Batt insulation under a brownstone roof?
I think the way they understand how to do insulation has changed in decades and now I believe they always vent your roof. (The way you phrased it read to me like putting batts under attached to the underside of the roof, which would probably cause condensation and moisture issues.)
I think blow-in insulation is actually preferred over batts, because it covers the joists and fills in the nooks and crannies, whereas batts have gaps around the joists. For a typical house you want something like a foot deep of insulation depending on the material.
I recently had it priced and for a “green” version that isn’t fiberglass it was like $4 a square foot which was way more than I could afford (it may pay for itself in theory but it would take decades). I don’t know if it’s allowed now but fiberglass has some nastiness and cellulose may have some chemicals too. if your ceiling is properly sealed around lights, vents, and so forth it may be a non-issue.
Lurker
in General Discussion 5 years and 10 months ago
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Guest User | 5 years and 10 months ago
Is it possible to insulate a brownstone roof effectively with fiberglass batts under the roof (in the cock loft)? And does it matter if some of the batt insulation is around ductwork and pipes?
We’re doing a gut renovation. We ruled out spray foam insulation because of off-gassing concerns, so the plans call for 2 layers of R-19 fiberglass insulation under the roof and 1 layer of R-13 fiberglass insulation in the exterior walls. It sounded good to me, a complete novice at brownstone renovations. Then a mistake was made and 2 layers of R-13 fiberglass insulation was put under the roof. To get us from R-26 to R-38, the GC agreed to put in a 3rd layer of R-13, so we’d end up with R-39.
That 3rd layer, though, is being put around the ductwork, possibly because there wasn’t room to put a 3rd layer above the first layer of sheet rock, so from top to bottom we have:
– roof
– 2 layers of R-13 (which I’m told is unfaced)
– 1 layer of sheet rock
– 1 layer of R-13 (unfaced) in between the ductwork and electrical wires
– 1 final layer of sheetrock, which will be our finished ceil ing
My first question is whether this arrangement will actually give us R-38. I’m concerned that where the ductwork is, we’ll still only have 2 layers of R-13. Will that have a significant effect on heat transfer?
I looked on the internet to see if I could find an answer. I didn’t really find one, but what I did find was all sorts of articles and comments about how insulating under the roof with fiberglass batts is a huge no-no, and can lead to all sorts of moisture problems. One poster on this forum told of a roof that sagged thanks to the batt insulation below. Our new roof has already been installed, so our only option at this point is to insulate under the roof, but is there a way to use batt insulation that won’t lead us to any of these moisture-related horrors?
I guess I’m not sure I understand why insulating under a roof is different from insulating an exterior wall. Why would there be more moisture under the roof than on the interior side of a wall? Or are we setting ourselves up for all sorts of moisture problems by having fiberglass batts in our exterior walls, too? (In case it’s relevant, our exterior walls are brownstone to the parlor level and brick above.)
Thank you in advance for any advice. At this point, I’m thoroughly confused and terrified our house will end up being an ineffectively insulated rotted mess.
Guest User | 5 years and 10 months ago
First, I am NOT an expert, but we did re-roof and insulate our cockloft not that long ago (with a low VOC closed cell spray foam). It was explained to me that one could definitely use fiberglass batting in your roof, but that you need to make sure there is a moisture barrier under the roof and a decent source of ventilation up there to ensure that the warm air that naturally rises doesn’t get trapped there and cause moisture (the reason that a roof is more prone to moisture than walls is that heat rises). When fiberglass batting gets damp it can compress which lowers its effectiveness and of course could lead to other moisture related issues.
Guest User | 5 years and 10 months ago
Thank you so much! This is very helpful. Not sure what to do about it, though, as we have no ventilation up there and the roof is finished.
It makes sense that heat rises so moisture could be an issue for the ceiling/roof, I wonder, though, how much of that concern is theoretical rather than real? For example, very little had been done to the house when we bought it — the people before us had lived there for 40 years and done virtually nothing to it — and when we took down the ceiling there were fiberglass batts in between the joists as fluffy and dry as can be. A few of the joists were rotted, but we had assumed that was from water infiltration from above (leaky roof) not moisture below.
I’d love to hear from anyone who has or has had fiberglass batts in an unvented cock loft. Have you or did you run into any problems? Has anyone been fine with this arrangement?
RobertGMarvin
in General Discussion 5 years and 10 months ago
We have loose cellulose insulation, blown in 40+ years ago. The company that did the job installed two mushroom vents from above, cutting found holes in the roof. I think any competent roofer can install vents. We had a third vent installed by a roofer several years ago when we put a vent fan in out top floor bathroom.