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nft 06-25-2010 07:29 PM

venting insulation on a flat roof
So, tomorrow morning my roofer is coming to install about 1000sq feet of torch down modified bitumen on my 1905 townhouse in DC over 2 roofs. Included in this, they are installing 2" insulation boards underneath the membrane.

Also, they will be installing 2-way breather vents on my roof.

The way they install them is to cut a hole in the insulation and put the vents over these holes and flash them into the MB.

Now, I have my doubts about this technique and I am hoping to check up on my contractor. I asked my engineer dad because of my doubts...and he said cutting holes in the insulation completely defeats the purpose and that there shouldnt be moisture build up between the wood and the insulation bc the house is old and fairly porous and that if there was a moisture issue a few ceiling vents (no attic, just some dead space thats not accessible) would solve it.

While I know my dad knows his stuff, they sell breather vents for a reason. However, it does sound extremely logical that cutting multiple 4 inch holes in the insulation creates an easy place for heat exchange to occur so that my heat in the winter will rapidly escape through these holes and the hot air of the summer will rapidly invade in the summer.

Does anyone know the theory behind this? is it worth it? I'm not paying extra for the venting (just the parts).

Please let me know! He's coming at 6am tomorrow.. I didnt get skeptical until I saw the vents and asked them how they worked.

seeyou 06-25-2010 07:45 PM

The vents in question are likely not 2 way vents, but actually ONE way vents. They will let any moisture under the membrane escape, but not allow any back in. How many vents are they adding? A couple of 4" holes in the insulation, while not ideal, is not going to diminish the insulation value near as much as it being saturated. Part of the price you have to pay for not stripping the old roof systems.

nft 06-25-2010 07:54 PM

The old roof system is being stripped off. Everything is being taken off down to the decking.

They are 2 way vents. Its going to be 3 or 5 vents total.

seeyou 06-25-2010 07:56 PM

HMMM. I don't know then.

OldNBroken 06-26-2010 10:03 PM

If they are one-ways, you don't cut the insulation out. Their main reason is actually to release any positive pressure from under the membrane. They have nothing to to with ventilating the space. No clue why you would be talking about "two-way" vents on the system you described. Pictures would help.

Grumpy 07-06-2010 09:20 AM

The roof is either a cold deck or a warm deck, but not both. I had this very same argument with an architect on a condominium specification just a couple months ago and ended up walking away from a potentially very lucrative project because I deemd the architect to be an idiot. I agree with your engineer, ventilating an insulated deck is a waste of money and defeats the purpose. I also agree with Seeyou and OldNBroken, that one way breather vents are not a bad idea. However I would not cut through the insulation with those one way vents.

I know this is now after the fact, what ever happened with this project?

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