Vapor Barrier

[Tiltrotor]

I am new to this forum. I just had a 36x53x11 pole building put up. The contractor said since I am installing and insulating my ceiling, there is no need to put a vapor barrier in. Is this True?

The building is up without it so, I am hoping he was correct.

 

[Jlbc212]

Water vapor is attracted to cooler surfaces and the vapor will condense into liquid when it reaches a surface temperature below the water vapor's dew point. This is what happens at the surface of an air conditioner evaporator coil. Unless you are located in a desert region you will definitely have water vapor in your garage. Most likely you will have temperatures below the water vapor dew point somewhere in or just beyond the cold side of the insulation. So the simple answer is "yes, you should have a vapor barrier installed on the warm side of the insulation." Have you placed already placed gypsum board or some other type of finish material over the insulation? You may be able to apply a paint that will restrict the passage of water vapor.

 

[dirttracker18]

Water vapor is attracted to cooler surfaces and the vapor will condense into liquid when it reaches a surface temperature below the water vapor's dew point. This is what happens at the surface of an air conditioner evaporator coil. Unless you are located in a desert region you will definitely have water vapor in your garage. Most likely you will have temperatures below the water vapor dew point somewhere in or just beyond the cold side of the insulation. So the simple answer is "yes, you should have a vapor barrier installed on the warm side of the insulation." Have you placed already placed gypsum board or some other type of finish material over the insulation? You may be able to apply a paint that will restrict the passage of water vapor.

Well yes but that is not it's primary purpose.

The VB is meant to stop air movement. Insulation only resists the exchange of temperature, from warm to cold depending on what you are trying to do (heat vs air condition or cool).

Air movement helps that difference in temperature move or lowers the effective R value of your insulation. If you do a heat loss calculation you will see that heat loss is more affected by the VB or lack there of than by the insualtion r value.

So yes you needed VB. What side of the wall it goes on depends on what part of the country you are in.

 

[8man]

I've never put vapor barriers on the ceiling, only the walls, and as said above which side of the insulation on the wall matters based on where you are in the country.

 

[kbs2244]

If the "attic" space above the ceiling is well vented then you do not need the VB.
We have talked about this a lot.
Research why they do not put a VB in a home ceiling while they do in the walls.

 

[MScott]

So yes you needed VB. What side of the wall it goes on depends on what part of the country you are in.

Why someone would submit a question like this which is so location specific without having their general location in their profile is beyond my comprehension.

 

[dirttracker18]

If the "attic" space above the ceiling is well vented then you do not need the VB.
We have talked about this a lot.
Research why they do not put a VB in a home ceiling while they do in the walls.

I have never seen a house built without VB in the ceiling.

However I am in a cold climate.

FWIW it is not only standard practice but code in this province.

As noted above in my previous post, it's primary function is not to stop moisture but air movement. A quick heat loss calculation for a building with and without in the ceiling will show you a distinct difference in the efficiency of that home.

VB is necessary on all exterior walls and ceilings in a conditioned building.

 

[Notgrownup]

I am in Eastern NC and i have no vapor barrier in my roof or ceilings, only in my crawlspace and whouse wrap. Good venting is key right?

 

[Jlbc212]

I am in Eastern NC and i have no vapor barrier in my roof or ceilings, only in my crawlspace and whouse wrap. Good venting is key right?

Yes, good venting is essential.

For those who commented that the vapor barrier is more intended to prevent air movement, there is a strong, valid reason why it is called a "vapor barrier."

 

[dirttracker18]

Yes, good venting is essential.

For those who commented that the vapor barrier is more intended to prevent air movement, there is a strong, valid reason why it is called a "vapor barrier."

Your misunderstanding based on the name is understandable.

However the tests and studies show it's ability to stop air movement is the primary reason to use it. If you stop the air movement you stop the transfer of moisture or vapor in the air.

In reality VB acts as an air barrier and seals the building envelope.

Once again, run the numbers to see the proof.

 

[dirttracker18]

Let's find some common ground to settle this debate.

No debate that warm air carries moisture.

When warm meets cold the water or vapor condenses and we have water or at least moisture if you will.

In order for warm moist air to meet cool air it must move through the insulation.

If there was a VB the air could not migrate from the warm inside to the cold attic (or via versa depending on where you are located, may be cool to warm attic)

Thus the prevention of air movement performs two functions.

1 the prevention of moisture migration

2 the prevention of air movement which depletes insulating value

 

[rburke65]

Then why is it not called Air Barrier?

 

[Jlbc212]

Let's find some common ground to settle this debate.

No debate that warm air carries moisture.

When warm meets cold the water or vapor condenses and we have water or at least moisture if you will.

In order for warm moist air to meet cool air it must move through the insulation.

If there was a VB the air could not migrate from the warm inside to the cold attic (or via versa depending on where you are located, may be cool to warm attic)

Thus the prevention of air movement performs two functions.

1 the prevention of moisture migration

2 the prevention of air movement which depletes insulating value

Good reply! Living in a cold climate I have seen water vapor condense in fiberglass insulation. It was soaking wet and rendered ineffective. I have also seen mold buildup on the interior side of the exterior wall sheathing between the fiberglass insulation and the sheathing. All because of the lack of a vapor barrier. You can have an effective barrier to prevent air movement that will be ineffective in preventing the movement of water vapor. If you have a Goretex garment, you should be able to understand the difference. Goretex is an effective barrier to air movement and the passage of water in liquid form, but it does allow the passage of water vapor. You can vigorously work or exercise in the rain, not get wet, and your sweat will move out through the microscopic pores of the Goretex.

 

[kenfain]

Vapor barrier goes under a poured slab. Pretty sure there's no air moving there!

 

[ADSR]

If there is no permit, don't use it. I'm a carpenter by trade and the only time i find rott or mold issues is because of the damn poly.

And poly always goes on the warm side BTW.

 

[JakeKohl]

I have never seen a house built without VB in the ceiling.

However I am in a cold climate.

FWIW it is not only standard practice but code in this province.

As noted above in my previous post, it's primary function is not to stop moisture but air movement. A quick heat loss calculation for a building with and without in the ceiling will show you a distinct difference in the efficiency of that home.

VB is necessary on all exterior walls and ceilings in a conditioned building.

Unless you are in a warmer humid climate - then you don't want a vapor barrier in the ceiling. You want the building moisture to be able to migrate to the attic and out of the building. Vapor barrier type and location depends greatly on the climate you are building in.

 

[dirttracker18]

Unless you are in a warmer humid climate - then you don't want a vapor barrier in the ceiling. You want the building moisture to be able to migrate to the attic and out of the building. Vapor barrier type and location depends greatly on the climate you are building in.

From the guy in SC? I am pretty familiar with the requirements for a cold climate, note my location.

It is code here (and all over Canada for that matter) and for good reason. If moisture is migrating up into the attic, so is your heat. No you do not want the moisture migrating into your attic.

Most buildings are not sealed well enough to be concerned with too much moisture in them. Those that are require an HRV, most new homes do.

 

[owenst7]

Unless you are in a warmer humid climate - then you don't want a vapor barrier in the ceiling. You want the building moisture to be able to migrate to the attic and out of the building. Vapor barrier type and location depends greatly on the climate you are building in.

I didn't realize the Anchorage, Alaska municipal building codes were written for a warm climate. It sure felt cold there to me.

 

[Angelfire]

An air barrier goes on the outside of the walls and is more often referred to as house wrap. Vapor barrier is a bit of a misnomer and is technically referred to as a vapor retarder. Depending on climate will determine what level of retarder you should use. Some locales only need the asphalt covered paper typically found on batt insulation. Other areas may need to go with visqueen. Many of us fall somewhere in between them.

 

[dirttracker18]

House wrap is definitely not an air barrier. Not even close. It is a one way moisture barrier. Air flow very freely through it.

In Canada, nothing less than 6 mil poly passes code for VB.

 

[Angelfire]

If you review the specs, you'd find many of these housewraps do indeed fit in the category of "air barrier". The housewrap is meant to repel moisture and air intrusion. Whereas a vapor retarder manages the vapor transmission from outside to inside and vice versa. The likes of Tyvek are most definitely moisture/air barriers providing they are installed correctly (which unfortunately they are not). So it still comes down to installing the correct retarder, not barrier, inside the home.

 

[Tiltrotor]

Like I said I am new to this. I am in Pittsburgh, PA. All around ****** weather. Four strong seasons. wet springs, moist summers, dry falls, cold winters.

 

[JakeKohl]

From the guy in SC? I am pretty familiar with the requirements for a cold climate, note my location.

It is code here (and all over Canada for that matter) and for good reason. If moisture is migrating up into the attic, so is your heat. No you do not want the moisture migrating into your attic.

Most buildings are not sealed well enough to be concerned with too much moisture in them. Those that are require an HRV, most new homes do.

My point was that where and how the moisture barrier is used changes depending on climate. I was saying that it is common practice (and meets code) to not put a moisture barrier in the ceiling/attic in southern climates. In many cases, in this (my) climate, it is advised to not put a moisture barrier in the ceiling structure. I wasn't telling you not to use it.

 

[Fishplate]

In the winter, it's cold outside and warm inside.

In the summer, it's warm outside and cold inside.

Which side do I put the air barrier on to prevent condensation in the insulation?

 

[WVBrady]

In the winter, it's cold outside and warm inside.

In the summer, it's warm outside and cold inside.

Which side do I put the air barrier on to prevent condensation in the insulation?

This is discussed in the link in post #27 in this thread:

http://www.garagejournal.com/forum/showthread.php?t=247027&showall=1