Vapor barrier woes.....

[machwon169]

I'm building my 50x50 right now... it's Going to be r13 in walls insulation to metal outside then 4 mil vapor barrier then the durapanels over tbag for the walls... that's fine...

My question is when doing the roof how and where do I install a vapor barrier....

My plan was put up the durapanels, lay in the insulation and then 4mil plastic on top of that? Will this work? But will it still condisate up there and drip down onto the plastic and I would never know cuz the f
Roof will be covered....

What would you do?

 

[brownbagg]

its a shop, forget the vapor barrier

 

[bullnerd]

First question you will be asked is where is the building located?

Vapor barrier goes on heated side of insulation.

I'm not a pro, just an enthusiast.

 

[Firebrick43]

First question you will be asked is where is the building located?

Vapor barrier goes on heated side of insulation.

I'm not a pro, just an enthusiast.

+1. Condensation dripping is greatly reduced if not eliminated if you insulate.

 

[Aquamoose]

Location is important. In Florida, the barrier gets placed on the INSIDE!


Sent from my iPhone using Tapatalk

 

[machwon169]

I'm in NJ....

I am worried that the unheated part above the roof will sweat and drip water onto the insulation in the roof

 

[Firebrick43]

If the area above the insulation is vented well it's not an issue. Sweating occurs during large temp swings and high mass items such as the floor and cast iron machinery/vehicles are still cold causing the moisture laden air to parcipitate as it comes in contact with the colder items. By insulating you slow the temp swings and block the high mass of the slab/items from the roof. Any condensations will be much reduced and will be removed if ventilated.

 

[hefnerconstructionlc]

This above, I have a stick built shop with unfaced r-19 between shop ceiling in this order- tin/insulation/decking/well vented attic space. I heat my shop in the winter, no sweating.
I use small wall mounted electric heater, which also would help to dry the air. Main thing is ventilation, I have soffits and used attic baffles, for air movement. The attic also have roof vents. Your set up will likely be different, but the principles remain.

 

[machwon169]

So I have a roof soffet so I'm thinking it's plenty ventilated....

So I guess I will just do roof. Open space then insulation and the metal ceiling panels

 

[Firebrick43]

Soffit venting is not enough. You need ridge vents, or gable vents, or turbine ventilators. Other wise no natural convection currents will form.

 

[machwon169]

I have a 50 foot cobravent up
There also

 

[machwon169]

I guess it's safe to say to save the $1600 on the bubble wrap stuff

 

[Voi]

So I have a roof soffet so I'm thinking it's plenty ventilated....

So I guess I will just do roof. Open space then insulation and the metal ceiling panels

There are roll insulations that are designed to not sag in cathedral ceiling type applications and leave an air space. But I don't think they come in lower R values to fit inside of the typical 2x4 upper chords. I think they're R-38 and are made for 2x10 rafters.

Are your upper chords 2x4 or greater? How much of an air gap will you have?

A properly ventilated cathedral ceiling is more important in cold, moist climates like yours. I'd make sure to get this right.

 

[bullnerd]

You can see some pics of how I did it in my SIG.

I'm in NJ too.

Yes, no bubble wrap.

I've gone up in my attic during these big temp changes that were mentioned and didn't see any water building up, even without any insulation up yet, just vapor barrier.

 

[HotrodHR]

The key is to have your ceiling insulted (attic floor) and ventilate the attic as mentioned if you plan to heat or cool your shop...

 

[yeldogt]

You will get moisture from the inside -- I would not put the VB on the cold side of the insulation. When using plastic, great care must be given to air tightens and making sure the other side of the plastic can breath. Many rotten building caused from poor plastic vapor installs.

 

[machwon169]

The air gap between the ceiling and the slope of roof is going to be about 8 feet of air space

 

[machwon169]

The upper cords are 2x4

 

[willymakeit]

See if there is a envelope consultant in your area. I deal with vapor and waterproof barriers on large below grade and multi story structures.
Buildings need to be thought of as a 6 sided structure to have a high performing structure.
Below grade, walls and roof as a total system.
They can tell you where the dew point is on your system along with how to properly seal it.
In typical systems a 1'' square hole will exchange 32 pints of water per year. With a sealed building and slightly positive inside air pressures great things can happen. Fail and its a nightmare.

 

[Voi]

The air gap between the ceiling and the slope of roof is going to be about 8 feet of air space

The upper cords are 2x4

I apologize. I thought you were trying to insulate under the roof deck.

Here are some articles.

http://www.greenbuildingadvisor.com/blogs/dept/musings/do-i-need-vapor-retarder

http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers

 

[machwon169]

Ok so I'm right in saying walls from outside-in...

Metal siding
Insulation
Vapor barrier
Metal linar panel

Then for the roof from outside in

Metal siding
Open space about 8 feet
Insulation
Vapor barrier
Metal linar panels

 

[ard]

Ok so I'm right in saying walls from outside-in...

Metal siding
Insulation
Vapor barrier
Metal linar panel

Then for the roof from outside in

Metal siding
Open space about 8 feet
Insulation
Vapor barrier
Metal linar panels

Sounds right to me.

You want to stop the transport of moisture laden air from a warm area, into a cool area- where it can then condensate out of the air and deposit. (NOT necessarily SWEAT, but over time lead to rust/rot/failure.) I am shocked to read someone say 'its a garage, skip it'. A building is a building, physics is physics- protect it.

If you imagine a wall cross section, picture the inside at (say) 65F and the outside at 32F (or whatever)...now look at your proposed structure- take guesses what the temp will be at various points in the wall. Just behind the metal liner panels it will be 60-62F (guess). Perfect place for VB. But keep going, as you get 'deeper' into the insulation layers, it gets cooler- then just under the outer skin it will be 34-32F. Bad place for VB.

 

[bullnerd]

Yup.

Where in NJ are you BTW?