Insulating flat roof with 7" cavity with roof tear-off

Hot-dry climate. Roofer removing all sheathing. Insulating from above while cavity is open. Plan for 5.5" open cell foam to the drywall and also covering dropped soffit area that houses ducts. There will be a minimum air gap of 1.5" (7"-5.5") between the foam and the deck at normal ceiling heights, about 1 ft over dropped ceilings.

Any concerns with this installation? electrical, mechanicals. performance etc?

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Some initial concerns over the approach were radiant heat buildup in the air space if we convert to an unvented assembly and associated premature roof deterioration. There are currently bird board vents at soffit edges. We were planning on sealing them and going unvented. Not sure of the exact makeup of the new roof, but I believe it will have a reflective topcoat. Maybe leaving soffit vents intact and allowing passage of air for ventilation would be best? (although there is no high/low vents with flat roof so not sure how effective it would even be.)

I wouldn't expect much ventilation to happen with a flat roof. Why not fill the rafter bays completely full? What R values are you required to hit? Not sure what to say about the drop ceiling areas... what's there to spray against?

We would not fill rafter completely due to cost. Foam = expensive and you hit diminishing returns on a pretty steep curve. On the dropped ceilings, we would be spraying directly against the heating ducts, drywall and dropped ceiling/wall junctures to complete an air barrier and insulate kneewalls created by drop.

I wouldn't expect much ventilation at all either. I would be concerned about that heat build up in the one inch air gap beneath the sheathing, so maybe we will have to add the additional cost of filling the cavity.

We coat AC units, plenums, flex and metal ducts on the roof and in the attic or air spaces with our Cool Roof coating.  I have done a few in Yuma.  They were flat roofs (Mineral capsheet) and stucco walls;  Hundreds of cracks in the stucco and no more cracks in 6 years,


While the roof area is open, I would suggest you coat / paint the tops of the ducts and sides if you can get to them.  If the AC unit is on the roof, definitely coat those surfaces too. 


Our coating is effective vs radiant and conducted heat.  Therte is a;ot of data on these types of applications.



Talking about insulating the roof, not coating stuff on top. The ducts are contained within the dropped ceiling and are under the roof.

Craig if you leave a space you need to leave some sort of opening from what I recall (think expansion & contraction) - if you spray directly against the sheathing you wouldn't. Of course this is also dependent on how you are finishing the roof - is foam going on top, hot mopped, rubber, or...(If you have foam going directly on top, the cavities need to be filled)

Performance wise - it will probably be better than what was there before

Electrical & mechanicals - besides them being encased in foam / harder to get to later should there be an issue, there really shouldn't be any issues. With that said, check the wires & everything carefully for damage & be mindful of large groups being bundled together.

David, I think he is talking about interior soffits / bulk heads which encase the ducts which is where a great number of issues generally are, as most of them just are framed with no drywall inside. I don't see an issue with foam being used around the ducts but make sure you check those areas carefully & fix any duct / air sealing issues

Roof coating - that gets back to what your roofing material is & the best product to go over it. With the sun beating down on that roof a "cool roof" coating is generally a good bet 

Not sure of your point in regards to leaving a space. Cavities are filled with foam all the time. The space in this situation is between the top of the foam product (applied to drywall) and the roof deck.

I will need to contact roofer about what he is applying to the roof.

Actually so I can stop guessing at what you are looking at & thinking - where is this property at (city is fine & maybe the style of the house, age)? Something I would have done/recommend for Phoenix could be completely different than what I might recommend in another "hot dry climate" area or even different structures.  

Yes cavities are filled all the time, but generally it is directly against the sheathing. The vents are there for reason - now if they are still needed in your case is still up in the air. On the steep curve of foam & the cost mentioned in another reply, you aren't even close to it with OC in this area 

This is in Tucson. It is a 1300 square foot, built 1972, slump block, flat roof home with bird blocking and 2x8 roof. I prefer to create an unvented assembly, but the primary question is since we CANNOT spray to the underside of the roof deck and instead insulate to the drywall lid, AND it leaves an air space, is it ok, to have an unvented air space against the roof deck or could that cause problems? I don't want to add an additional 1.5" of foam if I do not have to, since 5.5" OC is plenty. If there is a building science reason why I can not leave that air space in an unvented assembly, I would propose filling cavity. Alternatively, is the air space is ok if I maintain the bird board vents? (or is a there a compelling reason to maintain the ventilation?)

NEW ROOF: Apply one layer of Glasbase and two layers of Glasply with hot asphalt between each layer, surface with emulsion, and two coats of white roof coating

Hopefully someone else chimes in if they know diffrent, but based on my recollection, you need the gap & venting with that setup. Probably be worth checking on the GBA or BSC site... (be careful with older papers)

With open cell foam you may have issues with condensation on the supply ductwork. Depends on the construction and specific climate conditions. How is the foam is being installed over the soffit areas?

Not sure why the air gap is there, do you intend to have a radiant barrier over the foam? Rigid insulation over the new sheathing/decking with any extra insulation in the cavities would be a more typical construction for an unvented flat roof assembly. This helps eliminate thermal bridging and takes care of air and vapor movement (but you may not need to worry about vapor in that climate). 

Choosing a reflective roofing material is one of the most effective tools in a hot-dry climate.

good article:


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