Help me understand vapor barriers..

[cfk]

I'm perplexed by vapor barriers (cold climate where we are heating all winter). My understanding is that they keep the moisture thats in the warm interior air inside, so when the warm temps meet the cold temps somewhere in the wall, there is no moisture in that air and it cannot condensate.

When we built our house, the insulators put plastic on all the walls under the drywall, which makes sense, but none in the ceiling. Why not?

Finishing the interior of my shop right now and was told to make sure I put plastic on the walls AND ceiling. Then someone told me I have to be careful to not get it TOO tight because they tried that with old houses and the keeping all that moisture in the house was not good?

A friend of mine is in the process of finishing his shop and has insulation in the walls, but no interior steel yet (and no vapor barrier), and when he runs a heater in there, he gets frost on the inside of the exterior steel.. I was told that will no longer happen once he has plastic up.

He is also in the process of "remodeling" a finished shop that was built close to 30 years ago - took the interior steel down and there is no plastic on the walls or the ceiling, batt insulation in the walls, blown-in fiberglass in the attic, and there is no sign of mold, mildew, rot, anything anywhere in that building?

 

[The Cobbler]

there's ongoing debates regarding vapour barrier in shops/garages . both yay and nay

 

[brownbagg]

because moisture will rise and rot the drywall with mold, at least that the story around here

 

[jsaw]

I have heard that without a vapor barrier, condensation and moisture will get into the insulation

 

[alwaysFlOoReD]

As I understand it, vapor barrier is to keep moisture inside where hvac can deal with it. On old houses there was never the attention to moisture control because heating was cheap. Very cheap. And the wood siding had lots of gaps behind the siding to channel away moisture that came from the interior. I resided the north and east walls of a house originally built in 1905, and that had additions built onto it, with custom wood siding. The newest additions which were probably built 50 or 60 years after had the worst rot, because there wasnt the airflow behind the siding; people had caulked the gaps in an attempt to prevent air movement, probably hoping to cut heating costs. And also didnt make the vertical corners so that air could move out from behind the siding. The siding was just **** up to 2x6 corners and caulked....
This house had sawdust insulation. My research found it to be better than fiberglass insulation in R-value, other than the fact it settles and leaves the top 2' of a two story house with no insulation at all! The walls are built diffent back then, the studs ran from base floor to ceiling in the two story house, and a ledger board is let in the stud to carry the floor joists. So usually there is nothing to stop the insulation from settling quite a lot.

 

[Ak Jim]

Not sure why they didn’t put the vapor barrier in the ceiling, they should have. At least his point your best option is to paint the ceiling with a paint designed to limit vapor travel into the attic.

Moisture Vapor Barrier - Sherwin-Williams

Designed to reduce the loss of moisture through walls and ceilings.. Combines a primer and finish in one single fast drying coat.. Is specially formulated for manufactured housing facilities..

www.sherwin-williams.com

 

[yeldogt]

VBs became more important w/ fiberglas batt insulation and drywall. Fiberglass batt has very poor air sealing ability and will absorb water. Bad combination. Warm air can hold more water than cold .... so when you heat a house it will hold the water humans make. Heat is power .... so that warm air wants to get out .... as the air leaks out it takes that moisture with it right into the batt insulation. Wet batt insulation is a poor insulator and will rot the wall

We had a house with blown in insulation -- the type pumped into wall cavities. That product was a common retrofit product starting in the 50's -- it was a great air sealing product and obviously no way to add a VB. If air will not flow -- the need for a VB drops. Also --- old style wall construction (plaster/lath) could manage water better than drywall -- those traditional building products can absorb water and dry w/o causing any physical water or mold problems. This includes typical exterior construction --- boards or plywood covered with wood claps or cedar. Drywall and OSB are poor water managers.

Originally the fix was paper w/ tar applied -- this helped the fiberglass in two ways. Stoped air movement and the tar slowed the movement of water vapor. When the energy crisis hit in the 70's ..... people went one step further. Why not stop the movement completely ..... use plastic. It took about 10 years before the problems of plastic surfaced --- lots of rot caused by trapped moisture. Houses have lots of holes and no matter how good you are with the plastic t will have lots of holes from both installing the drywall and at every outlet and switch location. Air would bring water into the wall cavity and condense ..... some so bad the bottom plate would be wet with water.

Today you will find that plastic is not recommended ....... it's a 1 on the VB scale and they say it "can" be used in AK. Look it up.

The best VB's are the membrane type used over unfaced or the special perforated plastic products you see on the encapsulated batt insulation. They slow water movement ....... Tyvex does the same for the exterior -- it provides a bulk water protection all the while allowing a building to dry. Tyvex will also limit air movement if installed properly (almost never is)

ZIp type exterior products present another interesting concern IMO. They stop air movement and the wax in the OSB/ exterior coating is a further hinderance to drying. I wonder what will happen in the future with duel VB's and batt insulation. I think trapped water. I would prefer the use of rock wool w/ ZIP.

The key with buildings is for them to dry ... both to the inside and outside. I like using closed foam ..... no air movement. Buildings will dry from both inside and outside.

Do a search on a couple that did a flash and batt type house in a cold climate and did not condition the interior when they finished it out -- you can see how much water can flow through drywall and fiberglas batt. Whole thing was done incorrectly ... mess

 

[cfk]

VBs became more important w/ fiberglas batt insulation and drywall. Fiberglass batt has very poor air sealing ability and will absorb water. Bad combination. Warm air can hold more water than cold .... so when you heat a house it will hold the water humans make. Heat is power .... so that warm air wants to get out .... as the air leaks out it takes that moisture with it right into the batt insulation. Wet batt insulation is a poor insulator and will rot the wall

We had a house with blown in insulation -- the type pumped into wall cavities. That product was a common retrofit product starting in the 50's -- it was a great air sealing product and obviously no way to add a VB. If air will not flow -- the need for a VB drops. Also --- old style wall construction (plaster/lath) could manage water better than drywall -- those traditional building products can absorb water and dry w/o causing any physical water or mold problems. This includes typical exterior construction --- boards or plywood covered with wood claps or cedar. Drywall and OSB are poor water managers.

Originally the fix was paper w/ tar applied -- this helped the fiberglass in two ways. Stoped air movement and the tar slowed the movement of water vapor. When the energy crisis hit in the 70's ..... people went one step further. Why not stop the movement completely ..... use plastic. It took about 10 years before the problems of plastic surfaced --- lots of rot caused by trapped moisture. Houses have lots of holes and no matter how good you are with the plastic t will have lots of holes from both installing the drywall and at every outlet and switch location. Air would bring water into the wall cavity and condense ..... some so bad the bottom plate would be wet with water.

Today you will find that plastic is not recommended ....... it's a 1 on the VB scale and they say it "can" be used in AK. Look it up.

The best VB's are the membrane type used over unfaced or the special perforated plastic products you see on the encapsulated batt insulation. They slow water movement ....... Tyvex does the same -- it provides a bulk water protection all the while allowing a building to dry. Tyvex will also limit air movement if installed properly (almost never is)

ZIp type exterior products present another interesting concern IMO. They stop air movement and the wax in the OSB/ exterior coating is a further hinderance to drying. I wonder what will happen in the future with duel VB's and batt insulation. I think trapped water. I would prefer the use of rock wool w/ ZIP.

The key with buildings is for them to dry ... both to the inside and outside. I like using closed foam ..... no air movement. Buildings will dry from both inside and outside.

Do a search on a couple that did a flash and batt type house in a cold climate and did not condition the interior when they finished it out -- you can see how much water can flow through drywall and fiberglas batt. Whole thing was done incorrectly ... mess

Good info!

I should have specified - the vapor barrier they used in my house was Certaineed Membrain, which I guess is different than regular plastic?

Couple follow up questions to my original questions.. So when finishing out my shop, should I not have plastic on the walls or ceiling? That seems like the "standard" nowadays (which doesnt necessarily mean its right).

My buddy's shop, that gets the frost on the inside of the steel - thats normal? And just fine?

And you mentioned closed cell foam - no air movement - isn't that essentially what plastic is is doing?

Sorry again for all the questions - just trying to make sense of all this!

 

[BombShelter]

You should be able to look up your local code for the best recommendations. I am not an expert but I've seen some basement mold factories due to plastic sheet used on the walls. A few years ago they were going crazy with plastic barriers everywhere but we found out that moisture leaks from the outside were creating mold growth, especially with fiberglass insulation between the plastic and wall. Now the feeling is a little air movement is good to keep the area dry.

When I put the ceiling in my garage the code didn't require a vapor barrier, I haven't had any moisture issues and I keep track of it. I did use mineral wool for insulation since I've worked on too many projects with moldy, stinky fiberglass. The added costs are far outweighed by the benefits.

The plastic comes in many different styles, I see more for below grade and the good stuff is 6 mil, or more, rip stop plastic (it has checkered thread reinforcement).

You might be able to find better information on-line, HUD usually has some great information on house building and insulation, they have books you can download in .pdf form.

 

[rayra]

I find the guy's mannerisms kind of annoying but he shares a lot of info on modern building science. Plug "build show matt risinger vapor barrier" into youtube and you'll find some good videos on the information.

 

[theoldwizard1]

I have heard that without a vapor barrier, condensation and moisture will get into the insulation

Yes. And some experts will tell you, you should have a vapor barrier on both sides of the insulation. The paper on fiberglass insulation is somewhat of a vapor barrier.

 

[67CarGuy]

Some good info has already been shared here, and here's some more:

• The kraft facing on batt insulation is a vapor retarder, not a barrier. Important difference. It's not meant to stop vapor movement, just slow it down.
• Many newer buildings have unfaced batt insulation, as they use a Zip wall or similar exterior vapor barrier only. This is fine, provided the building is designed and built correctly.
• The issue with a vapor barrier (impermeable, water / vapor can't move through it; likely 6mil or thicker) is that any moisture that gets into the wall (outlets, switches, holes in the exterior, poorly detailed windows, etc.) has a really hard time getting out. So the moisture sits in the wall, getting things wet and generally causing bad things to happen.

If you're planning to cool the interior of the space, don't put up plastic on the interior. You'll have problems in your walls soon enough if you do use plastic on the inside. While a smart membrane will work, I'd rather just not deal with it at all. Let any moisture that does get into the wall dry to the inside, where your A/C can condition it out of the air.

 

[billconner]

A lot of good stuff here: https://www.buildingscience.com/document-search
I don't personally agree with all BSC opinions but it's usually on matters where workmanship is an issue.

For moisture problems in buildings, I'd recommending starting with sources - crawl spaces, slabs without vapor barriers, firewood stored indoors, and so on.

Exterior infiltration - a rain storm - that can be managed by drying to the inside or outside. A heating season of moist air hitting a condensing surface - like inside of sheathing - may not dry out soon enough.

As far as plastic - 6 or 8 mil poly - very good installation is key, and I only rely on it if doing it myself.

Want to avoid these problems, at least in walls? build with block and exterior foam.

 

[whatsitcalled?]

When and where to use a vapor barrier vs a vapor permeable air barrier is very much determined by the climate you live in. In many cases they say put the VB on the warm side of the wall. In MN that means the inside, in GA that's the outside. You never want to use a vapor barrier on both sides as it will trap moisture in the wall cavity. Architects will use a model to determine what the building needs based on climate and how long it will take the wall to dry out if vapor does get inside.

The big problem with vapor barriers is you have to take the time to seal them completely. If you live in MN or IA and have to put a vapor barrier on the inside but leave an area out around a power outlet or something you now have a pathway for warm humid air (in the middle of winter) to get in the wall cavity and then condense somewhere in the wall cavity.

Older buildings like your buddies shop are not as much of a concern as they were leaky enough to keep ventilated and dry out. The high ventilation (air leaks) just hurts from an energy conservation standpoint.

 

[yeldogt]

If you look into the specifics ...... You always want to use a vapor retarder. Allow buildings to dry ... plastic is a total barrier and is really never recommended except in the coldest of climates and then it is only "can be used"

Plastic traps moisture .....

Zip -- be careful --- follow the instructions and use the correct products. If I ever did a zip I would be using rock-wool only.

Again -- buildings that dry will survive. Always remember ... no matter what --- buildings leak.

 

[JunkBonds]

The big problem with vapor barriers is you have to take the time to seal them completely. If you live in MN or IA and have to put a vapor barrier on the inside but leave an area out around a power outlet or something you now have a pathway for warm humid air (in the middle of winter) to get in the wall cavity and then condense somewhere in the wall cavity.

You use these to seal these areas:

https://www.homedepot.ca/product/iberville-vapor-barrier-single-gang/1000176244

 

[Renegade1LI]

This pretty much sums it up.

Do I Need a Vapor Barrier? | CertainTeed

www.certainteed.com

 

[billconner]

This pretty much sums it up.

Do I Need a Vapor Barrier? | CertainTeed

www.certainteed.com

They do like the products they sell.

 

[billconner]

Better BSC overview, though long.

BSD-106: Understanding Vapor Barriers | buildingscience.com

www.buildingscience.com

 

[The Bean]

Plaster is an excellent vapor barrier! Skim coat your drywall with plaster finish. Done and done!

 

[billconner]

I found the permeance rating for plaster to be in the 10 to 15 range. Do you have a source showing different?

 

[yeldogt]

They do like the products they sell.

Notice the name quickly changes to vapor retarder

 

[yeldogt]

Better BSC overview, though long.

BSD-106: Understanding Vapor Barriers | buildingscience.com

www.buildingscience.com

understand .... they use retarder ... and look at plastic.

You always want buildings to breath

 

[cannuck]

When and where to use a vapor barrier vs a vapor permeable air barrier is very much determined by the climate you live in. In many cases they say put the VB on the warm side of the wall. In MN that means the inside, in GA that's the outside. You never want to use a vapor barrier on both sides as it will trap moisture in the wall cavity. Architects will use a model to determine what the building needs based on climate and how long it will take the wall to dry out if vapor does get inside.

The big problem with vapor barriers is you have to take the time to seal them completely. If you live in MN or IA and have to put a vapor barrier on the inside but leave an area out around a power outlet or something you now have a pathway for warm humid air (in the middle of winter) to get in the wall cavity and then condense somewhere in the wall cavity.

Older buildings like your buddies shop are not as much of a concern as they were leaky enough to keep ventilated and dry out. The high ventilation (air leaks) just hurts from an energy conservation standpoint.

Yes, you hit on the main points for housing requiring heated space. In the Great White North we use plastic pans stapled to the studs to mount electric boxes. Wiring comes into the pans and is sealed where it enters. Vapour barriers are all taped at every seam and taped down to the generous flanges on the plastic penetration devices. The aim is a 100% hermetic seal on the inside. Then, the house is ventillated by an air and heat exchanging box that does as the name implies. Get it properly sealed and there is no problem with the relatively short air conditioning season we have.

From what I have seen: air leakage is as high as or higher priority than insulation quality in making and energy efficient home (or shop). Of course: closed cell urethane foam is a fantastic solution to sealing a wall or roof 100%.

to the OP: vapour barrier on roof/ceiling really important with glass or rock wool batt insulation...or ANY other air permeable insulation (including open cell foam I believe).

 

[The Bean]

I may be in error. Spoke too soon. I have old house 1927 with plaster and lath, no insulation in walls. No moisture problems. Dries well. Sorry for confusion.

 

[billconner]

understand .... they use retarder ... and look at plastic.

You always want buildings to breath

Basis for avoiding double vapor barrier. Stud walls, fibreglass, siding - I'd always put poly on interior in heating climates. It gets trickier when you add continuous insulation on outside. Sure, 2" of foam on exterior will probably keep sheathing above dew point, so no poly, dry to inside. But what happens when the permeable drywall and latex is covered with something not so permeable?

I liked my first to houses - built in 1884 and 1904 - 1x12 sheathing, 2x4 will to attic studs, no headers (1x12s were fine), and plaster on wood lath. They breathed just fine.

A lot more pitfalls today.

 

[yeldogt]

Basis for avoiding double vapor barrier. Stud walls, fibreglass, siding - I'd always put poly on interior in heating climates. It gets trickier when you add continuous insulation on outside. Sure, 2" of foam on exterior will probably keep sheathing above dew point, so no poly, dry to inside. But what happens when the permeable drywall and latex is covered with something not so permeable?

I liked my first to houses - built in 1884 and 1904 - 1x12 sheathing, 2x4 will to attic studs, no headers (1x12s were fine), and plaster on wood lath. They breathed just fine.

A lot more pitfalls today.

That's why plastic is not recommended -- in a cold climate the warm moist are will find it's way in. Lots of houses ended up with rot when the moisture was trapped.

Also -- paneling was popular back then and that could be a disaster. Any moisture would work through the bats and condense inside the wall at the sheathing.

 

[Sumboodie]

VBs became more important w/ fiberglas batt insulation and drywall. Fiberglass batt has very poor air sealing ability and will absorb water. Bad combination. Warm air can hold more water than cold .... so when you heat a house it will hold the water humans make. Heat is power .... so that warm air wants to get out .... as the air leaks out it takes that moisture with it right into the batt insulation. Wet batt insulation is a poor insulator and will rot the wall

We had a house with blown in insulation -- the type pumped into wall cavities. That product was a common retrofit product starting in the 50's -- it was a great air sealing product and obviously no way to add a VB. If air will not flow -- the need for a VB drops. Also --- old style wall construction (plaster/lath) could manage water better than drywall -- those traditional building products can absorb water and dry w/o causing any physical water or mold problems. This includes typical exterior construction --- boards or plywood covered with wood claps or cedar. Drywall and OSB are poor water managers.

Originally the fix was paper w/ tar applied -- this helped the fiberglass in two ways. Stoped air movement and the tar slowed the movement of water vapor. When the energy crisis hit in the 70's ..... people went one step further. Why not stop the movement completely ..... use plastic. It took about 10 years before the problems of plastic surfaced --- lots of rot caused by trapped moisture. Houses have lots of holes and no matter how good you are with the plastic t will have lots of holes from both installing the drywall and at every outlet and switch location. Air would bring water into the wall cavity and condense ..... some so bad the bottom plate would be wet with water.

Today you will find that plastic is not recommended ....... it's a 1 on the VB scale and they say it "can" be used in AK. Look it up.

The best VB's are the membrane type used over unfaced or the special perforated plastic products you see on the encapsulated batt insulation. They slow water movement ....... Tyvex does the same for the exterior -- it provides a bulk water protection all the while allowing a building to dry. Tyvex will also limit air movement if installed properly (almost never is)

ZIp type exterior products present another interesting concern IMO. They stop air movement and the wax in the OSB/ exterior coating is a further hinderance to drying. I wonder what will happen in the future with duel VB's and batt insulation. I think trapped water. I would prefer the use of rock wool w/ ZIP.

The key with buildings is for them to dry ... both to the inside and outside. I like using closed foam ..... no air movement. Buildings will dry from both inside and outside.

Do a search on a couple that did a flash and batt type house in a cold climate and did not condition the interior when they finished it out -- you can see how much water can flow through drywall and fiberglas batt. Whole thing was done incorrectly ... mess

6 mil vapor barrier is SOP here. I don't think a new house would pass an energy audit without it.

 

[yeldogt]

6 mil vapor barrier is SOP here. I don't think a new house would pass an energy audit without it.

If you look on the federal sites .... plastic in Alaska is the only place where it hits the may be used. I'm sure the membranes are approved.

 

[billconner]

6 mil vapor barrier is SOP here. I don't think a new house would pass an energy audit without it.

Even with continuous (foam) insulation on exterior?

 

[Dagny]

Make the building tight and let the HVAC man control the climate with heat recovery ventilator and or whole building dehumidifier.

 

[Sumboodie]

Even with continuous (foam) insulation on exterior?

That's required between the studs and outdoor sheathing as a thermal break.

At least as far as I know. I'm not in the building trades, so not exactly keeping up with the latest trends.

 

[TTTTTT]

As canuck said and a fellow canuck 6 mil vapour is standard code. Vapour barriers, 6 mil plastic is kinda a misnomer. It's not so much to stop vapour but vapour creation caused by air leakage. Warm interior walls and ceilings are sealed with 6 mil, tuck tape, caulking, spray foam etc. Insulation should always fit snug to the studs and joists to stop all air flow . Where ever cold air can find a leaked air flow to the warm interior, it will condense to moisture. Then the possibility of mold and damaged insulation. A breathable membrane like house wrap is applied to the outside sheathing . This is standard for colder climates. Air flow is always the culprit. Again COLDER climates.

 

[billconner]

That's required between the studs and outdoor sheathing as a thermal break.

At least as far as I know. I'm not in the building trades, so not exactly keeping up with the latest trends.

Emphasizing for heating climatez, the continuous insulation I've seen is either over the sheathing or a part of it like the Zip products. My sense is when you do this, skip the warm side vapor barrier.

 

[billconner]

As canuck said and a fellow canuck 6 mil vapour is standard code. Vapour barriers, 6 mil plastic is kinda a misnomer. It's not so much to stop vapour but vapour creation caused by air leakage. Warm interior walls and ceilings are sealed with 6 mil, tuck tape, caulking, spray foam etc. Insulation should always fit snug to the studs and joists to stop all air flow . Where ever cold air can find a leaked air flow to the warm interior, it will condense to moisture. Then the possibility of mold and damaged insulation. A breathable membrane like house wrap is applied to the outside sheathing . This is standard for colder climates. Air flow is always the culprit. Again COLDER climates.

I respect your view this is the right way and 10 or 20 years ago I would have agreed completely. But if installing continuos insulation on the exterior, which is a vapor barrier, I question the wisdom of an interior impermeable (1 or less) vapor barrier, creating a walk that can't dry in or out. The continous insulation has been shown to keep the interior surface of the sheathing above dew point and thus no condensation of warm interior air.

 

[TTTTTT]

Vapour barrier always goes on the warm/ interior. Extra insulation on the exterior is fine too. It's still pretty standard here. Of course with closed cell spray foam, no need for vapour barrier, it provides it. Open cell foam, still needs a 6 mil vapour barrier. Recently this also what the building inspector required.

 

[billconner]

Recently this also what the building inspector required.

That makes it good design?