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Batt vs. Rigid-Board Insulation for Vented Attic

Joe_Adams | Posted in General Questions on

Hello everyone,

I am renovating my grandparent’s house, and one of the next steps is to insulate the ventilated attic.

The house is located in a marine climate, and it was constructed with CMU. The attic slab is made of concrete.

I have seen that the most common practice is to insulate ventilated attics with fluffy stuff (wools), lose or in blankets. So I was wondering why not insulate the attic with rigid boards? And if this is an option, should the boards be mechanically fastened to the slab or lay loose?

In case of going with fluffy stuff, how to prevent damage to the insulation in case of attic acesses ?

Thank you very much in advance

Joe

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Joe,

    The big advantage of loose fill or batt insulation is cost. You get a lot of R-value for your buck. Foam is more useful in roofs where there are depth limitations, or you need an impermeable insulation.

    People with loose fill insulation often make catwalks so they can access the attic without disruption. If there are trusses you can attach horizontal 2"x4"s between the diagonal webs, and then make a walkway out of several 2'x6"s on the flat. If there are rafters you can make a catwalk with 16" high pieces of 2"x10" acting as legs, with the same 2"x6" walkway above.

  2. Kyle Bentley | | #2

    Hi Joe,

    A picture is worth a lot here, if you wouldn't mind posting one. Is the slab flat? How is the roof attached? What kind of roof is it? Etc.

    That'll help everyone provide more accurate suggestions.

  3. Joe_Adams | | #3

    Hi,

    The attic is something like this. Some parts of the eaves are not open to the exterior (like in the pic). So I will open these parts.

    I do not have much space to insulate near the eaves and install the baffles properly.

    Is fire hazard any justification not to use flammable foams in vented attics?

  4. Bas Sommer | | #4

    Joe,

    an edit first:
    you can use fluffy stuff, boards (like Rockwool) or foam boards - all will work. Concrete is air and vapor tight enough for that.. Bords can be laid out side by side, if you tape the joints for solid boards - why not.

    For non-fluffy stuff, with two layers offset to each other you are on the safe side. You can also mix the type of insulation as long as voids and cracks are avoided.

    To the eaves put what is possible in thickness. Leave a bit of space /cut to follow the angle of the roof to leave a bit of ventilation space. Rockwool - at least the higher density boards - is not so sensitive to windwashing so that I would uses theses at least in that spot (same with foam boards). Towards the center more fluffy stuff will work (any blown fluffy stuff). Put a walkway on stands (make a frame with 2x4 and put OSB across) to keep room for the insulation. Any cracks here around the stands can be filled with any stuffed loose insulation.
    Some Rockwool boards are solid enough that one can walk there for inspections (better cover if regular access is necessary)

    the steel rods - better wrap some insulation around for like 16" above the insulation level - these will conduct quites some heat if in contact to the air. Anything goes - wrap it, uses boards with a notch along the length to cover them etc..

    Above the steel profiles - add one extra layer to gain the nominal thickness across them - best case

    regards - Bas

  5. Expert Member
    Zephyr7 | | #5

    Pretty much everything you’re likely to use will have some kind of fire retardant in it. If fire resistance is a primary concern, mineral wool is the best. Fiberglass would be second best. Both are a available in loose fill forms that can be blown in, which would be the easiest way to install it, and would also get a good, even insulating layer. As Malcolm mentioned, loose fill tends to be cheapest too, and a big part of that it tends to be the easiest to install.

    That roof looks like it’s a prestressed concrete supported system. Be very careful where you drill any holes in that since you can severely compromise the structure if you damage any of the internal reinforcement.

    Bill

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