BCAB #1003 - Vapour Barrier, Subsection 9.26.5.

August 19, 1988

BCAB #1003

Re: Vapour Barrier, Subsection 9.26.5.

With reference to your letters of February 18th and June 28th, 1988 regarding the application of Subsection 9.26.5. in the Building Code.

Basically we must take the Subsection as providing general principles for the installation of vapour barriers to prevent condensation within the structure. It attempts to cover situations envisaged by the authors, but we do not regard these as limitations, it is not possible either here or in the Code to identify each and every specific point which must be considered and accommodated. We must also make the point that there is obviously confusion among Code users between a "vapour barrier" and an "air barrier", but at this stage we must specifically address the vapour barrier requirements.

In general the vapour barrier must be as complete and as continuous as the realities of wood frame construction permit, the Articles in the Subsection pinpoint locations which we assume have given rise to difficulties, but others necessitate the application of practical judgement based on the general principles. As examples we would not consider major modifications in the methods of wood framing to be justified, rather we would expect the vapour barrier installation to accommodate such techniques; but minor changes, such as indicated in Article 9.26.5.5. are fully justified. On a general basis we would be concerned where lumber is left unprotected; we are unable to accept softwood lumber 2 inches thick as constituting an effective vapour barrier, and if this occurred would anticipate a high moisture content at these locations, probably higher than the 19 per cent permitted in the Code.

In the two letters submitted questions have been raised, and while we feel that the principles already outlined can be applied to these situations, we will provide further guidance:

(a) Article 9.26.5.4. is intended to ensure that joints in vapour barriers are supported, and the film must be sealed in a manner which would effectively make it continuous or be lapped, with the joints positioned over framing features such as studs. In the case of a wall plate we consider that the vapour barrier should be carried down over and be fixed to the plate, but sealing would not be mandatory at this location.

(b) If a crawl space is heated, then we consider it to form part of the heated area of the house, and Article 9.26.5.1. would apply.

(c) Article 9.26.5.7. is intended to ensure that service openings are sealed, in whatever manner effectively achieves this result, and while the question raised refers to drywall, this is a separate issue dealt with later. For penetrations through the vapour barrier we consider that, for example - pipes would be sealed around, presumably with resilient compound - electrical boxes would have the vapour barrier carried behind; or be set in plastic pans with flanges lapping the vapour barrier.

(d) Applying Subsection 9.26.5. to the header joists would vary. In the case of a heated crawl space, or at header joists between floors, insulation should be installed at the header, normally on the inside face, and this would require a vapour barrier on the warm side of the insulation in this location. The type of vapour barrier is covered by Article 9.26.3.4., but as indicated earlier we do not accept lumber for this purpose.

With an unheated crawl space, the header joist may or may not be insulated, this would depend on the location of the floor insulation. If installed at the bottom of the joists, the headers would require insulating, but if directly against the underside of the subfloor, then the headers would be outside the heated enclosure and a vapour barrier unnecessary.

(e) Behind the points raised in the June letter would seem to be the question of accepting drywall as a vapour barrier. As this has not been specifically raised we cannot provide a definite answer, but would suggest that if a product satisfactorily achieves the level of performance required by the Code, then it should be acceptable, provided of course that it lends itself to proper installation.

(f) You have submitted, in your February letter, a proposed change to the Building Code, but we must point out that this does not fall within our jurisdiction and suggest that it be taken up through the appropriate channels.

J.C. Currie, Chair