GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

How to insulate and vent this roof?

CHRISWALL | Posted in Energy Efficiency and Durability on

I would describe it as a lean-to roof against the tall side side of a shed building. We have just acquired this building and are faced with high energy costs and ice dams. [More information in Comment #1.]

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. CHRISWALL | | #1

    At present it is insulated with fiberglass batts between the ceiling joists, there is no venting.
    HVAC is in the attic as is a large blower that vents the kitchen stove and dishwashers. I would like to;
    1) seal HVAC ducts with metalized tape
    2) seal air leaks into attic
    3) put cardboard "walls" along the side of HVAC ducts to add blown in fiberglass
    4) add blown fiberglass over batts
    5) add venting

    I don't know how to vent this roof or what I should do with the stove vent
    thanks for any help

  2. GBA Editor
    Martin Holladay | | #2

    1. Since your attic includes HVAC ducts, the best approach is to insulate the roof assembly (the sloped rafters) so that you bring the attic inside the conditioned envelope of your building. There are two ways you can do this: you can either install closed-cell spray foam insulation on the underside of the roof sheathing, or you can install rigid foam above the roof sheathing. (If you take the second approach, you will need an additional layer of roof sheathing above the rigid foam, and new roofing.)

    For more information, see Creating a Conditioned Attic.

    2. The product you want to use to seal the HVAC duct seams is mastic, not metalized tape. For more information, see Sealing Ducts: What’s Better, Tape or Mastic?

    3. If you install blown-in fiberglass insulation on top of your ducts, the ducts can sweat during the summer (assuming that the building is air-conditioned). That's because the fiberglass insulation doesn't stop the humid attic air from reaching the cold ductwork. The result is condensation and wet ceilings.

  3. CHRISWALL | | #3

    Thanks. Creating a conditioned attic would be my first choice too, The problem there is to find a knowledgeable installer and of course cost.
    this building is a nature center in SE WI. The building was in disrepair when we acquired it and needed a new roof immediately so rigid foam on top would be destroying a new roof.

    this is the design on half of the building the other half is a cathedral ceiling with some rigid foam in place ( i cannot tell how much) one room hasNO insulation. As best I can tell it is 5/8 decking on 2x6 tongue & groove ceiling. Can we install 5.5 inches of rigid foam on the inside of that then 6 inch car-siding over that.? Foil faced polyiso?

    Thanks again for your help

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |