I've been working on insulating my flat roof (it's modified bitumen) and figured I'd share what I've done so far, maybe someone has better ideas or tips. First, I cleaned the surface really well—lots of dirt and debris had built up over time, you know how it goes. Then I laid down a vapor barrier to keep moisture out. After that, I put down rigid foam insulation boards (polyiso) because they're supposed to have good R-value and handle moisture pretty well. I staggered the seams to avoid thermal bridging, taped everything up nice and tight with foil tape, and then covered it all with another layer of modified bitumen membrane.
It seems solid enough, but honestly, I'm not 100% sure if this is the best method or if there's something better out there. Maybe spray foam would've been easier or more effective? Curious if anyone's tried something different or has any suggestions for improvements...
Sounds like you're on the right track with the polyiso boards—I've seen a lot of roofers use them exactly as you described. Spray foam can be great too, especially for tricky areas or uneven surfaces, but honestly, it's pricier and messier to apply. If your roof is pretty flat and straightforward, I'd stick with what you've done. Just make sure your drainage is good... standing water can really mess things up over time.
I've seen polyiso boards work well too, but have you considered the long-term durability compared to spray foam? I know spray foam can be messy and pricier upfront, but I've heard it holds up better against moisture issues over time. Also, curious if anyone's had experience with mineral wool insulation on flat roofs—I've read some good things about its moisture resistance, but haven't seen it used much in practice... thoughts?
"Also, curious if anyone's had experience with mineral wool insulation on flat roofs—I've read some good things about its moisture resistance, but haven't seen it used much in practice..."
Funny you mention mineral wool—I had a building a few years back where we decided to give it a shot. The roof was modified bitumen, older building, and we'd previously dealt with some pesky moisture issues. We liked the idea of mineral wool because it was supposed to handle moisture better and not lose its insulating properties as quickly.
Honestly, it did hold up pretty well overall. We had a couple of minor leaks over the years (mostly flashing issues, not insulation-related), and when we checked under the membrane, the mineral wool was surprisingly dry. It didn't soak up water like I've seen polyiso sometimes do. But... installation was trickier than expected. It's heavier, and the crew wasn't familiar with it, so that slowed things down and bumped the labor cost a bit.
All things considered, I'd say it's a solid option if moisture is your main concern—but make sure your installers have experience with it, or you might end up paying extra labor hours.
Honestly, it did hold up pretty well overall. We had a couple of minor leaks over the years (mostly flashing issues, not insulation-related), and when we checked under the membrane, the mineral woo...
Interesting to hear about mineral wool. Worked on a job last summer where the owner insisted on spray foam insulation under modified bitumen. Seemed great at first—fast install, good R-value—but we ran into issues later when moisture got trapped underneath. Ended up having to rip sections out, total headache. Mineral wool sounds decent, but yeah, crew familiarity definitely matters. I'd probably stick with polyiso myself, despite its quirks.
