Insulation Install. Do I need vapor barrier?

[SliderWoodWorker]

I am located in west Tennessee and I have a 12 ft X 24 ft. storage building with wooden exterior walls, a metal roof, and a 9ft. rollup style door. I am in the process of finishing it out to be my woodworking shop.

I have started insulating the 2 X 4 walls with "faced" R15 insulation, stapling the flaps to the face of the 2 X 4 studs. And I plan on installing 7/16" OSB as the interior walls. My intention is to be able to have an air conditioner for the summers and a heater for the winters.

My question is, do I need to install a "vapor barrier", given that I am using "faced" insulation and I live in Tennessee? And, if so, what type? Does it go over the insulation? Or was I supposed to put it in before the insulation?

SliderWoodWorker

 

[eegger]

I believe faced is the vapor barrier, if you used unfaced, you would add it via plastic sheeting. You do not want to used faced and plastic sheeting.

 

[Hohn]

I believe faced is the vapor barrier, if you used unfaced, you would add it via plastic sheeting. You do not want to used faced and plastic sheeting.

Faced insulation is a vapor "retarder" and not a vapor "barrier".

Vapor Barriers and Vapor Retarders — What's the Difference?

www.wrmeadows.com

 

[Hohn]

I am located in west Tennessee and I have a 12 ft X 24 ft. storage building with wooden exterior walls, a metal roof, and a 9ft. rollup style door. I am in the process of finishing it out to be my woodworking shop.

I have started insulating the 2 X 4 walls with "faced" R15 insulation, stapling the flaps to the face of the 2 X 4 studs. And I plan on installing 7/16" OSB as the interior walls. My intention is to be able to have an air conditioner for the summers and a heater for the winters.

My question is, do I need to install a "vapor barrier", given that I am using "faced" insulation and I live in Tennessee? And, if so, what type? Does it go over the insulation? Or was I supposed to put it in before the insulation?

SliderWoodWorker

Skip the vapor barrier, you did well to use faced insulation and stay with a retarder setup instead.

Barriers in most of the lower 48 are either a terrible idea or merely not useful. Only those folks who never see winter should consider a barrier, and even then it must be correctly placed (on the outside where it will stay warm when the space is air conditioned).

The common scenario is the barriers cause mold because they lead to condensation in the wall cavities.

If you put a barrier on the inside in the south, the cooler air conditioned space puts the barrier below the dew point of the warm humid air making its way through the walls. The will turn the wall cavities into a cesspool of black mold if the a/c is cranked and the outside is hot and humid.

Likewise in the far north in winter, if the barrier is on the outside, the heat leaking out of the space into the walls will carry moisture to the cold barrier and cause condensation.

The net effect is the misapplied barriers cause the walls to rot-- the difference is only whether it's outside in (up north) or inside out (down south).

By contrast, a faced insulation lets vapor move, but slowly. This allows the walls to dry out and prevents condensation.

In TN, you still get situations where you have some winter cold and you don't want a vapor barrier. Barriers are only for climate regions where you never use heat in winter (i.e. tropics) or blast the A/C in summer (far north).

*Not an ASHRAE member, just read a lot of Joe Lstiburek

 

[wolfhawk73]

Skip the vapor barrier, you did well to use faced insulation and stay with a retarder setup instead.

Barriers in most of the lower 48 are either a terrible idea or merely not useful. Only those folks who never see winter should consider a barrier, and even then it must be correctly placed (on the outside where it will stay warm when the space is air conditioned).

The common scenario is the barriers cause mold because they lead to condensation in the wall cavities.

If you put a barrier on the inside in the south, the cooler air conditioned space puts the barrier below the dew point of the warm humid air making its way through the walls. The will turn the wall cavities into a cesspool of black mold if the a/c is cranked and the outside is hot and humid.

Likewise in the far north in winter, if the barrier is on the outside, the heat leaking out of the space into the walls will carry moisture to the cold barrier and cause condensation.

The net effect is the misapplied barriers cause the walls to rot-- the difference is only whether it's outside in (up north) or inside out (down south).

By contrast, a faced insulation lets vapor move, but slowly. This allows the walls to dry out and prevents condensation.

In TN, you still get situations where you have some winter cold and you don't want a vapor barrier. Barriers are only for climate regions where you never use heat in winter (i.e. tropics) or blast the A/C in summer (far north).

*Not an ASHRAE member, just read a lot of Joe Lstiburek

I love watching Joe's talks.

I seem to remember his saying something about not installing a vapor barrier unless you live close to the Arctic Circle or essentially never see Summer.

And bring back tar paper.

 

[Hohn]

I love watching Joe's talks.

I seem to remember his saying something about not installing a vapor barrier unless you live close to the Arctic Circle or essentially never see Summer.

And bring back tar paper.

Basically.
IMO, here's the seminal Joe L article on vapor barriers. EVERYONE should read this if they haven't.

BSI-021: Thermodynamics: It's Not Rocket Science | buildingscience.com

buildingscience.com

 

[wolfhawk73]

Basically.
IMO, here's the seminal Joe L article on vapor barriers. EVERYONE should read this if they haven't.

BSI-021: Thermodynamics: It's Not Rocket Science | buildingscience.com

buildingscience.com

I haven't read him in a while. I forgot how funny he was.