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Permeable building in a cold climate

Mathieu | Posted in Green Building Techniques on

Hi – We are renovating a 1940 brick house in climate zone 4, Quebec, Canada. The question is about the necessity of a vapor barrier or vapor retarder in our assembly.

Previous building assembly : 
– Brick
– Air gap
– Tar impregnated wood fiberboard 
– Tar-paper, about 15 lbs
– 3/4″ tongue and groove boards
– 2×4 (empty)
– 3/4″ tongue and groove boards
– Tar-paper, about 15 lbs
– Lath and plaster
– Many coats of paint

We removed the asbestos-containing lath and plaster and got back to the interior boards 

Half-thorough building assembly :
– Brick
– Air gap
– Tar impregnated wood fiberboard 
– Tar-paper, about 15 lbs
– 3/4″ tongue and groove boards
– 2×4
Blown-in cellulose 
– 3/4″ tongue and groove boards
1 inch wood fiberboards nailed to the boards to get R3 thermal break

Original finishing option was Intello plus, furring, drywall, lime-paint.

We are exploring an unusual way to finish this wall, a technique used in Europe , which is to finish the walls with one last layer of 0.5 inch wood fiberboard and lime-plaster applied to it. No furring, no drywall, no paint.

The lime-plaster specialist suggested that it would be best to not use a vapor barrier to get a moisture permeable assembly. Interesting but not in-line with the building code and what I have assembled through my readings. He argues that lime-plaster with be air-tight and sufficient to not have excess humidity passing through walls. The moisture managing nature of natural building materials such as cellulose, fiberboard and lime plaster is another of its points. Our brand new mechanical ventilation system would also extract excess humidity from occupants. All fair points, but there is still some diffused water vapor that are going to inter the wall. Lowest perm rating here is 4 perms with the 3/4″ wood boards.

Let’s say we try this finishing option, I still ponder adding a VB such as an Intello plus before the fiberboard and lime-plaster (but a lot of screws will go through it when securing the fiberboard and there will not be the usual air gap).

Any experts with thoughts on that unusual assembly that would not have a VB in a northern climate ? My concern is dew point within the cellulose during cold periods of the winter. Maybe damage to the brick and mortar with excess vapor in the freezing season ? We do not have a 12″ hemp block like those textbook permeable assemblies…

Mathieu

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Replies

  1. DC_Contrarian | | #1

    A surface can be air tight and vapor open. That's the way drywall is, which is why drywall walls need a vapor barrier on the inside.

    You want a vapor barrier, in a heated building there is quite a bit of vapor drive toward the exterior even when it seems dry inside. If you want your wall surface to be able to dry to the outside you could paint it with a vapor barrier primer before painting.

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