Am I correct in putting the vapor barrier on top of the old roof and taping it to the existing ceiling vapor barrier. I am building storage on top of the old roof and putting wall niches in the 2×4 wall. The old roof in essence is an exterior space.
Edited 1/19/2009 7:18 pm ET by silvertip
Replies
See what others here say, but...your drawing is kind of confusing, but in general, you do not want to have a vapor barrier on two sides of any wall or other structure, as it will trap moisture. In a climate where heating is the main concern, again, in general, the vapor barrier should go towards the warm side, so I would think the existing VB was on the ceiling, say, between the drywall and the bottom of the truss, not on top of the roof. Are you calling the roofing felt (tarpaper) a vapor barrier? I don't see why you would need any vapor barrier quite where you want to put it, but I may not be understanding you.
To me the space between the roof sheathing and the insulation is a cold zone which in essence would be outside. I agree with you that about the vapour barrier on both sides. It is like the code here says that garages are unheated spaces so if they attach to the side of the house your vapour barrier would go on the warm side of the common wall between them. But if you heat both spaces the the vapour barrier is redundant. I kinda jumped the gun on my picture cause i think the roof on the existing structure would have to be insulated as well. My addition is two stories that attaches to the existing house which is which is 1 1/2 story so the top floor wall of the addition is about 5 feet higher than the existing structure. Picture two gable roof 90 degrees to each other. Basically the 2x4 would have to be an exterior wall but I moved the exterior wall further up the roof on the existing house. Thus I think the old roof deck would have to be insulated and have a vapour barrier.
Put your location on your profile. Where you are determines the answers you need.
But a vapor barrier is unnecessary almost anywhere. What is needed between a conditioned space and an unconditioned space is a continuos, uninterrupted air barrier. Vapor diffusion is almost never the problem - it's air-born moisture that causes 99% of the problems (excluding leaks, of course).
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 1/19/2009 9:18 pm ET by Riversong
That is almost a contradiction in terms. Every house has warm moist air in it that is heated thus the need for a vapor barrier.If were stand the roof vertically it would basically be an exterior wall with sheathing on the inside so will the sheathing act as a vapor barrier I don't think it does.
Building science has understood for many years that in a heating-dominated climate the indoor moisture that migrates into an exterior insulated wall or ceiling assembly is 99% due to air transport and about 1% due to diffusion through the drywall.
So no vapor diffusion barrier is needed (except to meet archaic codes), but a carefully-detailed air barrier is needed on the warm side in winter. This can be accomplished (but rarely is) by a poly vapor barrier with taped or caulked seams and no penetrations at electrical outlets or plumbing penetrations or around doors and windows or at bottom plates or sills. But a vapor barrier like poly can also trap moisture in the themal envelope because the moisture movement reverses in summer or when the sun shines and an envelope should ideally be able to dry on both directions.
Vapor diffusion is a good thing. Air movement is not.
Graphic attached is from Building Science Corporation (Joe Lstiburek)
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
"Every house has warm moist air in it that is heated thus the need for a vapor barrier."Except for those that have warm moist air on the outside and dry cooled air on the inside.And many parts of the country have significant time in each mode..
William the Geezer, the sequel to Billy the Kid - Shoe
So what we are saying is that the building code is incorrect in the assumption that we need a vapor barrier. Hypothetically if we surface mounted all our electrical boxes and removed all our windows, the drywall that we have on walls wouldn't allow more than a 1/3 of a cup of water in our walls through a heating cycle and would benefit in the summer months. I agree with the theory but then why does building science not address this issue why do we still want a vapor barrier if it does nothing to stop moisture from entering the wall cavity. Why dont we just put vapor boots around the electrical boxes and tape them to the drywall. Not trying to be argumentive but it seems that some bright spark must have thought a vapor barrier was a good idea.
i still think the jury is out on the use of vapour barriers, HOWEVER, if you use it the rule is always warm side in winter.
Building SCience does cover this."Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior and different climates require different approaches."http://www.buildingscience.com/resources/vapor_barrier_code_changesThe problem with term VAPOR BARRIER is usually means a product that is also a water barrier. And that can trap water in a wall assembly..
William the Geezer, the sequel to Billy the Kid - Shoe
"Put your location on your profile. Where you are determines the answers you need."His screen name is Silvertip. He must be from Canmore, Alberta. Easy as pie.
I did edit my profile and no I am not from Canmore but I did live ther for awhile.
well unless you won the lottery since you left you won't be moving back.
When I lived in Canmore we looked out our picture window and all you could see was a forest, not now. I don't even think the house I grew up in is still there. It is the same as Cochrane which now has been swallowed up by Calgary. Went through that area a couple of years ago. It is funny how small farm towns in around the major cities have been overrun by urban spawl.