Edumacate Me: Vapor Barrier On Fiberglass Insulation

[turbocruiser]

Fellas, I'd like to get some good Mud wisdom/experience/expertise please ... I honestly don't understand the usefulness of some specific vapor barrier on some specific insulation I purchased pretty recently. The insulation is called "comfort-therm" (Manning) and it is wrapped with a perforated plastic vapor barrier on both sides of the product. I'm used to the old fashioned offering with kraft paper put on only one side and even with that was always wondering how that would work to prevent vapor from entering through the exterior of the house; I guess I always assumed that the old fashioned offering would allow whatever vapor would transfer from the outside into the wall but not through the wall. With the new stuff I simply don't see how it works; I can imagine some vapor entering through the exterior, condensing on the first plastic face, transmitting through the fiberglass, condensing on the second plastic face, etc. I guess given the fact that our average humidity is around approx 26% here I would rather whatever "vapor" simply transfer through the wall without condensing at all. I know I'm missing something but I suspect that this product is really intended to take the itch away from installing fiberglass and I really could care less about that; what I want is the best way to insulate the interior and a way which wont collect or condense moisture. I was going to simply tear the plastic part off of the core and install the core alone. Can you all edumacate me on where I'm wrong, if I am. Thanks.

 

[nate-gk]

I am a carpenter and have wondered the same thing before. Most of the insulation we use is blown in (they staple fabric to the studs and then cut holes and shoot it in no vapor barrier) they also use plain insulation, for things like vaulted ceilings just the yellow glass no paper no wrap. IMHO the stuff you get at home depot and the like is wrapped so you don't have to touch the insulation itself. I assume homeowner joe is not used to working with it nor does he want to touch it. If it came in a bag i would leave it in the bag or go buy some unbagged commercial insulation (most likely cheaper)

 

[photogod]

The stuff wrapped in plastic with holes in it is not really a vapor barrier. That stuff is meant to let moisture pass and help the user not itch so much from the glass. If you use that stuff but want a vapor seal you need to lay plastic sheet or something like Tyvec over the studs after the insulation is installed and tack it in place before dry wall is installed. Even though your humidity is low there can still be a moisture problem if the temp differential is great enough to form condensation on the inside of the wall. That leads to mold and that is a bad thing. The plastic barrier will help eliminate the moisture transfer. The vapor barrier always goes on the "warm" side of the structure, inside wall, not outside wall. That is the way we do it in MN. It is code here. Hope that helped.

 

[RavenTai]

The vapor barrier goes towards the hot side, when air cools the amount of water it can hold decreases,

Lets say you are in Miami and it is 95 degrees and 95% humidity outside and 72 degrees inside, the inside face of the insulation is 72 degrees, this sets up a convection current in the wall the insulation slows this flow but does not stop it, were the vapor barrier towards the inside the outside air has access to the insulation, as the air infiltrates to the inner portion of the insulation it would would cool and condense some of its water making the insulation wet making it less effective and allowing things like termites to eat the moist studs and mold growth eventually destroying the wall.

Were the vapor barrier towards the outside instead inside air can flow into the insulation and do the opposite, as inside air is warmed by the outside heat it gets dryer, evaporating any moisture in the wood and insulation and the air will take this water vapor with it as it exits the wall.

Lets say you live in Canada, its -20 outside, 72 and inside, people and animals breath, showers, cooking, etc put humidity in the air, were this relatively warm and humid inside air to flow into the insulation and cool it would drop its moisture into the insulation and walls, same problem as above, but in this case the vapor barrier should be towards the inside,

attics are less critical, attics are general hotter and drier than the indoor or outdoor air even in the winter, at least here in the south that is the case, that may not be true in the north.


Now you say your insulation has a vapor barrier on both sides? ok that works, but it is perforated? I do not know for certain but I'll bet that insulation was designed to be used with something like home wrap as an external vapor barrier

 

[turbocruiser]

Excellent answers fellas thanks for that, one thing that I noticed after reading this thread and working with the product (Manville BTW not Manning which is what I wrote originally) is that the plastic is perforated on the outside facing side and "solid" (aka non-perforated) on the inside facing side and the staple tabs for attaching to the studs are also "solid". Their website makes mention of a vapor retarder as opposed to vapor barrier and really emphasize the itch free aspect about it as much anything else. Late last night I spent some time reading through their website which is incredibly informative. In my case I don't think it matters much what I will do with this "encapsulation" because I'll basically install a real vapor barrier (reflectix reflective insulation) on top of the studs and on top of the inside sheathing anyway. Does that sound sensible? Thanks again for all the advice, I appreciate it.

 

[nate-gk]

on the vapor barrier side of things opinions vary. I build in Washington on the Olympic peninsula pretty wet. what we do is layer one is siding then homeslicker if in a storm prone site (ie pounding high wind and rain) then tyvek that is taped at the seams then 7/16 osb on the studs then we have the insulaters blow in the insulation and sheetrock.

i have used other methods and this is by no means the end all just cost effective and efficient.

you really dont want to trap water vapor in the wall cavity with the reflective insulation i would make sure that its a permeable membrane before putting it on both sides of the insulation.

raven also has some good points if this is a remodel i would go that route. you haven't told us what stage of construction you are in or what is currently in place. that will change recommendations greatly.

 

[John Galt]

In the North we take it a few steps further with new construction.
2x6 studs at 24"o/c are used, then a continuous air\vapor barrier of 6 mil polyethylene is applied on the inside of the wall studs and the roof trusses. All joints and seams in the air\vapor barrier are sealed with acoustical sealant or red sheathing tape. The importance of the air/vapor barrier is to keep interior humidity from getting into the insulation. Then the ceiling is strapped with 1x4 and the walls are strapped with 2x3 horizontally at 16" o/c. The whole building is sealed as one big room before the interior partitions are erected. Only then is the electrical wiring run so all electrical is inside the continuous air\vapor barrier. Both the 2x6 space and 2x3 space is filled with insulation. The exterior is clad with Tyvek type air barrier which lets any moisture escape which might get into the walls. Plywood sheathing is NOT used, diagonal wind bracing is used instead for strength. The wall exterior is strapped diagiagonally with 1x4 and then the siding is applied.

 

[haystax]

I too have been researching insulation methods and what it amounts to is opening a big can of worms -

The most important factor in spec'ing an insulation method is your particular climate and location. I am little worried about the use of a continuous poly vapor "barrier" on the inside of the framing. Although poly is actually a lower perm rated vapor retarder it still doesn't breathe that well. A Certainteed product called "membrain" is available for interior use that acts more like Tyvek and has a higher perm rating than straight poly. It is relatively expensive but if you are convinced that you want an interior VR then it may be the safest bet. I am worried about creating a moisture load problem where in my even drier climate there may never be one.

I choose to go with a kraft faced, high density R-21 for the 2x6" exterior walls and a kraft faced, high density R-38 for the cathedral ceilings. Exterior is OSB + Tyvek taped & lapped + finish siding(T&G hor. cedar) Drywall will probably get glued/sealed to framing and that may also help w/ air infiltration to a certain degree.

 

[haystax]

In the North we take it a few steps further with new construction.
2x6 studs at 24"o/c are used, then a continuous air\vapor barrier of 6 mil polyethylene is applied on the inside of the wall studs and the roof trusses. All joints and seams in the air\vapor barrier are sealed with acoustical sealant or red sheathing tape. The importance of the air/vapor barrier is to keep interior humidity from getting into the insulation. Then the ceiling is strapped with 1x4 and the walls are strapped with 2x3 horizontally at 16" o/c. The whole building is sealed as one big room before the interior partitions are erected. Only then is the electrical wiring run so all electrical is inside the continuous air\vapor barrier. Both the 2x6 space and 2x3 space is filled with insulation. The exterior is clad with Tyvek type air barrier which lets any moisture escape which might get into the walls. Plywood sheathing is NOT used, diagonal wind bracing is used instead for strength. The wall exterior is strapped diagiagonally with 1x4 and then the siding is applied.

So you are building a double wall system w/ the 2X3? The utilities are ran in this area between the partitions and exterior? Why no plywood or OSB for exterior shear? What type of siding do you then apply?

Interesting details, how far North and what are the design temps and conditions that warrant this method? Are you using a super insulated "hot" roof or which method for roof/attic ventilation?

 

[John Galt]

I am little worried about the use of a continuous poly vapor "barrier" on the inside of the framing. Although poly is actually a lower perm rated vapor retarder it still doesn't breathe that well. .

Poly is not supposed to 'breathe', that's why we use it on the interior. It's VERY important to keep interior moisture from getting into the insulation and condensing on the framing where it can cause rot.
A properly designed wall 'breathes' more as it gets to the exterior. That's why we don't use OSB or plywood on the outside because it traps moisture. Continuous vapor barrier on the inside airtested for continuity, and Tyvek type product on the outside to let any moisture 'breathe' out.

So you are building a double wall system w/ the 2X3? The utilities are ran in this area between the partitions and exterior? Why no plywood or OSB for exterior shear? What type of siding do you then apply?

Interesting details, how far North and what are the design temps and conditions that warrant this method? Are you using a super insulated "hot" roof or which method for roof/attic ventilation?

I suppose one could call it a 'double wall', the horizontal 2x3 is nailed to the 2x6 studs, sandwiching the 6mil poly air/vapor barrier. This places the air/vapor barrier where it does not get penetrated by electrical and mechanical. Any type of siding can be used over the diagonal 1x4s, horizontal, vertical, wood, vinyl, you name it. The diagonal 1x4s @ 12" o/c provide all the shear strength needed, and provide an air space behind the siding that kills wind penetration and lets any moisture escape and not get into the siding.

North of 60 with a -40 design temperature and 12475 heating degree days
Ceiling insulation is typically R60 in truss space. Trusses are designed with a 'high heel' that allows for 16" of insulation at the outer edge over the walls.

We've been using this building system in Canada since the early 1980s.
More information at Welcome to R-2000

We found that the kraft paper vapor barrier on insulation batts did not prevent moisture from getting into the walls, and then it condensed and froze on the inside of the OSB, then the frost kept building in the insulation till the insulation was a useless 'ice sponge' by February. When doing retrofits we found rotted OSB in many cases on North walls where the moisture never dried out. Houses should be built air-tight to protect the insulation from moisture. We typically use dedicated heat recovery ventilation systems that provide just the right amount of fresh air for the occupants while recovering most of the heat from the exhaust air. Cost savings payback with our building system is typically less than 6 years. The houses are so much more comfortable that people don't see the payback as the major advantage.