Posted By acobb on 17 Dec 2014 10:05 AM


A peel & stick membrane acts as a vapor barrier as it has zero permeability. Therefore, if we follow your specification and the codes requirement of a vapor barrier (SIP tape) on the inside in all climate zones north of Florida, we have the worst of all possibilities. Zero capacity to dry! You have designed an assembly with a vapor barrier on both sides!

This discussion is equally applicable to all types of SIPs and all types of assemblies. I stand by my statement that it's a really bad idea unless you're building in a climate zone that does not require a heating system. In that case, your vapor barrier goes on the outside of the structure and your assembly can dry to the inside.

Al Cobb
SIPschool.org
training and consulting for the SIP industry

Al,


At 4" - 6" closed sell (polyurethane) or even open cell (polystyrene) core on a SIP has basically ZERO permeability. The latter EPS has a little more permeability but the polyurethane at that thickness is basically almost ZERO in terms of permeability. Therefore, a SIP design like that CANNOT dry to the interior even if it wanted to. That is why I am confused by your statements.

You stated, " Zero capacity to dry! You have designed an assembly with a vapor barrier on both sides!"

Unfortunately, that is the SIP described in my paragraph. Such a SIP has ZERO ability to dry because the 6"+ of closed cell PU foam or 8-10" of open cell PS foam is basically a vapor barrier. The vapor barrier is inherent in the SIP design itself due to the vapor impermeable core sandwich. The only way around it is to use a core with 4" -  6" of EPS and even so, the vapor drive through that will be so miniscule that it would take forever for something to dry out and rot would occur if the OSB got wet.

The ONLY solution that building scientists will agree on (if that is even possible since they argue between themselves about this issue quite often) is to create another OSB layer above the SIP and leave a 1/2" air space between the SIP and the extra layer of OSB. The problem with this is the very expensive costs $$$ it adds to a SIP roof. Nobody would want to spend that type of money on a SIP roof and then have to build another roof above the SIP roof.

SIPA doesn't push that because they would go out of business as would any other SIP industry related company. What kind of sales push would that be if you told the buyer they have to build 2 roofs $$$ if they use a SIP for their roof. Good luck selling that.

So where we disagree is that you claim SIPs are vapor permeable even with a 4" - 6" PU core or 8"-10" + of PS cores. I believe you are 100% wrong. Show me the studies that vapor drive will occur with 4" - 6" of closed cell polyurethane and/or 8-10" of open cell polystyrene.

CLOSED-CELL POLYURETHANE CORE:

Water Vapor Transmission ASTM E96 0.7 perms @ 1.5"

PER BUILDING SCIENCE STUDY:

Closed-cell (about 2 pounds per cubic foot density or more) spray foam applied in thicknesses of over 2” (50 mm) will control vapor diffusion to safe levels in all
climates up to 10000 HDD and interior winter-time relative humidities of up to an over 50%RH. As thickness increases the level of diffusion control increases.

Therefore a 4 -  6" roof SIP with PU has a vapor impermeable core.


Now all we need is the stats on 8 - 10" of polystyrene cores and what the water vapor transmission perm rate is for 8 - 10" of EPS. Anything < 1.0 perms is considered a vapor barrier by IRC.

Lbear.
I thought we were talking about drying potential and applying a vapor barrier to the cold side of a roof assembly. Now you want to change the course of the conversation to permeability? OK

First, EPS is a closed cell foam.
Yes, you are correct that the thickness of any closed cell foam will result in a perm rating that cause it to act as a vapor barrier.

"Therefore, a SIP design like that CANNOT dry to the interior even if it wanted to. That is why I am confused by your statements." You seem to suggest that drying has to be through the entirety of the assembly. When in fact, the outer layer need only to dry towards an environment that is, itself, dryer than the skin. Hence, if the inside skin needs to dry to the inside it doesn't need to permeate the entire core. Only the skin itself. Same is true with the outside skin.

" The vapor barrier is inherent in the SIP design itself due to the vapor impermeable core sandwich."
With a vapor permeability of .1-1.0, it's classified as vapor semi-permeable. Yes this qualifies as a vb but the industry, the code,and anybody who's done a blower door test knows that the Achilles heal of the system is the joints. If the joints are not properly sealed, vapor is allowed to enter the assembly and potentially migrate to a cold skin on the opposite side. And, since we both know that vapor travelling on air leaks is a thousand times more likely to carry vapor than it is by means of permeability, I fail to see your position behind arguing about perm ratings.

" the vapor drive through that will be so miniscule that it would take forever for something to dry out and rot would occur if the OSB got wet."
So you want to further delay the time it would take to dry by adding P&S so that moisture damage occurs faster???

"The ONLY solution" - "is to create another OSB layer above the SIP and leave a 1/2" air space between the SIP and the extra layer of OSB."
Not so at all. There are many methods to enhance drying that are both effective and affordable. But, None of them work if we let you put P&S directly to the SIP!

"Good luck selling that."
Not only do I sell it, but I install it, I teach it, and I show my clients how to do it with an eye towards durability, increased thermal performance, and increased sound isolation. It's a win,win,win.

"So where we disagree is that you claim SIPs are vapor permeable even with a 4" - 6" PU core or 8"-10" + of PS cores. I believe you are 100% wrong. Show me the studies that vapor drive will occur with 4" - 6" of closed cell polyurethane and/or 8-10" of open cell polystyrene. "
I never made such a claim. However, any material above .1 still drys to some extent.

What if the drying needs only to pass through the panel skin? Consider, if you will, the drop of moisture lay directly inside of the panel skin. Your concerns over foam permeability are moot. In that scenario, you need only to look at the permeability of the SIP skin. AND, if that drop of water found its way to that skin via air-transport and condensed on a cold skin, then fire your SIP installer and know that the drying can occur through the cold skin (albeit though slowly) or back the way it came via the improperly sealed gap that let it in in the first place. Again, foam permeability plays virtually no part in drying in either direction.


Where we disagree is that I want a high performance assembly to DRY in any way possible. And you want to seal in the moisture with a method that violates the building code.

Al Cobb
SIPschool.org
training and consulting for the SIP industry

Posted By acobb on 17 Dec 2014 05:24 PM

Where we disagree is that I want a high performance assembly to DRY in any way possible. And you want to seal in the moisture with a method that violates the building code.

Al Cobb
SIPschool.org
training and consulting for the SIP industry

So what is your solution/build-up of a SIP roof with a metal roof on top? Please don't tell me "building paper" because metal roofs with building paper underneath is a match made in hell...

The only viable solution that is out there and recommended by BSC is adding a vented roof on top of the SIP. What is the going rate $$ of adding furring strips and another layer of OSB on top of a SIP and then adding the proper vent channel details?

The truth is that SIP companies do a song and dance when it comes to this issue. They know the cost of SIPs is a negative to builders and if they have to mandate a cold roof design, that is adding injury to insult, a hard sale to make to home builders.

You might also take a look at the ThermaShell building system. It exceeds the R-values for SIPs, goes up even quicker than SIPs and is very resistant to wind (up to 173 mph), termites, mold and mildew. And you will benefit with reduced electricity costs for heating and cooling of up to 80%! www.thermashell.com

Posted By scottucs on 23 May 2015 11:36 AM
You might also take a look at the ThermaShell building system. It exceeds the R-values for SIPs, goes up even quicker than SIPs and is very resistant to wind (up to 173 mph), termites, mold and mildew. And you will benefit with reduced electricity costs for heating and cooling of up to 80%! www.thermashell.com

scottucs,
the product from what I can see does not exceed SIP R- values
and will not be resistant to anything unless cladded with plywood, and at that point not resistant to termites?
the technical specifications on the website are misleading making claims that they have an Energy Star" 5 star rating? Energystar does not give 5star ratings to building materials , They do give ratings to "whole house" buildings , of which, the insulation plays a small part of the overall rating, as does the: windows, doors, HVAC system, finished roofing, Hot water system, lighting, appliances and blower door results.
"EnergyStar" is very touchy about logo misuse and false claims involving their name/ratings

Hey Jim,

It's about 15 months since your original post.  Please let us know what you decided to do, why, and how your project worked out (or is going).  We are in a very similar situation and are most interested.

Thanks!

Hi LMN. I chickened out on SIPs but used SuperiorWalls for my basement. I'm pleased with both decisions.

I was not exactly sure of some window placements in some areas of the home. SIPs have to be planned out and it's much harder to change window and door openings. Sticks allow for flexibility. I designed my own home and soe mistakes were not apparent until walls were erected. That would have been a nightmare with SIPs but no big deal with sticks.

I also could never nail down a good cost comparison between stick and SIP construction. I'm pretty sure that IF I had done stick construction with 6" walls and sprayed in insulation, the SIPs wold have been comparable.

One of the "problems" in my areas is that, here in North Alabama, our energy costs are far lower than elsewhere. So high-efficiency SIPs and spray insulation has a "payback" time measured in 15 to 20 years. Sorry, that's just not worth it for me but certainly may be for you.

So, my home takes advantage of passive solar (still thinking about solar panels), R30 blown in the attic and standard fiberglass in the walls--but sealed up tight.

Thanks for dropping back in with an update.  The sips vs stick comparison is a point that is hard to put your finger on, especially if your utility costs are low. 

I found this study a fellow did in 2015 that evaluates that question (for a degree requirement it seems).  He compared the sips vs stick in a couple of different locations and with several different residential designs - his conclusion is that sips cost more in almost every case due to the cost of the panels largely.  He does have a nice little table near the end of the document that gives the price difference for each element of a structure (materials + labor) that's interesting.  Seems to pretty thoroughly analyze the issues.

It's located here

http://scholar.colorado.edu/cgi/viewcontent.cgi?article=2109&context=honr_theses