the interior under the drywall and/or a new properly designed siding detail?"

    A. Those are two possible fixes, but there are others. I'm not sure what you mean by "a new properly designed siding detail," but changing the siding type or adding a rainscreen gap would certainly help. Another possibility: If you are willing to remove the interior drywall, you don't have to add interior rigid foam -- instead, you could remove the interior polyethylene and replace it with MemBrain (or a similar product).

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