# wall vapor barrier question



## Anthill (Mar 23, 2013)

I've heard of guys sealing up a house, having moisture problems, then cutting slits in the VB in the attic to solve the problem.

The reason we live in sealed houses is so we can control the relative humidity and temperature inside. If anyone thinks little or no vapour barrier is better, just leave a window open year round. Turn up the heating/cooling to compensate and then you'll essentially have the same as a leaky vapour barrier. If a house is sealed, then there must be mechanical ventilation to control the humidity. Lots of people see that as a waste of heat and shut off the fans and plug the intakes, etc. Hence, the moisture problems. However, with a mechanical system, it is controlled. In bad vapour barrier system it is not.


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## ScipioAfricanus (Sep 13, 2008)

Golden view said:


> I've had the pleasure of adding on to a home I built in Fairbanks Alaska (-50 degree winters), so I could open up walls and see that there were no moisture issues in a wall assembly I always worried would hold moisture.
> 
> From inside to out:
> GWB
> ...



Don't worry too much about that, EPS is not a vapor barrier.

It "breaths".

Andy.


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## Golden view (Feb 16, 2012)

ScipioAfricanus said:


> Don't worry too much about that, EPS is not a vapor barrier.
> 
> It "breaths".
> 
> Andy.


Oops. I meant XPS. It too has a much higher perm rating than 6-mil poly, so the imbalance should make the wall breathe, and my wall assembly showed that it was working. 

However, there were many cases in Fairbanks where older homes were having problems due to retrofitting with rigid insulation, I assume because their vapor barrier detailing was crap.


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## Philament (Dec 9, 2014)

muskoka guy said:


> Here in Ontario, its code that we use 6 mil vapour barrier. It must be continuous through all plates and walls where they meet the cold.


I think you're confusing air barriers and vapour barriers. The air barrier must be continuous, vapour barrier has no such stipulations (9.25.3 and 9.25.4 http://www.e-laws.gov.on.ca/html/regs/english/elaws_regs_120332_e.htm)
6 mil poly is one way to meet the requirements for a vapour barrier, but it’s certainly not the only one. Foam insulation at sufficient thickness, paint, tested sheet goods, tinfoil... all vapour barriers according to 9.25.4.2

The use of 6 mil poly gets confusing because it's classified as both an air and vapour barrier and it can and often is used for both. Like ceilings below attics, it's both the air and vapour barrier, so it needs to be sealed, but in walls with an exterior air barrier, there’s no need to seal it. 
Things are getting a lot trickier now that we’re starting to put outboard low permanence foam board though as Golden View points out, but it’s not because of poorly detailed vapour barrier, more likely poorly detailed air barrier in combination with equal permeance on the interior and exterior. XPS or foil-faced polyiso and 6 mil poly don’t play nice together, walls have nowhere to dry.

As for how to seal pipes and wires in the wall...don't bother, just put a good air barrier on the outside and paint the walls inside with a low perm paint , caulk any place they penetrate the air barrier. 
http://www.cmhc.ca/odpub/pdf/66564.pdf


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## bbgcarpentry (Apr 11, 2009)

Yes even when you read the building science web sites thjngs are very unclear with regards to vapour barrier.

When people in there ac at super cold levels in the summer all of a sudden the house is cooler on the inside than the ouside,reversing the vapour barrier mmmmm


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## FrankSmith (Feb 21, 2013)

Not the focus of this particular conversation but one thing I always factor in is that I think gluing drywall is a good thing.


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## Anthill (Mar 23, 2013)

Philament said:


> I think you're confusing air barriers and vapour barriers. The air barrier must be continuous, vapour barrier has no such stipulations (9.25.3 and 9.25.4 http://www.e-laws.gov.on.ca/html/regs/english/elaws_regs_120332_e.htm)
> 6 mil poly is one way to meet the requirements for a vapour barrier, but it’s certainly not the only one. Foam insulation at sufficient thickness, paint, tested sheet goods, tinfoil... all vapour barriers according to 9.25.4.2
> 
> The use of 6 mil poly gets confusing because it's classified as both an air and vapour barrier and it can and often is used for both. Like ceilings below attics, it's both the air and vapour barrier, so it needs to be sealed, but in walls with an exterior air barrier, there’s no need to seal it.
> ...


I'm not sure I follow you totally. So if the exterior is sealed and the interior is not, what happens when the air inside is high temperature, high humidity and the exterior is low temperature low humidity? The moisture can then penetrate the wall until in reaches dew point. the temperature difference will find a point of equilibrium somewhere in the middle of the wall. Will this not result in condensation in the insulation? prime spot for mold? It makes more sense to me to stop the moisture before it condenses.


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## SLSTech (Sep 13, 2008)

Anthill said:


> ...The moisture can then penetrate the wall until in reaches dew point. the temperature difference will find a point of equilibrium somewhere in the middle of the wall. *Will this not result in condensation in the insulation?* prime spot for mold? It makes more sense to me to stop the moisture before it condenses.


No, moisture needs a condensing surface which will be either the drywall or the sheathing depending on conditions, not the insulation

Plastic VB is meant (for very cold climates only where AC is not used) to help stop diffusion & if done well enough, some air leakage (but that is secondary to its main purpose). Your primary air barrier will always be the outside sheathing followed up by the interior drywall or similar membrane (exclusion is attics - drywall is primary air barrier there).

If you are really worried about moisture in walls you make sure the bulk water issues are taken care of, air-leakage is minimized, and control the moisture in the house (25% humidity level during winter) - kitchen & bath vents being used, not having a tree farm inside, etc...


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## Chris Johnson (Apr 19, 2007)

I believe you want to maintain humidity levels inside the structure between 40-48% to prevent damage to finishes like wood floors, cabinets, furniture, etc


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## bbgcarpentry (Apr 11, 2009)

Interesting stuff


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