Add rigid insulation to a 2×4 ceiling with an existing polyethylene vapor barrier?
I’ve been creeping around this website for years. I love reading random articles and questions and learning this stuff for fun.
Zone 6. We live in Southern Maine (Scarborough to be exact, stop by and say hi!).
Our “play room” appears to have been built (or renovated) as part of the garage addition, date unknown. Our house is 1960ish.
– (Section A) About 300 sq.ft. of the room is a flat drywalled ceiling, 8′ tall. 2×6 framed ceiling. “Attic” space
above. Oddly, no real insulation in bays. Poly vapor is above framing with unfaced batts rolled over it. Lots a mouse poop in the void/bays between ceiling drywall and vapor barrier.
– (Section B) Going back, the next section is about 75 sq.ft., slopes about 2/12 down, drywalled ceiling, avg of 7’7″ tall. Rolled roofing. This has no access above, I assume it’s the same insulation as Section C.
– (Section C) Farthest section back is about 165 sq.ft. slopes same 2/12 down towards rear, drop ceiling tiles, avg of 6’6″ tall. Same rolled roofing. I can pop the tiles and see poly vapor on interior side of studs and unfaced batts between studs (normal like, not like crazy Section A).
QUESTION #1: In the winter I can feel quite the draft coming down from ceiling tiles in Section C. My plan is to remove the drop ceiling and put up drywall to act as a more proper air flow stopper. I don’t have much head room, but will gain about 2″-3″ when I remove the tiles/grid.
– Should/could I install some rigid insulation before installing the drywall?
– If so, what type?
– Should I be concerned about combining rigid (vapor barrier?) with existing poly vapor barrier?
– If so, should I remove the existing poly vapor (and all the mouse poop that I can see above poly)?
QUESTION #2: Has anyone ever seen a layout like Section A? The thought of crawling around this attic space for a day or two to demo all of this insulation, removing the poly, and blowing cellulose does not really appeal to me. Would I replace the poly? Or skip poly and assume cellulose will block air infiltration (and vapor)?
Sorry, I know this is quite the lengthy and (likely) confusing post). I have attached a layout, if that helps.
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Replies
Is the existing insulation on the interior (conditioned space side) of the poly vapor barrier in sections B & C?
It reads as if the insulation is on the exterior side in section A (?).
And, you reversed the ABC section order in the attached diagram?
In the section A, is the attic space vented to the outdoors?
A 2:12 pitch is really impossible to vent properly in a snowy marine zone like Scarborough. Get much ice damming? ;-)
Stack-up drawings pointing to the various layers, with the roofing on the top , and the finish ceiling on the bottom would make it a bit clearer than the 1000 words alternative. Or even then names of the layers stacked in their gravitational orientation, eg:
-roll roofing
-#30 felt
-3/4" CDX roof deck
-2x6 rafters, empty
-3.5" batts
-polyethylene sheeting
-2x6 rafters, empty (except for air and random mouse poop)
- drop ceiling tiles
...or whatever it actually is.
Rigid foam without facers is not a vapor barrier, and may be OK depending on the particulars. The polyethylene SHOULD be an air barrier as well, so you should probably figure out how to stop the outdoor air leaking before insulating.
As long as at least 50% of the total-R is on the exterior side of the poly vapor barrier the risk mold & moisture would still be pretty low even with lower cost fiber insulation on interior side of the vapor barrier, as long as there is reasonably air-tight gypsum board painted with standard latex ceiling paint, with all electrical & plumbing penetrations caulked tight.
Dana, thanks for feedback on question and pointing out where I could help myself with the post!
My "sketch" was labeled very poorly, I apologize. I listed them in order that you see from the interior up (down->up)... which makes little sense as listed it looked (top->down). I have corrected with attachment to this post.
My sketch was labeled Section C, B, A as I see from above, but going front front of room to back. This is awkward, but I didn't change (as it would confuse even more).
- C and B are insulated "normally" with vapor on interior side of insulation.
- A is also ordered this way... except the bays are empty and everything is "pushed" to the outside of the framing. I am not considering mouse poop as insulation, not sure of R-value.
I haven't had too much ice damming as this space is very under-heated with just a vented gas Rinnai space heater. I'm looking to add a mini-split sometime in the future, but I want to get my insulation correct first.
Sounds like I will/can start looking for un-faced rigid foam for the ceiling.
Thanks!
PS: Any thoughts on the crazy Section A situation?
Seth,
The existing insulation situation is a mess, as you know.
Sections C and B break the rules -- using fiberglass batts to insulate unvented roof assemblies. This is a recipe for mold and rot. As Dana noted, it's close to impossible to vent roofs with a 2-in-12 slope. To insulate these roofs from the interior, remove the existing insulation and install closed-cell spray foam against the underside of the roof sheathing. To insulate these roofs from the exterior, install an adequately thick layer or layers of rigid foam above the roof sheathing. (This approach requires a second layer of roof sheathing above the rigid foam and new roofing.)
For more details on addressing these roofs, see these two articles:
How to Build an Insulated Cathedral Ceiling
Insulating Low-Slope Residential Roofs
It sounds like Section A is a vented attic, so the best approach would be to remove all of the existing insulation and then install a deep layer of cellulose above the existing ceiling. You wrote, "The thought of crawling around this attic space for a day or two to demo all of this insulation, removing the poly, and blowing cellulose does not really appeal to me." That leaves you three options: (a) Live with the leaky ceiling the way it is; (b) Hire a contractor to do the work; or (c) Develop the courage to do work that does not appeal to you.
Martin, thanks!
When I'm in the attic of section A, I can see proper vents sticking out from between the insulation and sheathing of section B (thus venting into the vented attic space of Section A). I believe we'd agree this is likely not adequate/ideal venting.
I just had my roof replaced last year and there was only 1 small section of decking rot at the eave of flat roof. This appeared to be from leaking and not poor venting rot. I could be wrong though. As this area has been under heated and we haven't utilized much in the winter, maybe the deck hasn't had the chance to "properly rot". We have a toddler now, so that's why we're starting to use this area.
Follow up questions:
- Martin: It sounds like you don't think that taped rigid foam on the interior of the ceiling in section B and C would help with this poorly vented roof?
- It sounds like if I dig the fiberglass insulation AND vapor barrier out of Section A, I can install adequate cellulose and not install a vapor barrier. I "confirmed this" with this previous thread:
https://www.greenbuildingadvisor.com/community/forum/general-questions/29193/cellulose-insulation-vapor-barriers
Seth,
If you have a poorly vented or unvented low-slope roof insulated with fiberglass batts, the roof sheathing is at risk for moisture accumulation, even if you retrofit interior rigid foam. The risk is higher in cold climates than warm climates; it's higher in homes with high indoor relative humidity than in homes with low indoor relative humidity; and it's higher in homes with leaky ceilings than in homes with perfectly airtight ceilings.
Sealing the leaks in your ceiling and keeping your indoor RH low will reduce but not eliminate the risk to your roof sheathing.
To do it right, you need to adopt one of the strategies described in these two articles:
How to Build an Insulated Cathedral Ceiling
Insulating Low-Slope Residential Roofs
You are correct that neither building experts nor code officials require an interior vapor barrier for ceilings. (In your climate zone, there is a code requirement for a vapor retarder, a less stringent layer than a vapor barrier. You can use vapor retarder paint to satisfy the requirement for a vapor retarder.)
For more information on this issue, see Do I Need a Vapor Retarder?
If section A depends on air coming up through the venting baffles (not indicated in the stackup list) for section B, it's likely that even section A is not adequately vented. With 2:12 roofs the 1" code-min vent space clearance is a joke- nowhere near adequate. In a coastal Maine climate you'd want at least 2", and 4" would be even better, which doesn't exactly leave much room for insulating between the rafters even if they're 2x6s.
How much was the roofing job? It may be that the best thing to do here would be to add 4-6" of reclaimed foam on top of the structural roof deck, and stuff the rafters with batts, with painted half-inch gypsum or unpainted OSB as the interior side air barrier of the rafters in the section-A attic. Reclaimed roofing polyiso is dirt-cheap (usually cheaper than batts on an R x square footage basis) and there are multiple vendors in the biz, as well as private one-off deals. This guy in Portland looks like he's sitting on 30-35 sheets (about 1000 square feet, roughly 2x the area you're looking at) of 4" polyiso (~R20 derated for climate) with a "$500 or best offer" type pricing:
https://maine.craigslist.org/mat/5802840853.html
Even at $500 that's about $15-17/sheet, or about 20% of the cost of virgin-stock goods. But feel free to dicker, and take the whole lot- some of those sheets may be a bit too dinged up, and you can craigslist off the remainders when you're done.
Since you're on the warm edge of zone 6, with R20 foam above the roof deck it would be fine to install R21 fiberglass or R20 cellulose between the rafters, and it would come pretty close to IRC 2015 compliance on a U-factor basis. (If you used a less-derated value of R22 for the reclaimed 4" polyiso it would make it for sure.)
Install a 1/2" plywood or OSB nailer deck for the new roofing though-screwing the nailer to the structural roof deck with 5.5- 6" pancake head timber screws ( Fastenmaster HeadLok, or similar, $150-175 for a bucket of 250 screws at internet pricing) and put down #30 felt over the nailer deck under the new roofing. Stagger the seams of the nailer deck with the seams of the foam by at least a foot.
When re-doing the ceiling, get rid of the polyethylene and air seal off any & all venting before installing fiber insulation between the rafters. If the exterior wall of the section A attic is currently uninsulated, it too would have to be insulated if you're moving the insulation boundary from the attic floor to the roof deck.
DISCLAIMER: I dislike reading through threads and seeing people ask for advice and then disregarding advice. That said, I've been thinking about this a lot and I hope I'm not giving off the vibe that anything that has been suggested is lost on me. I love all the thought/input.
I agree spray foam would be the best option for this area. I'm also a little hesitant to spray foam as some wires are run in the ceiling and it seems like every 2 years I need to pull/move wires due to minor changes. I sort of regret spray foaming my basement sills for this reason. It's also a little more expensive, and I if I can do it for less...
I poked around above the drop ceiling and found:
- it's actually 2x6 framing, not 2x4 (garage which runs parallel is 2x4)
- some of the poly is torn wide open in areas near the wall where previous owners ran electrical wire through the ceiling.
- there are styrofoam/eps proper vents
- even in areas of ripped poly and batt insulation pushed to the side, there are no signs of framing/sheathing discoloration
Given the above and:
- this is on a slab
- this is far extension of the house with no water source for humidity within 50'
- this is not occupied all that often, maybe 2-4 hours on a good day
- the roof slopes up to another section ("A") which has gable vents, and a vaulted skylight which likely is a humid area magnet.
- there are no going to be no ceiling protrusions. There are 2 recessed lights now, which will be taken out. This thing really is an insulation nightmare.
I'm considering taking down the drop ceiling, installing 2" of polyiso (ideally 2 layers of 1" staggered and taped) up against the existing poly vapor (after repairing section of poly), and drywalling it.
I'd then have (stack up) drywall -> polyiso -> poly -> fiberglass insulation in framing bays -> proper vents -> plywood sheathing -> full ice and water -> rolled roofing
My reasoning:
- humidity levels should be fairly low here
- the existing conditions haven't led to a moisture issue and I'm hopefully making this insulation issue better with the polyiso.
- the polyiso vapor barrier up against the poly vapor barrier isn't a threat of "double vapor barrier", otherwise two layers of polyiso would be a double vapor barrier?
- using a vapor permeable rigid would only allow more moisture into ceiling and allow for the moisture to be an issue?
I'm not looking for a home mold warranty, but am I wrong in my thinking?
Seth. I think the key question will be whether your roof assembly can dry to the outside. See this article for more detail: https://www.greenbuildingadvisor.com/articles/dept/musings/walls-interior-rigid-foam
Just an FYI update, I discovered some ventilation conditions that would likely get worse as we start to heat/use this area more. I have torn out all the 2x6 fiberglass insulation batts and will be spray foaming between 2x6s.
Now my question is whether or not I sandwich some rigid board insulation between the joists and the drywall. If I do, it would be 1/2" of 1" XPS (semi-permeable) to knock down the thermal bridging a bit. Ceilings are super low or else I might consider thicker.
Seth,
You can install 1/2 inch or 1 inch of continuous rigid foam on the interior side of your rafters or joists if you want. As you wrote, this layer of rigid foam will have a small effect (slightly reducing thermal bridging through the rafters). The foam won't cause any moisture problems.
If you're looking to maximize performance with interior joist-edge strips, use polyiso, not XPS.
Polyiso will perform at or slightly above it's labeled R value when on the interior side of the assembly, and doesn't need to be derated the way you would need to if it were above the roof deck, and at a much lower average temperature.
Dana,
I was thinking the XPS would help to dry to the inside? I shouldn't have much moisture issue with the spray foam, but thought the the polyiso might be making a double vapor barrier (with the small amount of air void and exposed beams between the vapor barriers).
Am I wrong? In theory? In practice?
If there is still going to be interior-side polyethylene there is no drying to the interior to be had.
What spray foam?
Maybe I need a drawing of what you intended. I don't know what is meant by "...the small amount of air void and exposed beams between the vapor barriers..."
Seth,
My understanding is that you will be installing spray foam between the rafters. You neglected to tell us whether you intend to install open-cell spray foam or closed-cell spray foam.
If it is open-cell spray foam, the foam usually fills the rafter bays and is trimmed flush with the edges of the rafters.
If it is closed-cell spray foam, the foam doesn't fill the rafter bays (because closed-cell spray foam is hard to trim).
It is my understanding that you don't intend to install any interior polyethylene. (That's good.)
You seem to be worried about whether a continuous layer of 1/2 inch thick or 1 inch thick XPS on the interior side of the rafters would somehow trap moisture. It won't.