At 8lbs density rock wool is very air-retardent, sufficient to not lose R to convection at higher delta-Ts, but not sufficient for use it as the primary air barrier- use the structural sheathing &/or housewrap instead.

The issue with summertime moisture drives causing condensation on the cavity-side of the gypsum is real, made worse if there's a vapor retardent interior finish, particularly in areas with dew points in the high-60s & 70s, but not so much for most of the western half of the US where summertime dew point averages are in the 50s. It doesn't take much of a vapor retarder at the sheathing layer to reject that though, and rainscreen cuts down the acute drives of sun on wet-siding. Under stucco/brick/stone cladding you'd want at least a double-layered felt to get the permeance between the rainscreen gap and the structural sheathing under 2. With ~ 2-perms on the exterior and ~2-3 perm OSB sheathing the air conditioning will provide sufficient drying to the stud cavity through ~2-3 perm paint on gypsum even under "reservoir cladding" like masonry.

Tony: In Wichita KS you're smack in the center of US climate zone 4, where 1" of ANY foam outside the sheathing would be sufficient condensation protection even on 2x6 framing. ( I too would have done things differently on some projects less than a decade ago, if I only knew then what I know now.) When it's time to re-side or re-roof these are opportunity-moments, since thing that are cheap to do at the time become cost-prohibitive after the fact.

I stumbled upon another article that was a good read from the Cold Climate Research Center.

Safe and Effective Exterior Insulation Retrofits: Phase II. Jan. 15th 2012. http://cchrc.org/docs/reports/MTL2_final.pdf

It talks about insulation up in Alaska, but it offers some measurements as well as talk about building envelopes which use less than 30% of insulation on the outside of the house.

It was explained very nicely by Dana1 on the specifics for dew point condensation. This article provides some tests and measurements as well as describing the situations nicely. It also talks about batt insulation with and without facers and provides data points from real observations.

One thing I noted that they determine that using 30% or less of total R insulation on the exterior was a bad idea. It may be more prevalent in the extreme cold weather where they test, but it did make me leary of putting something on the outside that would limit the vapor from escaping due to my interior kraft facers. R5 on the outside is then only 35% of exterior insulation (I actually believe I have R11 batt leading to whole wall of R9.48). R7.5 is 45% on the outside.

My house seems to be a higher humidity level than is recommended in the winter. Right now it's reading 45% and I have the whole house humidifier turned off. I'm a little hopeful insulating the basement will stop some of the moisture from coming in. The basement is at an even higher humidity level. I have posted on insulation types in a separate thread. For that insulation, I'm leaning towards just 2" of XPS on the walls and then roxul R15 batts in the walls. It seems to be a pretty good choice. I didn't really get the response I needed to fully seal the basement for both air and vapor, so I'll leave some vapor permeability with the 2" of XPS.

It's center-cavity R, not the whole-wall R that's critical in terms of moisture accumulation in the sheathing.  At the studs the low-R of the studs makes even 1" of XPS a ~50% R value fraction of the R at the framing, but the center cavity the sheathing will run cooler. With R11 batts and R7.5 sheathing that's ~40% exterior R at center cavity, and that's what you need to run the dew point calc on.  With R10 XPS/R15 Roxul you'd be at the same center-cavity fraction, and ~R20ish for whole-wall R.

In most AK climates you BET less than 30% of total R on the exterior wouldn't cut it without an interior vapor barrier, in some places even 50% wouldn't!  The mean January temp in Fairbanks is about -10F (yup, below zero!) considerably below the heating design temp in Detroit, and even 50% exterior R wouldn't quite meet the bare minimum for going sans-vapor retarder there (particularly in a higher-humidity interior), and at only 30% you'd probably need at least a class-II vapor retarder.   This is borne out in the table a the end of your referenced document- with 42% exterior-R and no vapor retarder the sheathing was moldy, with 65%+ exterior R with no vapor retarder it was dry.

But in Detroit 30% of the center-cavity R on the exterior of the sheathing is more than enough.  Your climate is closer to that of central KY than central AK (yet still different enough to matter.)  Detroit's average winter temp is ~26F,  (~36F higher than in Fairbanks),  and the "drying-season" for the sheathing is 8-9 months long instead of 3-4.  It's ridiculous to build for a Fairbanks climate in Detroit (and conversely.)

The fact that it's currently 45% relative humidity indoors in late March after a week of record warmth is of no consequence.  (If it's that high in January through February at normal mid-winter temps, THAT's the time to be concerned.) The dew point of 70F 45% RH air is ~48F, and if the average outdoor temp of the last 2 weeks has been close to 50F, so foam or not,  there has been no additional accumulation of moisture from interior drives during that time period- in fact the sheathing has been DRYING on average (in both directions), and fairly rapidly during that stretch of summery weather.

Whole house humidifiers are a bad idea in most homes. Tighting up the home and controlling the moisture by varying ventilation rates in winter, and by REMOVING moisture with air conditioning or dehumidifiers is a better approach. Low interior humidity in a Detroit climate is a symptom of high air infiltration rates, and adding humidity to the air increases the moisture accumulation in materials along the exfiltration path.  That infiltration air is also a substantial fraction of the heating load in winter, and latent-cooling load in summer.

If there are no signs of moisture in the foundation wall and a good capillary break between the foundation wall and foundation sill, 2" of XPS is fine. But in your climate 3" of fiber-faced iso would be better. If there is no sill gasket 3" of unfaced EPS would allow the concrete to dry better to the interior, protecting the foundation sill. Otherwise, stop the foam a foot from the floor to allow any ground moisture wicking up from the footing to dry toward the interior.

For those of you who live in areas that have dissolve sulphate in the soils adjacent to the foundation, it is probably not a good idea to allow the foundation to dry to the inside. This will allow a continuous sulphate attack on concrete as the capillary action of the concrete draws the water in to be dry of the inside surface.

Posted By FBBP on 27 Mar 2012 10:24 PM
For those of you who live in areas that have dissolve sulphate in the soils adjacent to the foundation, it is probably not a good idea to allow the foundation to dry to the inside. This will allow a continuous sulphate attack on concrete as the capillary action of the concrete draws the water in to be dry of the inside surface.

A membrane or metal capillary break between the footing and foundation wall (as well as between the top of the foundation wall an the foundation sill) is a good idea in ANY new construction, and would largely mitigate that as an issue.

In high sulfate soils drying in ANY direction would be a problem without capillary breaks.

I was looking into fiber faced polyiso, but haven't had luck finding someone local who carries it. I need to dig a bit to a roofing supply company and see what i find. BTW, I do have a sill gasket. I did look up the Roxul ComfortBoard IS. I needed to buy 400pieces from my local lumberyard to place the order! So can't use that.

Well, I found this product called PolyGlass PolyTherm that is carried by a local supply company.

"rigid closed cell polyisocyanurate foam core bonded to glass fiber mat facers"

Sounds like the stuff that was talked about here, though it shows < 1.0 perm so i'm not quite sure it's the same stuff, since I thought I recall it was supposed to be around 2-3 perms.

http://www.polyglass.com/US-EN/product-detail.asp?IDSottoLinea=50&IDProdotto=150

Fiber faced iso is usually under 1 perm, but over 0.5 perms, and it's rating stays pretty much the same independent of thickness. Because it's entirely a function of the facers, and the facers aren't easily manufactured to a limited range, they use the <1 perm in the product description. Were it <0.5 or as low as 0.1, you can bet they would specify that. From a moisture-trap modeling point of view I use 0.5, not 1.

By comparison, @ 1" XPS is 0.8 perms (yep, < 1 perm!)

...and @ 2" XPS it's 0.4 perms

...which is about the same as a kraft facer, and similar to or lower permeance than fiber-faced iso.

You can go much higher-R with fiber faced iso than XPS as long as you're keeping it to no more than 2 layers (4 facers.)

1" XPS = .8 perm?

I just read that the 1" Dow foamular 150 XPS panel = 1.5 perms

http://www.foamular.com/assets/0/144/172/174/1b241d3e-6d7f-4c14-88db-ab256a190e08.pdf

The 250 panel is the same.

It varies by manufacturer & density- I tend to use a reasonable worst-case when modeling these. Other documents for Foamular 150 specify 1.1 perms, and specify it as a MAX, not typ.  

I recently saw a spec for another vendor's standard product at 0.8 perms typical & 1", (I'm trying to remember where and dig up a link for you), so that's what I tend to use.

Interesting. It seems your doc is from 2004 while the other is a little newer. It's enough to make you leary of trusting the numbers and being pessimistic like you were saying.

Yep- it's not clear if the 1.5 perms is a typ or a max, but I suspect the latter. There has been process and blowing-agent changes over the decades too, all of which can affect the permeance.

Many designers use 1-perm @ 1" as the short-hand for the order of magnitude for either closed cell spray foam or XPS, and that's probably good enough for most purposes. As long as you're in the ballpark the real-world differences aren't that meaningful. Like XPS the specified rating of the test samples delivered to the lab have quite a range across vendors & density, and the in-situ "as installed" range is probably greater still. But they're all way under the permeance of 1" EPS, and and way above kraft facers, even if the size of the ballpark is pretty wide.

Going with 3" of fiber faced polyiso for the basement.

Since the perm rating stays constant, then I would guess after reading through the perm ratings then 1.25/1.5" of polyiso or 1.5 of EPS would be the best option for the outside of the house.

when people talk about polyiso and they use terms like foil faced, glass faced ... is the glass faced polyiso just the normal fiberglass re-inforced polyiso or is it a special high end thickness glass coating? I was calling around and many have the fiber re-inforced polyiso, but is there a 100% fiber glass facer? It seems the regular polyiso is a mixture of fiberglass and cellulose.

Fiberglass reinforced iso has glass fiber in the iso to give it better structural capacity. Fiberglass FACED iso has fiberglass-reinforce paper, often asphalt-loaded in the outer skins, but none mixed in with the iso itself providing puncture-resistance and adjusting the (fairly high) permeance of the raw iso down to something useful for moisture control.

You could do 3" of fiber faced iso on the outside of the house in a single-layer, using 1-part foam or duct-mastic to air-seal the seams.

Switching to the previous sub-topic, while on the outside of the house the fiberglass-reinforced paper polyiso would see ok, I would think this same product would be a bad idea for basements even if there are no moisture problems.

Posted By alang on 02 Apr 2012 12:08 PM
Switching to the previous sub-topic, while on the outside of the house the fiberglass-reinforced paper polyiso would see ok, I would think this same product would be a bad idea for basements even if there are no moisture problems.

In basements you need to control the moisture flow in both directions.  A very low perm layer like foil facers or poly often comes with consequently higher moisture content to the concrete from ground moisture, leading to rot at the foundation sill, unless there are membrane or metal capillary breaks at  both the footing and between the concrete & sill (with foamy sill-gaskets being a poor third place "better than nothing" approach.)

With ~0.5-1 perm foam on the wall ground moisture wicking up the wall dries toward the interior, but the rate of vapor migration in winter to the cold section of above-grade portion of concrete is slow enough that the concrete still dries toward the exterior.  If you go with a foil faced approach leave the bottom 12-18" un-insulated (or insulated with EPS at the same thickness) so at the very least the moisture wicking up the foundation wall from the footing can dry toward the interior.  In very cold climates (US zone 7 or the cold edge of zone 6) a vapor-tight basement wall can even lead to spalling of the exterior above grade concrete during freeze/thaw cycles due to the elevated moisture content of the concrete from ground moisture wicking.

Fiber faced iso is designed to work under extremely low-perm membrane roofs in all climates, and neither the facers nor the iso  has any mold growth, potential (it's either not mold-food, or the treatments with asphalt etc. inhibits mold growth.)  In my own home I've had the opportunity to remove small sections of fiber-faced iso after a few years' use to be able to move plumbing drains, etc. and have found no mold or moisture damage.  (I'm in an average-temp part of US climate zone 5, in a region with a design frost depth of 48".)