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Leaking Flat Roof

July 19, 2011 I replaced my entire roof last year. I have one flat roof and it was replaced with a Firestone rubber roof. Five houses in my area used the same experienced roofer. This past winter the rubber roof leaked. The roofer came back and thinks he found the leak. It is leaking where the rubber ends and the shingles begin. My ceiling is water stained and cracked. I am using an experienced contractor that I have used before. He removed the sheet rock and I am leaving the ceiling open for a few weeks to see if there are any more leaks. The roofer told me first to insulate the ceiling with foam and then he told me not to insulate the ceiling due to the heat in the ceiling. I have not discussed this with the contractor. The Question: Should I insulate the ceiling? Thank you

Re: Leaking Flat Roof

No, I wouldn't do anything yet.

I think your contractor is concerned about VENTILATING the space between the insulation and the underside of a presumably wooden roof deck.

You see, if you put insulation between the ceiling joists under the flat part of the roof, then the wooden roof deck will be colder than it is now, and any warm moist air escaping into the space above that insulation can form condensation or frost on the underside of the roof deck. That frost doesn't do any harm, but when it melts there's going to be water up in the space between the insulation and the underside of the roof deck. You don't want any moisture in that space because you have no way of getting that moisture out of there. And, as long as there's both wood and moisture up there, there's the possibility of wood rot in the ceiling joists.

I own a small apartment building, and the insulation I have is OVER the wood roof deck. The flashing along my roof line allows for air to enter the spaces between the top floor ceiling joists that support the roof deck, and I often see warm moist air escaping from the building out the bottom of the metal roof flashing on extremely cold days. (I have no ceiling insulation between those joists.) It's a bit of energy wasteage, but that escaping air means that the wood supporting the roof is cold but it's DRY, and as long as it's dry, it won't rot. I'd much sooner lose that heat and not have to be concerned about rotting ceiling joists.

I think your contractor may be concerned that by insulating between your ceiling joists, then you're going to trap condensation between that insulation and the flat roof membrane, an not have any way to circulate air through there to dry out that moisture. Most likely, if you don't have any insulation between your ceiling joists, then the insulation preventing your house from losing heat through the roof is ABOVE your wooden roof deck, just like in my case.

The architect that designed your house would have considered that, and so I wouldn't change anything yet. That is, I wouldn't insulate. By doing so, you're going to create a space where moisture can accumulate, but can't be ventilated to get rid of that moisture. And, almost certainly you've got insulation above your wooden roof deck and so you don't need any between the ceiling joists anyhow.

PS: You don't need to know the rest...

If you followed your contractor's initial advice, and used fiberglass insulation, or even cellulose, then you could have creates a real nightmare for yourself. The reason why is that insulation like fiberglass and cellulose work by keeping air stagnant. The insulation prevents convective air currents from forming which would otherwise serve to dry out any moisture. So, wet fiberglass or cellulose insulation take forever to dry out. And, having wet insulation in contact with your wood ceiling joists creates ideal conditions for the wood rot fungus (aka: "Serpula Lacrymans" to his friends) to thrive and multiply. And, on top of it all, the vapour barrier you have between your ceiling drywall and the wet insulation is gonna keep the ceiling drywall dry, thereby diminishing the urgency of the problem in the homeowner's eyes. You essentially get a "perfect storm" of circumstances that can lead to very serious damage to a house.

Your roofing contractor is probably already aware that your "ceiling" insulation is above your wooden roof deck, and so there's little benefit but a big downside risk of serious damage by also insulating between the ceiling joists as well. He'll tell you not to insulate the ceiling next time you see him. If you want to save energy, add more insulation to what's already there on top of your wood roof deck. Leave your ceiling uninsulated.

Re: Leaking Flat Roof

Thanks, Nestor for the advice. I will talk with the contractor. Much appreciated.

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