Flat Roof Insulation Strategies (For Architects and Contractors)

Flat roofs can be insulated successfully using a variety of insulation products and strategies. Apart from the energy efficiency benefits, specifying the right type of insulation and locating the insulation in the right location within the flat roof assembly is critical to preventing moisture-related issues such as condensation, mold growth, and structural damage from rot. Moisture accumulation within the roof assembly can compromise its integrity and lead to costly repairs or premature replacement. Almost all flat roofs must resist interior air leakage and vapor diffusion into the upper parts of the roof assembly, as moisture can get trapped underneath the impermeable roof membrane. When selecting an insulation strategy for a flat roof assembly, one must consider air leakage, vapor diffusion, and the effects of thermal cycling.
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0:00 Intro
0:31 Strategy 1 (Rigid Insulation)
2:55 Strategy 2 (Rigid Hybrid)
4:55 Strategy 3 (SPF Hybrid)
6:09 Problems With Spray Foam
8:16 Flat Roof eBook
8:49 Strategy 4 (No Foam)
11:12 Strategy 5 (Exterior SPF)
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[@ASIRIDesigns]

*Get my climate specific guides to flat roof design available here:* asiri-designs.com/shop

[@JohnLee-db9zt]

Thank you. This is the most comprehensive easy to understand tutorial on flat roof construction I’ve seen.

[@markstipulkoski1389]

"Polyiso: At a mean temperature of 75°F, the polyiso sample also exhibits a level of thermal conductivity which correlates to its published R-value of R6. 0/inch. However, as the mean temperature drops to 15°F, the R-value decreases to approximately R2. 0, representing a significant 66% loss in R-value."
It also costs more than EPS or XPS. I would consider burying the the polyiso under a blanket of XPS so it will not be subjected to the most extreme cold, and thus, retain its marketed R-value.
Note, Polyiso's R-value begins its plummet around 55F. So, IMO, Polyiso is a warm climate insulation.

[@ASIRIDesigns]

Yup! We discuss this very topic in the book and the ability to combine it with more thermally stable insulation types like you mentioned. Good stuff!

[@eliinthewolverinestate6729]

I am doing warm roof on 6/12 pitch. Exposed rafters and roof board inside. I like mineral wool but it does cost. With post and beam timber frames we try to keep moisture going out of structure. I am used to seeing thick roof assemblies with purlin.

[@bigdougdog]

If I would like to have a decking surface on top of the flat roof it seems strategy 4 is the only real way I could do that. Would you agree or are there other options that could be used? What do you think the best way to insulate it would be? Thank you.

[@ASIRIDesigns]

Actually there are several ways to successfully accomplish a deck above a flat roof membrane, check out this video I released a few months ago: ua-cam.com/video/jVaOG2ffSFs/v-deo.htmlfeature=shared

[@pranavpatel2968]

I'm assuming all 5 strategies listed would be for an unvented roof design, correct? Is there a reason why in Strategy 2 and 3 you cannot run ductwork in the rafters/trusses/roof framing area? As long as the insulation is touching the underside of the roof deck in an unvented design, this should be an acceptable practice, correct?