Opinions on flash and batt method plus vapor barrier

[OptionalStop]

I'm planning on insulating the walls of my new pole barn using the flash and batt method. Location is upstate NY with cold winters. I'll be using 1" reclaimed polyiso foam on the exterior between the girts and 6" fiberglass over that, followed by plastic vapor barrier and my OSB walls. No matter how much research I do I can't seem to find a solid answer if I will be trapping moisture in the walls using plastic on the inside, and foam on the outside. The polyiso does have tar paper on both sides (I believe it came off a roof). I want to create my air barrier with the plastic on the inside. If I need to leave it off, will the polyiso foam sheets act as the air barrier and function as intended?

Is it true that only 1" of foam is not thick enough to create an air seal, so my plan will work to install plastic vapor barrier on the inside walls?

 

[NUTTSGT]

I believe you might find your answer in whether you're using open cell or closed cell spray. The closed cell should act as a vapor barrier.

Have you talked to any of the local spray foamers and sought their advice ?

 

[TTTTTT]

I went through this exact scenario a few years ago. 24x44 loft space, Gambrel roof design, 11' ceilings. I had 2" of closed cell spray foam put in and then the rest with fiber glass. I searched for awhile the question of adding 6 mil vapour barrier also. Couldn't get a definitive answer even from my insulation guy. I ended up putting it in, then my finished ceilings and walls. I figured with 2" of foam (about R12) and sealing everything. I have a total in some places probably R 40- R70. The 1000 sq ' heat amazingly with a wood stove and 18000 BTU mini split. No regrets. The only problem is until there is a serious problem with moisture you won't really know. The one good thing with the spray foam no venting required.
Good luck.

Sent from my ASUS_Z01HD using Tapatalk

 

[Jlbc212]

Don't confuse a vapor barrier with an air barrier. A vapor barrier prevents moisture in the form of vapor from passage through. In a cold climate a vapor barrier should be applied (attached) to the warm inside side of the insulation. This will prevent moisture inside the warm structure from migrating into the insulation where it will condense (change back into liquid) when it reaches the point in the insulation or exterior wall surface where the temperature is at or below the dew point.

An air barrier inhibits the passage of air, but not the passage of water vapor. An air barrier is usually applied between the surface of the exterior sheathing material and the finish siding material. A simple explanation for the use of an air barrier is an air barrier is used to prevent drafts, the movement of air from outside into the exterior wall.

Ideally no water vapor from inside the structure should enter into the exterior wall cavity. But some vapor may sneak through, such as around or through an electrical box mounted in the exterior wall. An air barrier (or anything else - like a closed cell foam or aluminum foil) should not hinder that moisture from eventually passing through to the outside air.

 

[TurnipTruck]

I was told that 3” of closed cell is essentially a vapor barrier. If your salvaged foam is closed cell AND about 3” thick, I would not add a polyethylene barrier, ESPECIALLY if the two barriers were separated by any distance, such as your 6” Fiberglas. If the two barriers were immediately adjacent, then maybe.

 

[theoldwizard1]

I'm planning on insulating the walls of my new pole barn using the flash and batt method. Location is upstate NY with cold winters. I'll be using 1" reclaimed polyiso foam on the exterior between the girts and 6" fiberglass over that ...

That is NOT WHAT THE INDUSTRY CALLS "FLASH AND BATT" !

The "flash" is closed cell sprayed on foam. Big difference ! The spray on foam seals all crack/crevice where air/moisture can enter. The spray foam can net into areas that are next to impossible to place a vapor barrier and insulation.

I am not knocking reclaimed poly-iso. It is just NOT the same thing and will not do as good a job as 2"-3" or spray foam and then fiberglass.

 

[ForceFed70]

Vapor barrier is ment to prevent condensation from occurring inside your walls. Warm moisture heavy air cools in the wall and condensation occurs. The vapor barrier will stop that from occurring.

Long story short - Vapor barrier stops the moisture from entering the wall in the 1st place rather than "trapping it inside".

 

[speed bump]

Green building corp has a pretty good guide explaining building sheathed walls.

https://buildingscience.com/file/3709

The short answer is it is preferable to have an interior vapor barrier and dry to the outside in primarily heated climate. In a mixed climate (50/50 heating/cooling) you want to be able to dry the wall either way, and in a primarily cooled climate you want your moisture to dry into the interior. Now assuming you want to use something like closed cell foam you run into a problem that in a primarily heated climate you want to drive moisture to the exterior and you are supposed to put a vapor barrier on the inside. In that case you need to have enough foam to mange where your condensation point in your wall is and keep it in the foam. If I remember correctly a 80° f delta in the desert climate of Casper Wyoming meant I needed 2" of rigid foam.

Since you live in a fairly humid climate I would be more paranoid about condensation in your walls especially if you are planning on heating and cooling it full time.

 

[joe--h]

In my morning mail from JLC

https://www.jlconline.com/how-to/in...)&he=69bb4d43cef34b0a2b6c1ef4fbb478df0a10f121

Flash-and-Batt Insulation: Good Idea or Bad Practice
By Martin Holladay
Login or Register to download the PDF version of this article. (54.98 kB)

Q. Is the flash-and-batt technique — where an inch or two of spray-foam insulation is applied to the interior side of roof or wall sheathing before fitting rafter and stud bays with fiberglass batts — a good way to insulate?

A.Martin Holladay, editor of Energy Design Update, responds: Even though thicker is always better with any type of insulation, applying a thin layer of spray foam is a good way to get air-sealing benefits at considerable cost savings over full-thickness spray foam.

Some spray-foam contractors dismissively call the technique "flash-and-dash"; they point out that fiberglass batts may fail to remain in contact with the spray foam, creating an air space and the potential for convective air currents through the insulation. But I think this is a relatively insignificant problem, particularly if the cavity is fairly airtight. Besides, it's easy to minimize the chance of a potential air space by simply choosing a thicker batt. In fact, batts that are compressed slightly as they are installed will yield higher R-values than ones that just fill the cavity.

Another concern is that in a heating climate, the flash-and-batt method creates a vapor retarder on the wrong side (the cold-in-winter side) of the fiberglass batt. But whether the spray foam actually becomes a vapor retarder depends on the type of foam used. Open-cell foams — that is, foams with a density of about 1/2 pound per cubic foot — are very vapor-permeable. However, since many low-density-foam manufacturers,including Icynene, recommend against the flash-and-batt method, most proponents use closed-cell foam with a density of about 2 pounds per cubic foot.

One inch of closed-cell foam has a permeance of about 2 perms, while 2 inches has a permeance of about 1.2 perms, so closed-cell foams are effective vapor retarders.

But does installing a vapor retarder on the cold-in-winter side of a wall create a problem? Actually, research has shown that exterior foam can safely be used as part of a cold-climate wall or roof — as long as the foam is thick enough. As a rule of thumb, walls with exterior foam sheathing or flash-and-batt closed-cell foam will avoid condensation problems as long as the foam is at least 1 inch thick in climate zone 5 (Pennsylvania, Iowa, Nevada) or 2 inches thick in climate zone 7 (northern Minnesota).

Since exterior foam reduces a wall's ability to dry to the exterior, it's important to choose an interior vapor retarder that allows drying to the interior — such as kraft-paper facing or vapor-retarder paint — instead of sheet poly.

 

[DC73]

I'm planning on insulating the walls of my new pole barn using the flash and batt method. Location is upstate NY with cold winters. I'll be using 1" reclaimed polyiso foam on the exterior between the girts and 6" fiberglass over that, followed by plastic vapor barrier and my OSB walls. No matter how much research I do I can't seem to find a solid answer if I will be trapping moisture in the walls using plastic on the inside, and foam on the outside.

As others have mentioned, your approach is not flash and batt. Flash refers to spray foam. The reason for the spray foam is to air seal in addition to providing some good r-value.

Your approach may work but I'd pose your question on the Q&A forum at GreenBuildingAdvisor.com. There are building science gurus who hang out there who can best advise you.

The use and location of vapor barriers are climate dependent. Most builders are getting away from a plastic vapor barrier and are instead installing a smart membrane. The smart membrane allows the walls to breathe yet still prevents excess moisture migration.

DC

 

[benjamintmiller]

Your approach is actually called "cut and cobble".

 

[maxpat82]

IMO unles you spray foam all the joint of the reclaimed polyiso, you don't have a effective vapor barriere..and not a good insulation either.