Basement Remodeling Vapor Barrier Question

[twoply]

Hey everyone!

I'd like to discuss what is needed in this picture in regards to insulation/ vapor barrier.

This is a generic picture I pulled off the internet, but it represents what we run into around our Northern Ohio area.

The owner is self performing the work and wants to install foam board in the rim cavities and faced insulation in the walls.

The real question I have is about the need for vapor barrier. Is it necessary? And if so, where should it be installed?

It seems if you install the VB between the stud and drywall, moisture may move from the moist masonry wall into the wood studs and promote mold growth.

If you install the VB right on the masonry wall it may trap moisture against the masonry wall and never have a chance to dry and promote mold growth.

I'm arguing with myself even. Can I get a little help here?

 

[TheCommish]

Check with Building science corporation, Lipsack (sp)

 

[steveray]

Listurbek...I believe....Code wise, there isn't a lot of guidance or info/ requirements (we don't really get into vapor drive). But BuildingScience has a lot of good info...

 

[Francis Vineyard]

Recommend reading TB 6 and TB 21 for best practice; https://www.insulate.org/tb-review/

To my knowledge the code does not prohibit vapor retarders or barriers where not required. The 2006 edition of the Residential Energy Code provided the code official discretion but apparently has been removed without substitution;

"The building design shall not create conditions of accelerated deterioration from moisture condensation."

 

[tmurray]

Unless you are building a basement in the desert, your vapour drive will be from outside to inside. Now the ideal thing is to place the vapour barrier against the foundation wall to stop vapour from entering the stud cavity and then allowing any moisture in the cavity to move into the living space through diffusion where it can be exhausted through the mechanical systems.

Now, you mentioned foam board. What kind? Typically, expanded polystyrenes are vapour open unless they are faced with another product. Extruded polystyrenes used in construction are either vapour barriers or so close they might as well be one. If a product that acts as a vapour barrier is installed against the foundation wall, I would recommend there be no vapour barrier used inside and mechanical ventilation be used to control the relative humidity in the space.

Realistically, you can have foam acting as a vapour barrier on the outside and poly on the inside, provided there is enough insulation value in the foam to move the condensation point into the foam for the majority of the year.

 

[steveray]

I do not believe that you would get mold on the concrete with a VB up against it...

 

[classicT]

R702.7 Vapor retarders. Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4.
Exceptions:
1. Basement walls.
2. Below grade portion of any wall.
3. Construction where moisture or its freezing will not damage the materials.

 

[mtlogcabin]

The wall needs to dry to the inside extruded polystyrene less than 1" with no foil facings or polypropylene skins, latex paint only on the gypsum board, if more insulation is needed frame a stud wall and used un-faced fiberglass.
Builders Guide to Cold Climates pgs 190,191 Joespeh L Stiburek

Thicker polystyrene will not allow the wall to breathe properly. Absolutely no sheet polypropylene or vapor barrier should be installed on an interior basement wall

 

[Francis Vineyard]

Other than tub/shower is there a code provision that would prohibit the installation of a vapor retarder in exterior walss where it could promote moisture damage and deterioration?