Insulation: Vapor Barrier

[midwest farms]

I think I have read ALL of the insulation threads but have a few questions around proper insulation. I know this has been beat to death so I will keep it brief and would anticipate the responses to be the same:

Description:
--Building is an existing pole barn. Not using spray foam due to cost.
--Using 2 layers of 2" rigid foam panels (total of 4") to insulate between poles-poles are 9' apart-not bookshelved (attaching to back side of perlins.)
--Using great stuff or similar spray foam around panels to create airtight seal like this:

--Finishing with metal on walls and ceiling. I have the ceiling details figured out.
--Located in Illinois so hot humid summers-colder winters. Shop has in-floor radiant heat. Will not be cooling in summer excluding fans.

Questions:
1. Do I even need a vapor barrier since I am foaming around the rigid foam board? If I use a vapor barrier I assume this should go on the interior wall?
2. Should I foam around the insulation panels nearest the outside wall or the closest to the interior wall. I want to avoid the pitfalls of a "double vapor barrier" while completely sealing the space.

Thanks!

 

[ilovevocs]

Their are several perspectives on building envelope deisgn. In order to determine the true need for a vapor barrier one would need to conduct a dew point analysis.

However, shooting from the hip with the infomation you have presented this is my OPINOIN.

Since your finishing with metal and not drywall your structure will not have a functioning air barrier. If you take the same approach with the roof as the walls and do not include a venitlation cavity you would be essentially creating a compact roof assmebly. The rule of thumb for compact roof assmeblies is When i doubt leave it out. The vapor barrier negates the self drying prciniples of the compact roof assembly in conditioned spaces. You didnt mention cooling so this pricinlple is likely less prevelant than in a fully conditioned space. Im going to sum this up, again this is just my opinion.

I would include a vapor barrier in the absence of the air barrier. Be certain that it is properly detailed as to negate any air infultration into the building envolope. Nothing will create more problems or trap more moisture and an poorly detailed vapor barrier.

 

[midwest farms]

Thanks for the reply. I was following the same logic. I would rather leave it out and have a less efficient envelope than create problems. I was not in a hurry to do the project but the temp changes are creating a lot of condensation on everything in the shop (cars, toolboxes, ...anything). Right now it is an open cavity with vented soffit. I need to get this insulated to avoid damaging and rusting my tools!


The ceiling will consist of metal ribbed panel (r panel) attached to the bottom of the trusses. This will have a vapor barrier and blown in cellulose on top. The ceiling cavity above the insulation is very well vented and I don't anticipate condensation problems in this space.

I was contemplating whether the foam board and spray foam sealing would act as a vapor and air barrier without the need for an additional barrier.


Thanks!

 

[readhead]

If you are having a lot of condensation issues now a dew point analysis may be in order. You only have condensation if there is a source of moisture and your source may be excessive. Is it new concrete with no vapor barrier? do you park wet vehicles inside? What is the temperture inside?

 

[ilovevocs]

If you are having a lot of condensation issues now a dew point analysis may be in order. You only have condensation if there is a source of moisture and your source may be excessive. Is it new concrete with no vapor barrier? do you park wet vehicles inside? What is the temperture inside?

My comments were in refernce to a new structure. Im not personally involved in many retrofit applications where the vapor barrier is a choice after the fact. I'm an architect turned roofing contractor so my experince runs the gaumet from design to implimentation. You would be supprised by the volume "design professionals" that improperly specify or detail air / vapor barriers and vapor retarders.

If your using a closed cell foam insualtion in conjuction with the spray foam i dont think you will have issues, however their are too many attributes to consider to make this statement with any confidence. If your insualtion was not of the closed cell type air infultration and vapor drive may be more of a concern in the degradation of thermal insualting value. Thus the inclusion of LTTR values for resitance ratings as apposed to intial r value. This is not the only factor driving these standards but none the less it should be considered.

This is likely the best / most accesable resource on the net for those that care to research a little further. http://www.buildingscience.com/

 

[midwest farms]

Thanks for the replies:

To address a few comments, the condensation only happens a few times a year when we temp swings from very cold to very warm in a matter of a few hours, especially when the dew point is high. This is usually accompanies by high winds as well. Anything metal is wet. The concrete floor is insulated and has a vapor barrier-its not a structural problem, just a weather change in a that building is well vented at the top which allows for heavy moisture intrusion especially with the slab being cold and this cold radiating to everything on it. With even a remotely well sealed building, this will be a non issue once the insulation is up.

Thanks for the replies. They have been very helpful. I'm going to spray foam around the rigid foam which should effectively seal the envelope (at least the walls). I'm ready for spring!

Thanks!