"Curious if anyone's tried spray foam insulation directly beneath modified bitumen instead—seems like it might offer better moisture control without relying so heavily on ventilation...?"
I've actually seen spray foam used under modified bitumen on a neighbor's flat roof, and it seemed to work pretty well. He was dealing with constant condensation issues before switching. Spray foam creates a tight seal, so moisture doesn't have much room to form. But I'd still be cautious—make sure the roof decking is completely dry beforehand, or you could trap moisture inside and make things worse down the line.
I've considered spray foam myself but ended up going with rigid foam boards under my mod bit roof. Spray foam definitely seals better, but like you mentioned:
"make sure the roof decking is completely dry beforehand"
I wasn't confident enough in my roof's dryness to risk it...
- Interesting choice going with rigid foam boards... definitely a solid option, but I'm curious if you considered green roofing at all?
- Spray foam does seal well, but like you said:
"make sure the roof decking is completely dry beforehand"
- Moisture concerns are legit, but a green roof can actually help manage moisture levels naturally by absorbing rainfall and reducing runoff.
- Plus, it adds insulation value and extends the lifespan of your mod bit roof by protecting it from UV and temperature extremes.
- I know it's not everyone's cup of tea, but I've seen some impressive results in terms of energy savings and roof longevity.
- Did you rule it out for any particular reason, or was it just not on your radar? Curious to hear your thoughts...
Green roofs definitely have their perks, especially with moisture management and insulation. But they're not always practical for everyone—weight load can be a real issue depending on your structure. I've seen cases where the existing framing just couldn't handle the extra weight without major reinforcement. If structural capacity isn't a concern though, it's worth considering... the energy savings and extended roof lifespan can really pay off in the long run.
Yeah, green roofs can be great, but you're right about the weight issue. If that's a concern, you might want to look into rigid foam insulation boards instead. They're lightweight, easy to install, and provide solid R-values. Usually, you'd install them directly over the modified bitumen membrane, then cover them with a protective layer like gravel ballast or even a reflective coating to protect from UV rays.
One thing to keep in mind—make sure your roof has proper drainage before adding insulation. I've seen a few jobs where people skipped checking drainage, and water ended up pooling, causing leaks and headaches down the line. Also, don't forget to seal the seams between insulation boards carefully; gaps can really cut down on efficiency.
If you're looking for something straightforward and effective without the structural hassle, rigid foam boards are probably your best bet.
