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Community and Q&A

Surprise Roof Replacement and Beefing up Insulation

etoole | Posted in General Questions on

Six months ago, I bought an old house in Western Mass (Zone 5a). A week ago, I noticed a sinking area in the roof just below a skylight. None of the repair options were satisfactory so I plan on replacing the whole roof (see roof details for more). I’ve found a good roofing contractor that will rip two layers of shingles off, replace bad planks with plywood and add a vented roofing system.

Please help me to understand if improving the insulation should be done in conjunction with this work or if it can be done later. Because of the sinking section, time is of the essence.

Roof details:
Cathedral ceiling, unvented. AFAIK the decking is the old (original?) planks. Two layers of shingles, with current layer put on 17 years ago. I’m not sure what the insulation situation is, but it’s not great. I’ve seen evidence of exposed pink fiberglass but I’m not sure if the thickness is only the depth of a rafter or has been built up. The previous owners were very eco-concious but not well-heeled.

One idea is to rip off the decking, have spray foam put in from above, and put new decking and roof over that. This would be a substantial enough change to the current plan that I may need to find a new roofer. It would also take time to coordinate and I probably can’t afford it.

I’m also thinking I might just have the roof replaced and improve the insulation and air barrier on my own from inside. I would expose the rafters, cover them with foam board, and put new drywall over it. Probably do it one room at a time and use recycled insulation over the next year or so.

Thank you for your help!

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Replies

  1. krom | | #1

    The one room at a time thing seldom works out that well..

    another suggestion would be strip the shingles, repair significant rot, then add foam above, with 2x nailers sheathing, and shingles. Air seal the foam to the top of the wall.
    nailbase, could be used instead of stacking layers

  2. Expert Member
    AKOS TOTH | | #2

    The only two ways of doing an unvented cathedral ceiling in zone 5 is SPF or rigid above the roof deck. Converting to a vented roof is sometimes possible but a lot depends on roof shape. Something like a skylight is already not a good start as it would be blocking vent path from soffit to ridge.

    Energy efficiency wise the best is sufficient rigid insulation above your roof deck for condensation control. You can read about the ratios here:
    https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies

    This is usually not cheap but it would fix your venting situation and bump up your roof R value by a fair bit.

    SPF is good for air sealing your existing ceiling but won't help with venting. Since the SPF is not applied to the roof deck, you still need to figure out a way to vent the roof.

    Sometimes venting can be added to complex roof shapes after taking off the old deck by cross strapping the rafters. This allows for air flow in two directions so there is a continuous path from soffit to ridge.

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