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Old 01-20-2018, 09:26 AM   #1
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Insulation and Moisture


I'm in the process of insulating a detached garage. Where I live the temps often drop to -30C in the winter and frost builds up inside the garage. I assume that the moisture from inside the garage is being pulled to the walls and freezing; not moisture from outside getting in.

My plan is to wait until temps are at least 0C before I add the insulation and vapour barrier. I'm just wanting to ensure I do not need any additional vapour barrier (or some other barrier) between the outside wall and the insulation.

I do plan on purchasing a 4000w infrared heater with thermostat that will keep the garage around 10C in the winter.

Thanks for all your help.
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Old 01-20-2018, 09:44 AM   #2
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Re: Insulation and Moisture


Quote:
Originally Posted by kjames05 View Post
I'm in the process of insulating a detached garage. Where I live the temps often drop to -30C in the winter and frost builds up inside the garage. I assume that the moisture from inside the garage is being pulled to the walls and freezing; not moisture from outside getting in.
My understanding is the same. I've heard people say that "frost is coming
in" but I don't think that's what's going on. Moisture within the building
will condense on the cold surfaces. With the VB on the inside of the wall
the relatively warm moist internal air can't move into the insulation where
the temp gradually drops to the dew point and condensation occurs.
If there's to be any housewrap on the outside it should be tyvek/typar
which will allow outward venting.
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Old 01-20-2018, 09:48 AM   #3
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Re: Insulation and Moisture


I don't think that a vapor barrier is needed. I would use those foam boards under the insulation.
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Old 01-20-2018, 10:09 AM   #4
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Re: Insulation and Moisture


Do your plans for the garage include a drywall covering on the inside. Where a vapor barrier is intended to stop moisture vapor it often will not do well at stopping air leakage and any air from inside the garage making its way into those walls will have the results you already see.

Minimize the moisture, snow and rain, you bring in, ventilate to the outside when possible to dry it out, and cover the insulation with a rigid air barrier.

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Old 01-20-2018, 11:45 AM   #5
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Re: Insulation and Moisture


A well done plastic vapour barrier is an air barrier - in fact most of the benefit comes from being an air barrier. Air carries moisture, leakage through the wall leads to condensation.

Without the barrier, adding batt insulation can make condensation problems much worse by making exterior surfaces even colder.

It's absolutely necessary with batt insulation, not only to protect the structure but to actually get the rated r-value.

Additional vapour barriers on the cold side are very harmful, trapping moisture; you need drying potential to the outside since no barrier is perfect.

The only acceptable insulation type which can stop moisture and air flow both ways is closed cell foam.
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Old 01-20-2018, 12:39 PM   #6
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Re: Insulation and Moisture


A well done plastic vapour barrier is an air barrier –
Granted, but that is not its primary purpose and only functions well when it is in full contact with a rigid surface. Not this application, but exposed plastic over fiberglass will pump in and out allowing air to circulate within the wall. We can benefit from its air blocking abilities but should not rely upon them in all cases.

in fact most of the benefit comes from being an air barrier. Air carries moisture, leakage through the wall leads to condensation.
True to a point, but only because they have learned that air sealing is a better way to reduce moisture movement. Within the scope of the larger picture many plastic vapor barriers suffer significant damage during construction from staples to other penetrations that never get properly repaired after the fact. Thus a barrier like the drywall or external sheathing can serve the air barrier function better but also must be well sealed.

Without the barrier, adding batt insulation can make condensation problems much worse by making exterior surfaces even colder.

That is why I have been suggesting the 1" of rigid insulation as it is sufficient to ensure the inside surface that basement air might reach will be above the dew point.

It's absolutely necessary with batt insulation, not only to protect the structure but to actually get the rated r-value.

The final layer of drywall should serve as the air barrier.

Additional vapour barriers on the cold side are very harmful, trapping moisture;

Correct
you need drying potential to the outside since no barrier is perfect.
And, in a basement there is very little drying potential to the outside with most of the surface being below the grade. Thus we omit the vapor barrier on the inside to get that desired drying potential.

The only acceptable insulation type which can stop moisture and air flow both ways is closed cell foam.

Extruded polystyrene is a closed cell rigid foam insulation that is rated as a class II vapor retarder with a perm rating between 0.1 and 1.0 thus it will still allow a small amount of moisture vapor from the foundation to dry to the inside
https://energy.gov/energysaver/vapor...sion-retarders

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Old 01-20-2018, 03:01 PM   #7
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Thanks for all the replies. I've heard other people say a vapour barrier is not necessary but I can't see that being the case as moisture will build up between the exterior wall and insulation. I assume this is what leads to mold forming.
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Old 01-20-2018, 03:11 PM   #8
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Re: Insulation and Moisture


Quote:
Originally Posted by kjames05 View Post
Thanks for all the replies. I've heard other people say a vapour barrier is not necessary but I can't see that being the case as moisture will build up between the exterior wall and insulation. I assume this is what leads to mold forming.
Will you be having a building inspection?
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Old 01-20-2018, 05:45 PM   #9
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Re: Insulation and Moisture


James, I don't understand your question: "I've heard other people say a vapour barrier is not necessary but I can't see that being the case as moisture will build up between the exterior wall and insulation."
The purpose of leaving out the vapor barrier is to allow the wall to dry to the inside as it cannot dry to the outside.

Explain where the water will be coming from.

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Old 01-20-2018, 05:53 PM   #10
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Re: Insulation and Moisture


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Will you be having a building inspection?
The garage was built by the builders (inspected by the city) and in my city I'm not required have a permit and inspection when installing insulation and drywall.

Quote:
Originally Posted by Bud9051 View Post
James, I don't understand your question: "I've heard other people say a vapour barrier is not necessary but I can't see that being the case as moisture will build up between the exterior wall and insulation."
The purpose of leaving out the vapor barrier is to allow the wall to dry to the inside as it cannot dry to the outside.

Explain where the water will be coming from.

Bud
This was more of a statement and not a question. I saw on a related forum someone said vapour barriers are unnecessary in detached garages. Given that it's a garage and I'll be tracking in snow and water from the vehicles, the moisture content inside the garage will almost always be higher than the outside air. Without a vapour barrier the moisture would seep into the walls and eventually cause mold and mildew to develop. At least, that's my thinking.
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Old 01-20-2018, 07:26 PM   #11
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Re: Insulation and Moisture


Quote:
Additional vapour barriers on the cold side are very harmful, trapping moisture;
Correct
you need drying potential to the outside since no barrier is perfect.
And, in a basement there is very little drying potential to the outside with most of the surface being below the grade. Thus we omit the vapor barrier on the inside to get that desired drying potential.
This is a garage so batts with vapour barrier will do fine.

Quote:
The final layer of drywall should serve as the air barrier.
It's not a good air barrier, especially at the edges. No way to seal.

A garage may not have drywall added.

A vapour barrier, tuck taped and sealed where it meets the studs makes for a good air barrier.

The impact of vapour diffusion is minimal, most moisture problems are caused by air movement - discovered after vapour barriers started being used.

Doesn't matter what they were originally for, today they're used to stop air movement.

Combined with continuous air, vapour permeable barrier on the exterior, vapour barrier and batts can work fine.

With foam, drywall must be put on top because of the extreme fire hazard foam poses.
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Old 01-21-2018, 07:18 AM   #12
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Re: Insulation and Moisture


OK it's Sunday so a good day for confession

My apology, I was wrong. Actually I was lost, my mind was in a different thread dealing with basement insulation, note I mentioned a basement. I went back and read from the beginning and saw my error. Being a detached garage you (and others) are correct that insulation covered with a vapor barrier to protect from the garage moisture is best.

There I feel better.
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