Flat Roof – Torch Down + ISO Board Underneath for Rain Noise Mitigation?
We’re about to put a roof on our home addition which has a flat roof in California. Our roof was approved with min r-30.
Underneath the base plywood rough roof structure is 2×6 rafters which we will either put iso board or spray foam, I’m thinking just boards to mitigate any off gassing risks.
On top though, we are thinking of doing torch down, but can we put ISO boards instead of a vapor barrier to increase the r value even more? This is all to make sure there’s as little rain noise as possible. This seems like an inappropriate use of ISO boards unless we use the sticker versions of the roof materials, but still is uncommon, so we likely will just do full torch down. I went to home depot yesterday and was tapping on the roof materials and it does seem like rain noise won’t be too much of an issue.
Underneath the roof for the ceiling, can we do a combination of ISO + rockwool? Or is spray foam entirely better?
The roof will look like this:
cap sheet
modified bitumen
base sheet
plywood (roof structure)
6″ sprayfoam/iso board
3″ – 0″ air gap because of slope (what do we put here)
drywall interior ceiling
room
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Replies
People still do torch down?
Anyway, a solid roof [IE T&G + ISO board] is terrible with rain noise.
Once you get a standardish rafter with fluffy or whatever, it is much better.
The cut and cobble rigid install in between the rafters is a very risky assembly and often fails.
If you want the roof to work, you need either closed cell spray foam underneath or rigid over the roof deck.
The good news is that you are in warm climate, so you don't need a lot of either as R5 will get you condensation control. So that is R5 rigid above or about 1" of closed cell spray foam bellow . The rest of the insulation can be fluffy batts which is which helps a bit with sound. It will still never be as quiet as a regular attic though.
If you want better sound, hang the ceiling drywall on either resilient channel or hat channel with isolation clips. The channel with clips is much more effective for this type of impact noise.
It's fine to put polyiso over the roof deck and under the roof membrane -- this is the normal way most commercial buildings are insulated. You do have to be mindful of the interior:exterior R value ratios though with your assembly to avoid moisture issues. Your potential issue here is not a problem with the polyiso itself in this application, the problem is that you have an unvented roof assembly, so you need to be very careful with your design to avoid moisture problems.
One of the things we do sometimes in commercial buildings to quiet down the roof is to put a layer or three of 5/8" drywall in the stackup on top of the roof deck. This is fine as long as the drywall is protected from water by the roofing materials. Note that this adds a LOT of weight to the roof, so be sure to allow for that if you try this.
BTW, I agree completely with Akos that you don't want to do a cut'n'cobble installation of rigid foam below the roof sheathing. That's asking for trouble -- closed cell spray foam is the only safe option with an unvented assembly unless you are putting the R value on the exterior with the rigid foam using uncut panels over the sheathing.
Bill
Closed cell spray foam and polyiso are both dense enough to transmit sound. Open cell foam is better, but the best is mineral wool, cellulose or wood fiber insulation. The "fluff" absorbs sound and limits transmission.
If you're going for a sound-abating assembly, why not use 2" or so of Rockwool Multifix boards outboard of the roof sheathing (designed with a substrate for hot mop and torch applications and famously great at sound abatement) and then batts or blown in in the roof / rafter cavity?
You get best of both worlds IMO - exterior insulation, sound abatement, no foam, and relatively cost effective/comparable compared to ccSPF / labor of cutting polyiso etc.
Rigid mineral wool is still fluffy insulation just a bit more dense. You can't build an unvented roof with just fluffy even in mild climates. The issue is the condensing plane will be somewhere within this exterior rigid layer, if air can get to it, it will condense and create moisture issues.
The exterior rigid insulation needs to be air impermeable for the roof to work, foam is pretty much your only option.
Good point, in that case a hybrid assembly could work for acoustic damping - 1" polyiso and 1" min wool w/ hot mop. abv.
I realize we hit r30 with the r30 rockwool bats, the joists are actually 8" minimum - 13" and the bats are 7.25" with compression which is how it can be installed. and rockwool has the best properties if we choose a good vapor barrier.
The issue isn't R value. In your mild climate, you don't need all that much insulation.
The issue is moisture buildup and roof deck failure over time. Even in your warm climate, you can't build an unvented flat roof with just fluffy insulation. The only two options are like I said earlier, closed cell spray foam or exterior rigid.
A flat roof like yours already has an outside vapor barrier which is the torch down membrane. You don't want another vapor barrier in your assembly as you can now trap moisture.
Around me flat roof installers are used to dealing with exterior rigid and torch down can go right over an overlay on top of it. On the inside, you install as much fluffy (mineral wool, fiberglass, cellulose all work) as you want followed by drywall, no vapor barrier. That is a simple, robust roof with no moisture issues.
So I have to say, I would think using a membrane of one type or another would be preferable in 2025
Torchdown cannot tolerate standing water and does not have the longest lifespan.
I am a fan of EPDM because it requires no special tools, but there are several other membranes some of which are available in white to minimize heat loading
Maybe this is just regional bias, but I have never even seen torchdown in newer buildings.
Also, considering this is California, you should consider a white membrane to reflect light/heat away from the roof to decrease cooling costs, your major comfort cost driver. Both EPDM and PVC have white alternatives. And, if this is a residential addition, you may find that these membrane come in large enough sheets to be nearly seamless over your roof - a big plus for durability and workmanship issues. I also endorse the general approach of polyiso above the sheathing and fluffy underneath. I find that cellulose has the best sound absorbing qualities, followed by Rockwool and fiberglass. Hanging the drywall from resilient channel is a great approach, though a bit pricey and it does result in lowering the ceiling a bit. If you want to maximize sound reduction with or without the resilient channel option, filling the entire sloped ceiling cavity with blown-in cellulose is very inexpensive and has plenty of R-value.