Can one install spray foam from above when replacing flat roof and roof decking?
Our parameters are:
1) We have low depth roof trusses, maybe 2×8.
2) Ceiling plaster on the underside, which we don’t want to disturb
3) Roof is vented with soffits on 3 sides (this is an end row house), currently likely no insulation and no other vent. Home is 100 years old
4) There is one parapet on the front of the house
5) Zone 5
6) Our contractor never used spray foam and never installed rigid foam in the roof
We would like to install the insulation from above, after removing the roof decking. The option to install insulation on top of the roof deck was considered, but not very viable due to row house situation, matching height of next door roof.
Unvented roof options call for installing a thick layer of spray foam on the underside of roof decking. Typically, the foam is sprayed from below. Will this option also work if we fill the truss cavity from above, and then install roof decking? Can the spray foam insulation not be in contact with the roof decking?
Do we need to worry about the weight of the insulation on the plaster ceiling? We think of installing 5” of spray foam.
How would we deal with weather, etc?
Any other job-sequencing considerations?
Normally, the contractor would be done with the roof in one day – what does he do now when he has to call in a spray foam contractor?
Do we keep some venting space above the foam or should all the vents be closed off?
We also thought of installing rigid foam blocks in between the roof rafters, but fear that the air sealing would be too hard to accomplish.
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No, you can't use spray foam to build an unvented assembly "backwards" like this. The reason is that the spray foam has to adhere directly to the underside of the roof sheathing to prevent moisture issues, and there is no way to get that type of application if you have to spray from above. Any air gap invites moisture accumulation and all the associated problems.
What I would do here is to use some high density batts, topped off with some 1/2" (or thicker) polyiso foamed in place at the edges. The polyiso would be at least 1" down from the roof sheathing to leave a vent channel. If you use polyiso exclusively (instead of batts), you could get up around R38 or so this way, but it would be more work to install.
BTW, the use of "high density" batts is important here since you need to maximize your R per inch in the limited space you have available. Regular low-density batts just won't cut it here.