Sheltair Scientific Ltd
Languages: English | Pages: 28 pp
Bibliographic info:
Canada, Canada Mortgage and Housing Corporation, June 1991

This report outlines a research project _funded by the Canadian Mortgage & Housing Corporation (CMHC) Research Division for the purpose of investigating problems related to crawl space construction in -British Columbia. Research was carried out ·between June/1990 and February /1991. A number of groups, including the Technical Advisory Committee of the B.C. Canadian Home Builders Association (CHBA), the CMHC Regional Office, and the New Home Warranty Program (NHWP), had identified problems with crawl space moisture control and ventilation strategies. Causes of the problems included uneven code enforcement and general failures in· local building practices.

The widespread use of passive crawl space vents has long been seen as an effective defence against moisture build-up in crawl spaces. However, because these vents are often left open in the winter, and have inoperable or loosely fitting covers, they cause excessive cold air movement through the crawl space. This translates into a high energy penalty in heat loss from piping, forced air ducting and through ground floors. Ambiguous wording in the building codes has forced many building inspectors to prescribe the use of these vents in all situations - including heated crawl spaces. To some extent, vents were seen to be a solution to high moisture loading caused by a lack of proper moisture barriers over wet soils or by blocked or inadequate drainage systems.

Research included a literature review, a survey of present crawl space construction practices in B.C., and the selection and investigation of 10 case study houses in Greater Vancouver and the Fraser Valley. The ten house sample included crawl space constructions of similar types, with different types of heating and crawl space ventilation systems (ie. vented or non-vented forced-air or radiant heated). Four of the houses investigated had experienced some degree of moisture problems which had required expensive remedial measures. Field investigation revealed that 3 of the 4 problem houses had inadequate moisture barriers. Moisture incidence was directly attributable to the lack of an effective moisture barrier, despite the presence of a 50 mm concrete slab.

A procedure previously developed for testing airtightness of attic spaces, was employed on crawl spaces in 9 of the case study houses. Test results showed that crawl space air leakage, excluding installed ventilation, was high enough to assure natural air change of 0.35 ACH with only a small temperature differential and slight wind. In most cases, there was a large difference (up to 500%) in the ground floor or "interface" airtightness between forced-air heated houses and radiant heated houses, because of the physical link created by the air-ducting system. It was also discovered that none of the vented crawl spaces had sufficient number of vents to meet the code requirements for installed vent area of 0.1 m 2 /50 m 2 of plan area.

Inspections of crawl spaces revealed extensive cracking of the concrete skin coat in all houses.

Cracks did not reveal damage to polyethylene moisture barriers where they were present. The absence of concrete curbs for interior support walls has lead to structural decay in two of the houses. High wood moisture content was commonly measured in areas behind the joist header insulation.

It was concluded that some revision of the national and regional building codes would be required to reduce incidence of crawl space moisture problems. Such revisions would have to address the crawl space issue in a more comprehensive manner. To some extent the use of crawl space vents in B.C. housing has been an attempt to compensate for inadequate or ineffective drainage systems and moisture barriers. A better approach may be to eliminate the vents in heated crawl spaces, and apply any cost savings towards improving drainage systems and moisture barriers.

Some residential building sites require more effectively designed moisture control systems in order to control high ground water levels. Perimeter drainage tiles should .be designed for easy testing, and clean-out, and should be connected to the floor slab area through drains installed in the footings. Fine granular sand presently used under slabs in the lower mainland does not provide an effective capillary break and could be substituted for coarse granular fill with no additional cost. Better application of insulation materials on the interior wall surfaces of crawl spaces is required to raise thermal efficiency and reduce surface condensation. Heat radiation and air leakage from duct work in a typical forced-air crawl space should be credited when calculating heating requirements.

More information is still required to fully define the minimum requirements of a crawl space moisture control system, including research into such issues as; moisture-proofing tops of foundation walls, air mixing systems for crawl spaces in houses lacking forced air duct work, the impact of crawl space vents and construction techniques on soil gas entry, and the airing out of crawl space moisture in new houses during the building dry-out period.