How do I compensate for that?

By adding so much exterior foam that at a given interior %RH and outdoor temp, condensation won't occur. But I argue that while condensation is the worst problem, even high %RH can be a problem (it can cause mold). So put the polyiso on the inside (in a cold climate) and let the exterior breath. Or at least replace the exterior polyiso with unfaced EPS. And in all cases, air seal well (gaskets and tape, interior and exterior barriers) and make sure that the building doesn't get positively pressurized; this will push moisture into the wall. The typical bathroom fan should ensure that during showers.

Posted By Surfsup on 07 Nov 2013 09:42 AM
Yes but I was wondering what happens when you have a section of wall that has barrier on both sides (external barrier and a shower wall that is waterproofed)...I am wondering if there is enough vapor permeability along the wall to reach the gypsum to make this okay...? Starting from the inside:

Drywall
Membrain or equivalent
2x6 framing with R21hd batt
1/2" sheathing OSB
Polyiso 3/4" (zone 5 )
Tyvek housewrap outside the polyiso per Dow recommendation
Windows furred out with 1x2s flange mounted to the furring strips


If I have a shower on an exterior wall, the rockboard, waterproofing and tile will cover up the membrain on that shower wall. How do I compensate for that?

Not all "waterproofing" materials are vapor barriers, some are.  Waterproofing is about limiting capillary draw of liquid water, not water vapor. You want something that is at least somewhat vapor permeable (0.5 perms or higher, but not more than 1.5 perms). Read the specs.  Almost all ceramic tile vapor permeable- glass tile often isn't.

If the most vapor retardent layer you have between the shower and the sheathing is MemBrain, the bursts of high humidity from showering can add a bit too much to the moisture load of the sheathing.  To compensate for that you could put a ~0.5-1 perm fan-fold XPS "siding underlayment) with vinyl or polyolefin facers (read the specs on the permeance- it varies vendor to vendor, product to product- some are over 1.5 perms, which is too open for your purposes.  Some fan fold XPS roofing underlayments are in the 0.1 perm range, which is way too vapor-tight.)

A couple of fan-fold XPS specs:

Pactiv

Owens-Corning

I was planning to use mapei mapegum liquid waterproofing around the shower enclosures on top of durock with tile over that.

Posted By Surfsup on 08 Nov 2013 11:33 AM
I was planning to use mapei mapegum liquid waterproofing around the shower enclosures on top of durock with tile over that.

That stuff test at permeability of 650 under Standard UNI 8202 part 23a testing, but I don't know how to compare that to US-perms under ASTM E 96 test conditions.  (Your googling is probably as good as mine for looking that up.)

in zone 5 if I put 3/4" polyiso on exterior wall and on the interior I sprayed 5" open cell between the 2x6 in the cavities, would this cause dew point mold on the sheathing?

Posted By Surfsup on 15 Nov 2013 08:47 PM
in zone 5 if I put 3/4" polyiso on exterior wall and on the interior I sprayed 5" open cell between the 2x6 in the cavities, would this cause dew point mold on the sheathing?

Yes.

You'd need at least R7.5 to keep the OSB dry enough, and more is better. (It's been enshrined in code.)

If you used a "smart" vapor retarder like Intello Plus or Certainteed MemBrain  between the o.c. foam and gypsum you'd be in pretty good shape though, except for not well ventilated bathrooms.

If you use a higher density semi-open foam with a permeance of less than 1 @ 5" you'd be good to go though. (That would be the case for Icynene MD-R-200). But it's pretty pricey, and not nowhere near the same performance uptick of going thicker with the exterior foam.

Well I can't do more than 3/4 due to siding installation issues and no one in illinois carries either membrain or intelloplus. Hell, no one's even heard of these products. Checked 8 lumber suppliers. Called certainteed, their only "dealer" in itasca IL doesn't carry it anymore.

I'm looking at demilec agri foam for the TJIs and 2x6s. That, from memory, has a perm of 5? So if I can find a smart VB, will that work with 3/4 ext rigid polyiso?

"no one in illinois carries either membrain or intelloplus"
What you say is true in most of the country, and is true for many of the newer products that are important in high performance homes, so order it from someone who carries those products. Mail order used to be unusual, but Amazon is doing 60 Billion a year in sales with no local stores, so finding things locally can't be used as an excuse. Maine Green Building in Portland Maine, The Small Planet Workshop in Oregon and 475 Supply in NYC (and others) all carry these products and they'll all ship quickly to Illinois or anywhere else.

Yes, 0.7lb Demilec runs about 5 perms at 5 inches, which is low enough to be protective of the sheathing with only 3/4" iso on the exterior, if you promise like hell to keep the interior dew point below 40F in winter (that's ~35% RH max @ 68-70F).

Yes, a smart vapor retarder would give you some margin on that:

http://www.foursevenfive.com/index.php?main_page=product_info&cPath=70_76&products_id=219

That's the plan, to use the Demilec Agribalance:

siding
3/4" polyiso
sheathing
2x6 walls
2x6 cavity filled with Demilec Agribalance
gypsum

http://householdresource.com/BuildingScience.html

This link states I don't need an additional VB. See snippet below. Do I even need the membrain or intelloplus?

"Code approved Vapor Retarders
SEALECTION Agribalance and SEALECTION 500 are in fact both code approved vapor retarders at 3.5", 5" and 10". "

Not to change the subject but they are also claiming their Agribalance is recommended for use in the TJI 14" cavity for the rafters - 11" R49. They say open cell is better than closed cell because it will absorb water, but also drain it as well (breathe a little).

So really two questions, with this insulation do I need the membrain? is it ok to use ocSFP on the roofdeck 11" between TJI 14s 24" o/c?

Don't buy the marketing hype for the vapor retardency of the wall foam- it's absolute crap! At those thicknesses yes, they are class-III vapore retarders, but so is standard latex paint!

The REAL code requires class-II vapor retarder (between 0.1 and 1.0 perms) on the interior side of the insulation when wood sheathing is used, unless you have at least the code prescriptive R-value for the insulating sheathing, which you don't. At 3.5" and 5" those products don't cut it AT ALL (the paint is more vapor retardent.)

The 0.7lb Agribalance comes close at 11" at about 2 perms, (only TWICE the code max :-) ), and I wouldn't sweat the roof deck application much. It'll work, even if it's not quite code, unless you screw it up with something like high solar-reflectance metal roofing above the roof deck.

The walls definitely need either a smart vapor retarder (preferred) or "vapor barrier" latex primer on the wall board (problematic) to meet code (even if you would still be OK if you keep interior on the dry-end of the range of human-healthy interior humidity in winter with only 3/4" of exterior iso.) A flash-inch of ~1-perm closed cell with the rest of the cavity filled with cellulose would be a more resilient solution than an an open cell fill if you're skipping the MemBrain.

Ok, i'll do the intelligent membrain. I'll order it any which way.

Regarding the roof, i am doing a metal roof. 3:12 pitch, light gray color!

So now ocSPF 11" wont work?

light gray color!

Is that compatible with the neighborhood? Sometimes neighbors react to those lighter colors, particularly where sun and reflection is an issue.

No restrictions. Can be red if I wanted...

http://www.berridge.com/colors/

The light grey I refer to is their "Zinc grey" in the standard colors. I might do charcoal but even with the kynar, I understand darker colors won't last as long (plus I'd rather reflect the heat rather than absorb it)...

I think I have the roof figured out now per my other post:

SS metal roof
#30 felt or synthetic
Sheathing
TJI 14" joist cavity 24" o/c
2" rigid polyiso board ripped to 1' 11-3/4" wide wedged between each TJI leaving 1-3/8" vent
(R-12) 1" closed cell SPF for air seal (+R-7 for R-19)
HD batt or spider to fill the rest of the cavity which is 9-1/2" left open
gypsum

R38 is required so 38-19 = R19 batt

With the 9.5" though I can use a R30 HD batt which is only 8.25" thick for a total of R49.

Well, just because there are no neighborhood covenants doesn't mean that just any color is going to create harmony with the neighbors.

You might look into the "copper penny" color. I believe it is a cool roof color, too. Apparently, it isn't used much and there has been no end to the compliments on it.

ETA:  It would be the "COPPER-COTE" color on the mfr you provided.

Posted By ICFHybrid on 24 Nov 2013 10:11 AM

light gray color!

Is that compatible with the neighborhood? Sometimes neighbors react to those lighter colors, particularly where sun and reflection is an issue.

What's the solar reflective index (SRI) of the finish?  (It should be in the specs somewhere.)  If it has an SRI over 50 it's potentially an issue, over 70 DEFINITELY an issue.

Mounting the roofing on purlins (the moral equivalent of a rainscreen gap in a wall assembly) with a somewhat vapor permeable underlayment you'd be fine.

The labor cost of ripping and sealing polyiso into TJIs it high- you'd be better off going with 2" of 2lb foam (~0.5-0.8 perms) applied directly to the underside of the deck with the rest as blown fiber (batts don't really cut it with TJIs- the widths are wrong and they leak at the edges.)  A greener option would be to go with 3" of either of Icynene's water blown semi-open foams the 2lb MD-R-200 (1.3 perms @ 3"), or the 2.2 lb. MD-R-210  (a bit under 1 perm @ 3"). Either runs about R5/inch.

SRI 30 for Charcoal Gray
SRI 40 for Zinc Gray

http://www.berridge.com/colors/sri-values-for-berridge-cool-roof-colors/

I was planning to do the ripping of the 4x8 polyiso sheets myself. Shouldn't be a big deal. With the vented opening, I should be able to use whatever panel color I want yes? It's vented...!?

If I blow directly to the sheathing, future sheathing/roof work will be a nightmare. This is why I think the vent is probably best at this point. Its about 90 sheets. Will take a little time but I'll rip em all in a couple hours. Install won't take too long either. It's all shed roof so it's all nice straight cuts.

Copper Cote looks fun but not for long term. Wife won't like it but I will show her. The renderings look bad.

Yes, if vented the color matters a lot less. With an SRI of <40 I wouldn't sweat it.

But venting above the roof deck by mounting the roofing on purlins is less risky, and make it easy to get a reliable air seal.

In a zone 5 climate 3" of R6.5/inch polyiso above the roof deck lets you skip the interior vapor retarder, and fill the rafter bays with cellulose. It's more foam, but better thermal performance (thermally breaking the rafter elements), and less labor.

But thats labor i'd have to pay for...and every dollar counts. When you ask subs to do something they are not familiar with, prices go high real quick.

Posted By Dana1 on 25 Nov 2013 11:09 AM

What's the solar reflective index (SRI) of the finish?  (It should be in the specs somewhere.)  If it has an SRI over 50 it's potentially an issue, over 70 DEFINITELY an issue.

Is the SRI the same as a "Light Reflective Value" (LRV)?

There is:
SR = Solar Reflectance Value
SRI = Solar Reflectance Index
E = Thermal Emittance Value

I would guesstimate that the LRV is the same as the SR or Solar Reflectance Value, not the SRI.

Posted By Dana1 on 27 Nov 2013 01:51 PM
Yes, if vented the color matters a lot less. With an SRI of <40 I wouldn't sweat it.

If unvented, would a SRI of 34 pose a problem? If so, how?