Flat Roof Insulation Strategies (For Architects and Contractors)

[@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: kzbin.info/www/bejne/oIfEgHpom8uGfNUfeature=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?