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What to do with an open cell spray foam attic in cold climates?

ranson | Posted in Energy Efficiency and Durability on

I’m looking at a house where the attic was insulated with open cell foam to the depth of the rafters.  The house is a 1950s era house near Worcester, MA. Zone 5. I don’t know what the sheathing is, but if it’s original, it predates OSB. The HVAC system is in the attic. There’s solar on top of the roof.

How can I remediate the two issues that I see?

1. I don’t want the sheathing to rot
2. It’s probably not enough insulation.

Also, how can I tell if the sheathing has already rotted?

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  1. Expert Member
    NICK KEENAN | | #1

    Can you stick a piece of wire in and see how deep the rafters are?

    Open cell foam is safe for roofs if it's thick enough. If it's not thick enough what I would recommend is making it thicker. You'd probably need to fur out the rafters to get the foam to stick in an even layer.

    Generally, if the sheathing starts to rot it telegraphs through the roof, you'll see sagging from the outside between the rafters. If he house is due for a new roof anytime soon it's not a big deal usually to just add a layer of 1/2" plywood over the sheathing the next time the roof is replaced.

  2. Expert Member
    Dana Dorsett | | #2

    Hi Neighbor! (Wormtown, here.)

    How air tight is the HVAC system? Is the living space air-leaky to the attic? If the ducts & air handler are reasonably tight and reasonably isolated from air movement to the living spaces, ventilating the attic to the outdoors in winter with a small ductless heat recovery ventilator (HRV) such as the Lunos E2 will lower the dew point of the attic air to below the outdoor air temperature, limiting the moisture accumulation in the sheathing.

    Measuring the pressure difference between the attic & living space under varying conditions (air handler on/off, warm days, cold days, etc.) with a micro-manometer would be one way to tell if living space air is fully communicating with attic space air. (Neutral- near-zero is best.)

    Short of that (if air sealing isn't practical), limiting the living space air to ~30-35% (lower if you can tolerate it) in winter with an HRV will also limit moisture accumulation in the roof deck. If it's possible, also install a layer of MemBrain "smart" vapor retarder over the open cell foam. At 30% RH and lower MemBrain becomes a Class-II vapor retarder, severely limiting the rate of moisture transfer. When the roof deck heats up in spring (or sunny days) releasing it's moisture through the foam, the RH next to the MemBrain soars, releasing that moisture into the attic space air. (This solution alone would be good enough in some climates.)

    The sections of roof with the largest risk are the north facing or fully shaded pitches, since solar heating of the roof drives off moisture. The parts of the roof covered with solar are by definition shaded at very close range, but also substantially protected from both radiational cooling down to the dew point and exterior moisture drives- it's a complicated model with lots of moving parts, but the solar may be helping more than hurting, whereas shading by trees or adjacent buildings/hills etc. definitely lower the average roof deck temp, increasing moisture uptake.

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