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Blown Cellulose on Flat Roof

True_North | Posted in General Questions on

Hi everyone,

We own a ~70yo commerical property that has been converted into a multi unit residential building (decades ago).

Concrete block construction on a slab. 

The structure’s original roof design is a flat tar and gravel design. At some point over the past 5 decades they added a shingled hip roof on top of the flat roof. Since our ownership we have added a 10kW solar assembly on top of the hip roof.

Soffit vents c/w ventilation baffles exist, but unfortunately no ridge vent is present, only box vents throughout.

No insulation exists between the ceiling joists and the flat roof. 

We are desperately in need of insulation throughout, but due to tenants living throughout adding interior wall insulation is not feasible so that leaves attic.

Question: Is blown Cellulose an appropriate option to add on top of the flat roof? If not, maybe 2″ base of ccSF with cellulose on top to desired R value?

Any input is greatly appreciated,
Katarina.

 

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Replies

  1. Malcolm_Taylor | | #1

    Katarina,

    Yes. What you end up with is very similar to an unconditioned trussed attic, which is the most common type of roof used in North America, and performs well. You will need to:
    - Install baffles at the eaves to allow the air from the soffit vents to get past the new blown insulation, and block off the area around them so the insulation doesn't fall into the soffits.
    - Make sure all plumbing vents and any exhaust vents are extended up onto the new roof, and don't terminate in the attic space.
    - You are still going to have an uninsulated space where the flat roof rafters meet the outside walls, which you can only insulate by opening up these areas either from above or below. Whether that is worthwhile is a judgement call.

  2. True_North | | #2

    I suppose my largest concern is the potential for cool air to migrate into the space between the ceiling joists and flat roof, and cause condensation on the tar and gravel side of the flat roof causing potential mold on the bottom cellulose layer.

    Is this creditable?

    1. Malcolm_Taylor | | #3

      Katarina,

      Because all the insulation will be on top of the existing flat roof, it will never get cold enough for the moist interior air to condense. The dew point is somewhere in the cellulose above.

      You are much more likely to get that problem now, with no insulation.

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