Interior insulation (in stud bays) when we have exterior insulation?
This is a follow up to the question we posed here https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/26143/vapor-retarder-interior-if-exterior-has-vapor-barrier but our problem is a little different. Here are the relevant details of our house:
We’re in Zone 5A, building a new house which has already begun construction. We’re nearing time for insulation, and realizing that we cannot afford the blown-in cellulose we were planning on using with a Membrane vapor retarder.
Our exterior has plywood, tar paper, 1″ of foil-faced polyiso foam, and red cedar shake. The framing is 2×6 wood construction. The interior will be heated with wood stoves only.
The only insulation that fits our budget now is to use fiberglass, and all the companies where we are (Cape Cod) only install batts. My questions are: 1) is it okay if we install fiberglass batt insulation? (The best quote we received was for R-21 for the walls using unfaced batts with polyethelene vapor barrier, and R-38 in the ceiling eaves). 2) what kind of vapor retarder, if any, should we use/specify?
Looking again at the best fiberglass quote, I see that it says explicitly “unfaced batts with polyethelene vapor barrier” – is that a problem?
Thanks!
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Replies
If I were in your situation, I would install 2x4 roxul batts with no vapor barrier. I'd put something against the batts to make sure they stand up over the years. That way you keep the sheething warm enough to control condensation. I would can foam behind all outlets and switches with fire block foam. Same with all penetrations.. I would make sure my drywall was airtight.
Is your house a cape cod? How is the roof being insulated? Can you do any of the work yourself to save money? Home depot delivers and has volume discounts.
Thanks, Stephen! The house is a two story gambrel, not sure how the roof will be insulated (we were planning on using the same insulation contractor throughout. each one quoted us differently on how they would do it.) the attic is vented. I'll talk to my husband about the possibility of doing it ourselves (er, himself). It's a good idea, he's done eveything else!
Stephen, what do you mean by "put something against the batts"? Thanks!
Cathy, roxul batts are friction fit and stand up pretty good on their own. But in case they don't, home depot sells thin metal braces that are designed to hold batts in place. You could throw a couple in each bay for piece of mind.
Back to the roof. How deep are your roof rafters? Meaning how many inches of Insulation are you going to put in them? And are you planning on putting any rigid insulation across the interior of the rafters before the Sheetrock?
Tells us more about how the roof is vented.
Tell your husband to watch the roxul videos. Very easy to install. They also have planning software to help you order.
Make sure home depot sends the order to their bid room for volume discounts. They deliver it shrinkwraapped so it can sit outside as needed.
Roxul R23s will have a very slight performance edge over R21HD "cathedral ceiling" type fiberglass, and I personally find them easier to trim and install. They also have fewer outgassing and airborne micro-particle issues to deal with than with fiberglass batts. But the high-density fiberglass IS a decent performer (better than low-density R23s stuffed into the same 2x6 cavity.
Last time I looked R21 fiberglass and R23 Roxul were priced nearly identically at the local box store. A quick online check today (not a distributor quote) had R23 Roxul at about 90 cents per square foot to R21 fiberglass at about 70 cents, in single-bundle quantities, but in a whole house quantity it's probably narrower. (If the Greenback rises against the Loony it may flip the other way.)
POLYETHYLENE VAPOR BARRIERS ARE A DISASTER...
...in your stackup, since in combination with the foil-facers on the exterior polyiso it would form a classic moisture trap. Any of the "smart" vapor retarders will work, otherwise you'd be better off with no vapor retarder and a layer of vapor-retardent latex on the gypsum. But polyethylene sheeting is an absolute "NO!!". No competent insulation contractor would even quote it that way, unless they weren't aware of the exterior iso, but maybe you didn't clue them in ahead of time(?). Even kraft facers at 0.3-0.4 perms are a bit more vapor tight than ideal. Smart vapor retarders run 0.5-1 perm during the dry air of winter, 1-2+ during the humid summer, which gives the assembly ample path to drying. Vapor barrier latex runs 0.5-0.7 perms, and keep it on the thin side, eh?
No matter what batt solution you go with, inspect it VERY critically for gaps & compressions before closing it in. If it's not slightly proud of the stud-edge plane it wlll have a performance-robbing convection path to bypass the insulation. Tugging it out a bit where it got pushed in just before the smart vapor retarder or sheet rock goes up does a lot. If the installers scrunch it behind wiring and plumbing and smush it in around electrical boxes rather then trimming and sculpting for a good fit both behind & beside the box, they're hacks, but it's possible to go along behind them fixing errors of omission and commission before you drywall it. Once the gypsum is up, you're stuck with it, so take your time and do it right.
Also, if you haven't already, caulk every stud to the sheathing and between any doubled-up stud plates (or have a contractor do it), and foam-seal any lateral stud penetrations for wiring & plumbing, etc. Stopping air movement within and between stud bays is very important for maintaining performance in your high average wind speed location.
In a vented attic on the Cape I'd go with "stabilized formula" cellulose, applied damp. The adhesives in the cellulose keep it from being moved around by soffit air currents when it's really howling out, and it's WAY more air-retardent than any fiberglass or rock wool solution.
Cathy,
Zone 5A with 2x6 construction should ideally have at least R-7.5 of rigid foam outside of the sheathing to minimize the risk of condensation within the wall cavity. 1" of polyiso @R-6.5 would be OK with 2x4 construction as only R-5 is required to mitigate the risk. This wasn't in response to your question ... just something to consider as it could become problematic at certain times of the year.
Hi Dana, thanks so much for your response! I probably did fail to mention the exterior insulation to the fiberglass company so I will go back to them about that and see how their quote changes.
One concern I have with what you recommend for the attic is that no company I received quotes from quoted anything other than X + batts of some sort (cellulose company quotes cellulose for walls and attic and batts for stairs and basement; fiberglass company quotes fiberglass batts for the whole thing; spray foam company quotes spray foam for walls and attic, batts for stairs and basement ceiling). So I'm not sure how to follow your advice...should I get two different contractors? (Not sure if we'll be able to afford that!)
Also, your advice from our previous post was that the best option for our vapor dilemma was to insulate the interior with 1" of closed cell, then the rest with open cell, I believe. I may go back to the spray foam contractor to see how much that would cost. Is that still something you think might work for us?
Basically, from the quotes I got, the costs are: fiberglass batts < spray foam < cellulose < airkrete.
If we do Roxul, I'm guessing we'll have to do it ourselves (neither of us know how, but I'm guessing their are videos online) because I think I called every contractor on the Cape and a few off Cape, and no one quoted me for that, and everyone knew I was looking for the greenest options.
Thanks everybody for your help!
I would put 2x4 batts in to keep the ratio of batts to foam r in check. Yeah it sucks to have 2in of empty space in the bays but it would suck more to have a swampy wall. Perhaps communication between the bays could be mitigated with weather stripping or caulk against the stud face under the drywall.
A 2" empty space in stud bays is both a thermal bypass and a fire-spread hazard. NEVER do it, even where it's code-legal.
I'm a little surprised that spray foam is coming in under cellulose on price, but I have seen that more than once in recent years in central MA.
There are many industry-generated instructional videos on how to install batts correctly (and I wish more insulation contractors actually bothered to watch them.) It's not tough, and dedicated DIYers are perfectly capable of doing as-good or better than a pro (unlike spray foam, which can be a disaster in the hands of a newbie in a hurry.)
DIY dry blown cellulose in attics with rental blowers is also fairly straightforward to get right, but not damp sprayed. If you buy "stabilized formula" and dry blow it you can still keep it from being blown around if you mist-spray the top of it at the first couple of feet nearest the soffit venting as you go. If open cell foam is cheaper than damp sprayed cellulose for the attic floor, that's a very decent option, since it won't move around. MA requires IRC 2009 code-min, which would take about 11-12" of open cell foam, and it would air seal the attic quite well. You'd still need to keep code clearances from flues and air seal flue chases with metal fire-block caulk. It's common to wrap flues with R15 rock wool batts to establish clearance to combustible foam or cellulose. With the open cell option you'd be required to cover it with an ignition barrier (sometimes intumescent paint flies, sometimes not, depends on the local inspector's take) if you want to use it for storage.
If you went with a contractor on batts for attic insulation at *IRC 2009 code-min, you can get it to actually perform to spec with a DIY overblow of 4-6" of cellulose (misting it near the soffit vents to make it stay in place under wind washing). The air retardency and infra-red opacity of the cellulose over-cap effectively blocks summertime roof-deck radiation from penetrating the IR-translucent fiberglass, and blocks wintertime convection between the fiber layer and the cold attic air. That would also put you slightly above IRC2012 code min, which would be appropriate at your anticipated whole-wall-R.
For the basement insulation, I'm assuming the quotes are for insulating the basement ceiling with batts, not the walls? If yes, a DIY rigid-foam or foam + studwall-batt solution insulating the walls to at least R12 would be higher performance than anything you did between the joists. I'm surprised the foam company didn't push 2" of closed cell foam from the slab up to the top of the band joists (which would be GREAT, but runs $2/square-foot or slightly more.) An inch of EPS trapped against the foundation wall by a 24" o.c. 2x4 studwall with unfaced R13s gets you to about R15 whole-wall for not huge money as a DIY, and would beat (IRC2009 code-min) R30 between the joists on performance, and lower the mold-hazard in the basement by keeping it warmer & drier both winter & summer.
Dana, fire spread point. Good point. Didn't think of that.