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Community and Q&A

Repair a poor roof assembly

CraigRo | Posted in General Questions on

Back story:
Anchorage AK, CZ7. Hot roof garage, existing home from the 70’s. 2×8 roof trusses. New shingle roof, but existing water damage forced me to remove Gyp and fiberglass batts. Furred out the 2×8’s with 2×12’s for greater insulation thickness and reinforce several rotten trusses from the water damage. 

Attempted to convert to cold roof, R38 batts, used blocking to keep an open air path. 6 mil vapor barrier stapled to the bottom. Then I stopped….bad move on my part, I know. Should have finished taping, sealing, and gyp. 

Really cold weather, no snow, 0F on the roof. Suddenly we have 25F and 12+” of snow. Now I have water dripping through the insulation and pooling on the plastic. From an inspection and pulling back the plastic, my fears confirmed. I have ice crystals on the bottom of the roof decking, and ice chunks on the shingle nails sticking through. My cold air blocking is also moist. Now as a reader of GBA, I have a strong feeling the moisture is from my lack sealing my air barrier properly. 

Here’s my question (thank you for reading the discussion):
It’s still winter, and trying to dry the deck underside is challenging at best. How do I seal this, dry this, and be able to continue without further damaging my home? What is the best way to move forward?

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  1. user-2310254 | | #1


    I will give your post a bump back to the top of the list. Good luck with your project.

  2. user-723121 | | #2


    Describe in more detail your ventilation strategy. I would say you are fortunate to find out your current assembly falls short, before you have installed the drywall.

    1. CraigRo | | #3


      The garage roof was an unvented design with R30 batts. I increased the depth to increase R-value and add a vent space. I used the basic vent spacers from Home Depot and stapled them in. I did not foam air seal the channels. The roof is a 3/12 pitch, simple A line with the garage sloping into the house attic. Garage has a cathedral ceiling, single slope into the house, 8 ft. to 11 ft.

      If not for the brand new shingle roof, I would have added a vented roof on top of the existing sheathing and more easily increased R to at least current code (R49).

      1. user-723121 | | #4

        I am a bit confused as to this area you are working on, garage or living space? In any case, moisture laden air is getting by your air barrier and reaching a cold condensing surface, the roof deck. If your air barrier was continuous and free of leaks this would not happen.

        1. CraigRo | | #6

          The space is the garage. The garage roof area vents into the living space attic (unconditioned). There is a ridge vent, added with the new singles. Existing venting was two end Gable vents.
          I taped up as many penetrations as possible this weekend, no new drips have appeared. Hoping it will dry out slowly.

  3. Expert Member
    BILL WICHERS | | #5

    Do you have open (unblocked) soffit vents at the bottom and a ridge vent at the top of those vent channels you installed? If you have vent channels but no vents at the ends of those channels to allow air to circulate, your vent channels can’t do anything to exhaust any moist air that makes its way into them.


    1. CraigRo | | #7

      I do have unblocked soffit vents and a ridge vent. I attached a picture of the house (old from years ago) for reference, so you can understand the roof line.

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