Zone 5 Chicago
2x6 wall, external sheathing
R7.5 external rigid insulation

If I have Applegate blow in cellulose into this wall assembly, will I need an internal vapor barrier?

Also, we had a brutal winter with -15 to 10 degrees for 6 weeks straight. I assume this will condense, or will it? Will the cellulose block humidity? If the external wood sheathing condenses, over time once it warms up will it be able to dry out sufficiently through the cellulose?

you do not want to install a vapor barrier, especially with exterior foam. You may want to install an AIR barrier, but the wall must be able to dry to the inside, since it will not be able to dry to the outside. With vapor barriers on both sides, any moisture in the wall assembly can lead to MOLD and ROT.

It won't condense, it will end up in the wood fiber in the form of adsorb, until/unless it hits such a saturation level that frost/dew begins to form on the surface of the wood (which takes quite a bit, not just a few weeks of super-cool weather).

You might have had lows that were -15F to +10F for most of 6 weeks but your average temps were much higher, and it's the average temperature over the period that ultimately determines how much moisture ends up in the sheathing, not the overnight lows. I doubt there were more than six DAYS (and not all consecutive) where the daily average was below +10F. And despite the occasional brutally cold weeks with the record setting lows, they were followed by weeks of above-average temps, the binned hourly average temp for the Chicago area for the 12 coldest weeks.

Pull up a temperature history graph and show me which 6 straight weeks even the nightly lows were all below +10F:

http://weatherspark.com/#!dashboard;a=USA/IL/Chicago

Just eyeballing it with the cursors I'd hazard the that the binned hourly mean temps for any 6 week period was within 5F of the historical averages.

But whatever.

It's OK to put an interior side vapor retarder on the interior of a cellulose-insulated wall with exterior foam if it's a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus and if that R7.5 is XPS or foil-faced iso that would be pretty cheap insurance. Here's the break down:

If that R7.5 is 2" of unfaced Type-I EPS (labeled R7.8) and under back-ventilated siding you could get away with cheap polyethyene on the interior, but it wouldn't be necessary since the EPS outperforms it's labeled R value substantially when the average temp through the foam is under 40F (and even the warm side of the foam would be averaging 40F or a bit below). At 2" the permeance of Type-I EPS is about 2.5 perms, with very decent drying capacity toward the exterior.

If it's 1.75" of Type-II EPS (labeled R7.4) under back-ventilated siding it would still have 1.7 perms of outward drying capacity, and like other polystyrene it gains performance when it's average temp is below 40F, and would be hitting about R8 when it's 0F outdoors. It's enough (though wouldn't meet code on labeled-R) and wouldn't need an interior vapor retarder.

XPS @ 1.5" /R7.5 the permeance is only about 0.8 perms, which isn't a lot of drying capacity but some. But it's performance is again better than the labeled R7.5, and would be running better than R8 during mid-winter coolth so you have some dew-point margin there. Again, no interior side vapor retarder necessary, but it has considerably less drying capacity than EPS options. If you're nervous enough to want to put in a vapor retarder might get away with interior poly, but a smart vapor retarder would be safer, since poly would block all drying toward the interior, and you may end up with condensation on the poly during the summer season if the sheathing isn't dry enough by then.

If that R7.5 is 1.5" of foil-faced polyiso (labeled R would be R9) it has ZERO exterior drying capacity. It's performance will be no better than R7 during the low temperature extremes, but climbs rapidly toward R8+ when the daily temps are averaging above 30F, accelerating the drying toward the interior. It's enough, but JUST enough for a zone 5 climate that you wouldn't need an interior side vapor retarder, but unlike the EPS options an interior poly would be asking for disaster (all walls leak somewhere, at some point in their lifecycle) but MemBrain or Intello Plus would be just fine. With the buffering capacity of the cellulose you don't really need the interior vapor retarder, but if you decide to install a smart vapor retarder to be able to sleep better, when the cellulose & sheathing are unloading their winter burden the limiting permeance for drying toward the interior will be the interior paint, not the vapor retarder.

So, what foam?

Thanks guys. Dana, I was planning to use XPS 1.5" thick to get that margin. I was also thinking to use the intelligent membrane (intelloplus or membrain) as you said but someone said not to, it would be a mistake. Does this smart barrier ever lose its capability to breathe, years down the road?

I'm also trying to get bids on JMSpider. Why is it no one in Chicago has ever heard of these methods/products described here? The JM wholesaler had only one contractor in the last year buy that stuff. Crazy.

I would use 2 inches of unfaced EPS, cellulose and good air sealing (using tape) on both the interior and exterior walls (air sealing has a lot more to do with wall moisture than permeability and R value do).

XPS @ 1.5" will be running less than R6.5 after 4-5 decades as it loses it's HFC134a blowing agent over time (warming the planet in the process, since it has about 1400x CO2 type global warming potential, which is why it's been banned in Europe as an automotive refrigerant.)

With 2" of EPS it's pretty much the same R at age 50 as it was on day 1. Blown with pentane (about 7x CO2 GWP) EPS sheeting has lost most of the blowing agent before it hits the distributor's loading dock (unless it has foil facers on both sides). With big slabs of foot-thick EPS it can take months rather than weeks, but the labeled R is it's fully-depleted R, unlike XPS which is labeled at something like a 20 or 25 year average performance.

It's not necessary to install a smart vapor retarder, but not a "mistake" in the sense that it reduces the resilience of the assembly. It still adds resilience by slowing down the moisture diffusion through the wall when the interior is kept at 35% RH or lower during cold weather, but it has a negligble impact on drying rates. When the sheathing & cellulose warm up releasing the adsorbed moisture load the air inside the cavity rises to above 50% RH, at which point the smart vapor retarders are running 10 perms or better, and it's the 3-5perm interior latex paint that determines how fast it leaves the cavity, not the vapor retarder.

There is no reason to believe the vapor retarder becomes less permeable over time, but like all polymers it will slowly break down, and probably become more vapor permeable. But that's a WAG, my gut feel, no data behind it.

Installing a true vapor barrier like 6 mil poly or foil wallpaper WOULD be a mistake.

Again, thanks guys.

jonr, I can't do 2" EPS or I would...

Dana, I do plan to use the smart VR if I use anything. Question, if I have this in a bathroom area with a shower, the humidity will temporarily rise in that room during a hot shower and the smart VR will open up and let some of the vapor into the wall right, until it "equalizes"...and then as the room dries out, the vapor retarder will then let the vapor out of the wall and back into the room to dry. I am curious if there is enough vapor moving through to even be concerned about this...?

The amount of moisture migration via vapor diffusion is all about the averages, not the peaks. If you have an HRV that is pulling air from the bathrooms &/or recirculating it with the rest of the conditioned space whenever it's running AND use local exhaust ventilation during showering periods, the average RH in bathroom will be pretty close to what it is elsewhere the house, despite the short term peaks. If you have 2-4 family members who insist on 20-minute showers every day it might add up.

In the case of over-the-top shower duty cycles, 1/4" fan-fold XPS siding underlayment that has a facer under air-tight gypsum or tile-backer to bring the permeance of the bath room walls to between 0.6-1 perm, or hitting it with ~0.5 perm vapor barrier latex primer might be "worth it".

http://homedepot.owenscorning.com/docs/foam-products/FOAMULARFanfoldProductDataSheet.pdf (~1.0 perm facers)

But really air-tightness on the interior side is going to be more important than the vapor permeance here.

With the exterior rigid, doing the work myself, I might install HD21 batts with the intelloplus over that. I will be sure to seal the sheathing well, caulking everything. I figure the air movement will be severely mitigated by the sealing effort, so HD21 batts will be economical. The intelloplus or membrain will prevent moisture, so I hope this is acceptable. Will save considerable money.

sounds good

Good tape and gaskets should outperform caulk (but I'd like to see more data). For example, as a 0.01" gap opens up to 0.1" as the humidity or temperature goes down. Try to avoid staple or nail holes in the membrane.

Posted By Surfsup on 01 Apr 2014 07:14 PM
With the exterior rigid, doing the work myself, I might install HD21 batts with the intelloplus over that. I will be sure to seal the sheathing well, caulking everything. I figure the air movement will be severely mitigated by the sealing effort, so HD21 batts will be economical. The intelloplus or membrain will prevent moisture, so I hope this is acceptable. Will save considerable money.

Rock wool R23s cost about the same as fiberglass HD21s, and is a somewhat superior product IMHO.  Rock wool doesn't melt in the event of a fire, and has fewer issues with particulate fibers ending up in the conditioned space air, etc..

The additional R of being R is "in the noise", after thermal bridging- the quality of the batt installation makes more difference than the additional center-cavity R2, but it doesn't hurt.

With a smart vapor retarder approach and air-tight interior side construction you're golden- the assembly will be pretty resilient.

OK great I think I'm happy now! I've been stressing this insulation so much...sheesh. Get one shot at it right? I'll look into rockwool R23 - I assume you mention it b/c it will fit in 2x6 (5.5").

"Try to avoid staple or nail holes in the membrane."
I will tape over any holes in the membrain/intelloplus but the drywall guys will come in too. So I realize it won't be perfect.

I might just do the same with the cathedral roofs. I might install 1/4" ply at the top of my TJI 14in rafters to leave the airgap, seal it well with caulk then use R30+R15 batts to get R45 and put intelloplus/membrain over that...

Posted By Surfsup on 02 Apr 2014 06:59 PM
OK great I think I'm happy now! I've been stressing this insulation so much...sheesh. Get one shot at it right? I'll look into rockwool R23 - I assume you mention it b/c it will fit in 2x6 (5.5").

"Try to avoid staple or nail holes in the membrane."
I will tape over any holes in the membrain/intelloplus but the drywall guys will come in too. So I realize it won't be perfect.

I might just do the same with the cathedral roofs. I might install 1/4" ply at the top of my TJI 14in rafters to leave the airgap, seal it well with caulk then use R30+R15 batts to get R45 and put intelloplus/membrain over that...

Yes, rock wool R23s are designed for 2x6 standard framing, either 16" o.c. or 24" o.c. variants can be had, and it's a bit over a buck a square foot at box store pricing, compared to 70cents/foot for R21 fiberglass at this week's pricing from the same box store outlets.  In bulk sources from distributors catering to contractors the up-charge for R23 rock wool vs. R21 fiberglass is usually narrower.

A few nail holes in the membrane barely affects it's vapor permeance, since the hole is filled with vapor-impermeable metal (the nail). Tears and unsealed seams affect the air tightness, which is far more important.  A bead of acoustic sealant between the membrane and framing at the edges matters, and seams where membrane edges overlap have to be over a framing element, and sealed with either acoustic caulk or the appropriate tape. Taping it in the middle of a stud bay supported only by the fiber insulation where it can flex isn't good enough.

The bigger detail PITA is air sealing the electrical boxes to the membrane.  The Canadians are way ahead of the US on air-sealing elecrical boxes, since it makes an even bigger difference to building sustainability in most Canadian climates than in the US, so you may have to train your contractors (or import some Canucks for that part. )  Note that the Canadian method in that 4 part series doesn't have the same long-term air tightness as flanged air-tight electrical boxes since the tape isn't supported by framing, but it's not bad, and probably good enough if you're really careful. It's a lot less taped seam than where full-height seams need to be sealed for overlapping full sheets.

Dana - the videos are a good example of why contractors have to retrain the kids after they come out of trade school.
"Those who can, do. Those who can't, teach!"

If you look closely, he doesn't seal the wrinkles in the plastic. Acoustic caulk is better. After all the time he has spent playing with the bits of poly, he didn't make sure he pulled the poly away from the box so the drywaller is going to cut it to shreds when he roto zips out the box.
VAPOUR HATS VAPOUR HATS VAPOUR HATS.
Read the comments on the first video.

This is how a trades man would do it, sort of ;-) https://www.youtube.com/watch?v=1vu1EsHA6x4

It takes both a leak and a pressure difference to get infiltration. So consider a room-by-room pressure check comparing interior to exterior pressure. With the heating/AC and just ventilation running.

See here for some caulk/foam vs tape info.

Rigid air barriers typically outperform membranes/films. And both together (or any multiple barriers) outperform either.

Here is an interesting variation on airtight electrical boxes although I expect that Airfoil boxes work better.

Roxul pricing isn't bad but compared to standard fiberglass $0.40 more is really 55% more ($1.12 vs $0.70) which percentage-wise if I paid 40% more for every product, I'd be broke. So it is a consideration but the guys that quoted Applegate insulation gave me a quote for under $10k to do the whole wall(4600SF) and roof (3000SF) which I'll take.

As far as the videos, I can't do any of that. I'm not sure where you guys reside but that is not code around here. All wiring has to be in metal conduit. There's no way that can happen. And working with metal conduit makes things more complex to air seal. I think I will blow Tuff Stuff into the hole inside the box. That's the only thing I can think of that would be easy. I will try to wrap each box with plastic and tape around it then tape to the intelloplus/membrain...

FBBP do you know what the plastic box is he is using in that video? I can't seem to google it...

Google Vapour hat or vapour box. Typically .30 to 80 cent. They are available in most box stores in Canada because this has been the code for over two decades. When we first did them, we all did them like the "instructor" did them but within 6 months the vapour hats where available and we no longer used bits of plastic sheeting. As they are not code in many southern locations, you might have a harder time finding them but they should be available from a web source.

Because of the wide flange, it is easy to get a good seal with acoustic caulk. Just cut out the hole in the wall vapour barrier slightly larger then the steel box (so the roto zip doesn't catch the poly) and then stick the caulking in between the v.h. and the wall poly forming a good size bead. If you then press the wall poly into the caulk, you will have a seal that will last forever. I would not use tape.

As v.h. and acoustic caulks have been around for almost three decades, we often run into old ones in renos. And we still cuss the darn black stuff because after all that time, it is still sticky and always winds up on your hands. You can literally reseal old poly joints by pressing them back together with just the old caulk that is still there.

https://ontario.westburne.ca/freeTextSearch.action?freeText=vapour+barrier

I'll have to talk to my electrician on this...yes I can't find anything in the USA. took me 20 minutes to find that link in Canada.