Fiberglass insulation and the air barrier
Hi Martin, I thought it would be best to move this discussion from the unrelated post. My apologies to Jason. Just so others know what started the discussion, I cut and pasted the Minnesota code requirement below.
CHAPTER 11 [RE]
ENERGY EFFICIENCY
Provisions for energy, energy conservation, or references to the
International Energy Conservation Code are deleted and replaced with
Minnesota Rules, Chapters, 1322 and 1323, Minnesota Energy Code.
R402.1.1.7Fiberglass batt interior insulation.
Fiberglass batt insulation shall comply with the following:
1. 1.The above-grade exposed foundation wall height shall not exceed 1.5 feet (457 mm).
2. 2.The top and bottom plates shall be air sealed to the foundation wall surface and the basement floor.
3. 3.A vapor retarder and air barrier shall be applied to the warm in winter side of the wall with a permeance not greater than 1.0 in accordance with ASTM E 96 procedure A and a permeance not less than 0.3 in accordance with ASTM E 96 procedure B meeting the following requirements:
1. a.the vapor and air barrier shall be sealed to the framing with construction adhesive or equivalent at the top and bottom plates and where the adjacent wall is insulated;
2. b.the vapor and air barrier shall be sealed around utility boxes and other penetrations; and
3. c.all seams in the vapor and air barrier shall be overlapped at least 6 inches (152 mm) and sealed with compatible sealing tape or equivalent.
After re-reading the requirements of section 3, I’m under the impression that if your going to use the A-test, a perm of .99 passes, but 1.01 does not. If your going to use the B-test, 0.29 passes and 0.31 fails. Both tests fall under the class 2 vapor retarder. (I’m unsure which ASTM test is used for classifying the building materials as 1,2,or 3). As you indicated, poly is .06 to .08 perms, less than .3, so it can be used. Spray foam insulation, vapor retarder paint over drywall and craft faced insulation will also work. Smart vapor barriers would work if they stayed below 1 perm on the B-test, but during periods of higher cavity moisture, the perm is above 1. According to the code, can’t be used. The only good thing about the wording being so confusing, most inspectors don’t have a good handle on the perm requirement either.
I just had a discussion with my local building inspector. He indicated that poly does meet the requirements, and is the most common air barrier he inspects. He did indicate he would be open to vapor retarder paint and air sealed drywall as long as the manufacturer has independent testing that confirms that the perm rating meets the code requirements and the home passes the blower door test. He has not had anyone ask. The conversation led me to believe that he feels the poly is the only way to achieve a blower door test of less than 3. That is also the mentality of most of the building contractors in the area. I’ll be talking to another area inspector soon. As a side note, I had a discussion with Peter Yost at the IBS on this same topic, he gave me the name of a contact that may help answer my questions, but all I have to go on is Peter K from Minnesota. Any help with the contact would be appreciated.
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Replies
Randy,
You are probably accurately describing the intent of the Minnesota code writers. But you are not accurately describing the code as written. As I wrote in the other thread on this topic, this is an example of bad code writing.
The code states that the required vapor retarder must pass certain tests. The characteristics of the required vapor barrier are described this way: "with a permeance not greater than 1.0 in accordance with ASTM E 96 procedure A and a permeance not less than 0.3 in accordance with ASTM E 96 procedure B."
Note the use of the word "and," which I have rendered in bold font. The code writers could have used "or," but they chose "and." As any reader of Supreme Court decisions realizes, conjunctions matter.
The required vapor retarder must be "not less than 0.3 in accordance with ASTM E 96 procedure B." What does that mean? It means that a vapor retarder rated at 0.29 is unacceptable. A vapor retarder rated at 0.31 would be acceptable.
For those who missed the first half of the discussion (it appeared here: "How to insulate and finish basement when there are signs of efflorescence?"), I'll cut and paste one of my comments addressing code writers:
"I will take this opportunity to indulge in a rant.
"In the past, I have often criticized the opaqueness of most building codes. They aren't written in clear English. This is a problem, because the intended readers -- builders and architects -- can't implement the codes if the meaning is unclear. Technical writing is not rocket science; editors exist, and a code authority could, in theory, hire an editor experienced at technical writing to help with code clarity. But code authorities don't do that.
"Here is a prime example of bad code writing. It is written in such a way -- mixing up maximum permeance requirements per one ASTM test with minimum permeance requirements per another ASTM test -- that educated readers of Green Building Advisor are left scratching our heads. Imagine the average builder confronted by these requirements. How many will understand?"
Jon,
You're right that it's a perfectly reasonable code, assuming that the code writers know what they were doing. If I were a cynic, I might imagine that the code language was suggested by a lobbyist working for a smart retarder manufacturer. But we all know that never happens, right? Lobbyists never suggest wording to legislators, right?
Here's the thing: Did the code authorities really mean to make polyethylene illegal? If so, did anyone tell the local code officials that interior polyethylene no longer meets the code?
6 mil polyethylene comes in at about 0.06 perms using either method A or method, B, and would thus be DISALLOWED under a literal reading of part 3 parsed in US-'merican English, since it is less than 0.3 perms. It's too vapor tight in a wet-cup test.
"3. 3.A vapor retarder and air barrier shall be applied to the warm in winter side of the wall with a permeance not greater than 1.0 in accordance with ASTM E 96 procedure A and a permeance not less than 0.3 in accordance with ASTM E 96 procedure B''
In MY dialect of English it reads that the vapor permeance has to be 0.3 perms or above in a wet cup test, but still no more than 1 perm in a wet cup test. Changing the conjunction to "or" does't really fix it, since materials north of 50 perms could then qualify under ASTM E 96 procedure B. It has to be both, ergo "and".
MemBrain clearly fails, but so does 4 or 6 mil polyethyene. Intello may squeak into that window with cherry-picked but still reasonable limitations on RH for the test conditions, but I don't know that to be the case with certainty. Intello runs ~1.5 or more perms @ 70% RH, but only ~1 perm @ 60% RH, and is probably still above 0.3 perms @ 30% RH:
https://foursevenfive.com/wp-content/uploads/2015/07/IntellovsOSBvsPlywood2.jpg
https://foursevenfive.com/wp-content/uploads/2015/07/INTELLO_membrain_Majpell.jpg
The notion that it takes polyethylene sheeting in the wall stackups to duck under 3ACH/50 is just silly, of course.
But even the smart vapor retarders fail that test unless the test conditions are restricted to a window. ASTM E96 tests are done at a couple of relative humidity (RH) points to determine the slope, but as far as I know there isn't a hard-specified RH requirement for labeling purposes, only that the test reports must indicate the RH levels used in testing. A 30-60% RH range probably works for Intello, but it absolutely does NOT for MemBrain.
Must say that I'm surprised that such a simple and clear sentence can be so misinterpreted. Perhaps the only solution is for code to include more examples.
> In MY dialect of English it reads that the vapor permeance has to be 0.3 perms or above in a wet cup test, but still no more than 1 perm in a wet cup test.
That's not at all what it says. Membrain ( 10 wet) is allowed.
I suggest that what is says "= .3 wet" is a reasonable requirement that allows some drying and limited wetting. It allows fixed permeability membranes (eg .5 perms) and smart membranes (eg Membrain). So what would be better wording for such an intent?
Jon,
I agree with your interpretation. The code excludes the use of polyethylene or OSB as interior vapor retarders.
I just sent an e-mail to Mr. Don Sivigny, construction code representative for the state of Minnesota, and invited him to join the discussion. Hopefully we will get a response.