Vent With Insulation?
We live in a co-op building in Prospect Heights with a flat roof and are investigating blown insulation. We currently have a bid for cellulosic from Brooklyn Insulation that does not include vents. They sent us an article, a contractors guide based in Pennsylvania, which claims that vents are not necessary with dense packed insulation…
We live in a co-op building in Prospect Heights with a flat roof and are investigating blown insulation. We currently have a bid for cellulosic from Brooklyn Insulation that does not include vents. They sent us an article, a contractors guide based in Pennsylvania, which claims that vents are not necessary with dense packed insulation jobs. However, a roofer guy told us we do need vents. One thing the article said is that they were not necessary in mild climates like PA. Could this be the difference? Any advice?
Paint – an effective vapor barrier? Then why does paint always fail on masonry/brick – materials which are permeable and constantly absorb and lose moisture – ? Granted there are some thermo-elastic coatings that claim to work on masonry – but I doubt that’s what renomandru has on that attic ceiling.
I opted to not vent when Brooklyn Insulation blew in cellulose insulation, believing that it was not necessary. For one, My roof is so short and flat, that there would have been no room for air to circulate, making the vents ineffective.
Also, multiple layers of paint on your ceilings are essentially effective vapor barriers in and of themselves. I question how much moisture will get into a typical townhouse attic from the living space.
Also, cellulose, unlike fiberglass, has the ability to absorb and give off moisture well unlike fiberglass. I decided there was no convincing argument for cutting two large holes in my water-tight roof.
I hope time will prove me right.
Cellulose insulation is primarily made of paper, and I would definitely recommend venting the assembly.
Kyle Page, AIA, LEED AP
Sundial Studios Architecture & Design, PLLC
doubter,
When I had cellulose insulation blown into the roof of my 20′ X 45′ house the installers put in two aluminum mushroom vents> That was over 30 years ago and I’ve had no discernable problems caused by condensation.
SundialStudios,
The OP refers to cellulose insulation (actually “cellulosic”, but I’d think that’s a typo). Is this considered “open cell” and and needing vents?
It is recommended that you vent if using an open cell blown insulation. But venting, or a vapor barrier, is not required if using a closed cell insulation. Basically, the air bubbles that form the closed cell insulation are too small for water to bead within once the dew point has been reached, so venting is not required. Google ‘open cell vs. closed cell insulation’ for more fun technical info.
Hope that helps.
Kyle Page, AIA, LEED AP
Sundial Studios Architecture & Design, PLLC
oh shit. we just did the blow in cellulose with a flat roof, no vents. is it recommended to vent after the fact? how large are the vents and how many needed to cover how much space?
I’ve insulated many attics and roofs and my understanding is that the only type that does not require venting is when foam insulation is blown directly up against the roof rafters. This type of insulation is also the only type that does not require a vapor barrier when applied to wall studs.
I did my brownstone and put 2 vents in. Federal Conservation did the job. The vents were $200 each, installed.
I’m not an expert, but I don’t understand how vents would not be necessary. Lacking a vapor barrier, water vapor will permeate through the ceiling and, in cold weather, condense out and wet the insulation, greatly reducing it’s effectiveness and, potentially, rotting wooden joists. Vents allow the vapor to escape.