Collaborative research into ventilation of wall cavities

By - , Build 97

A recent trip to North America provided opportunities for further refining the BRANZ moisture research programme. Some of the papers presented at a building physics conference are highlighted here.

BRANZ’s research complements North American research on leaking buildings and both show how important an understanding of infiltration paths is to moisture management in walls. In Europe there is a growing interest in insulating heritage buildings. Insulating these old stone walls changes the flow of moisture, so hygrothermal modelling is being used to ensure that moisture is managed at all points in the wall.

The effectiveness of drainage paths is being talked about more than ever. Walls designed to manage water are closely inspected for retained moisture. Drainage plane studies have highlighted this issue (as have our own studies) and we can expect this aspect of rainwater management to get a lot more attention in the future.

Building physics conference

The emphasis at the Third International Building Physics Conference was moisture-related issues (rain leaks, vapour diffusion from inside buildings and hygrothermal modelling) but there were also many papers on energy, insulation and ventilation.

Several interesting papers were concerned with the moisture and energy performance of roofs, and one discussed convection in bulk insulation and wind wash effects.

There were several papers on double facades, which ventilate the space between glazing panels to improve the thermal efficiency of the building. In summer, exhaust air carries away heat from sunshades (inside the glazed cavity) and in winter, heat losses from exhaust air substitute for heat losses from the occupied space. These systems were being studied in detail in test chambers. Some double facades included phase-change materials as well – improving the potential of the wall but complicating its integration with the building and local climate.

A paper from Waterloo University (Straube and Finch) reported on large-scale condensation on the insulated cavity side of sheathing. The building (multi-storey in Vancouver) had been retrofitted with a rain-screen cladding and the vapour barrier behind the wall lining had been eliminated. Condensation in a wall that has had its vapour barrier eliminated is of course interesting because it questions some fundamental thinking on moisture movement into and out of cavity walls. In this case the problem was sub-standard indoor moisture control and this was cured with a better ventilation system.

Another paper modelled the effect of two insulation systems applied to the interior of a historic building in Amsterdam. Two options were explored: using vapour-tight cellular glass and capillary active calcium silicate. The latter system turned out to more effectively manage water (mostly rainwater) because it could dry to the inside of the building. This is an interesting application of the idea of wicking water from a possible condensation plane in a wall to a point where drying can take place. The quantities of water involved were unlikely to challenge the internal conditioning system.

A research group at Concordia is using water trays in cavities to provide a constant and known source of moisture for drying rate measurements. Trials show evaporation rates to be very sensitive to temperature and air movement as expected, but overall, the method appears to be as good as any of the other approaches taken to date. One advantage over our approach with absorbent layers is that it takes the air/water interface away from the building materials and eliminates some uncertainty about whether contact with the building wrap is liquid or vapour.

One paper made a start to measuring the effectiveness of drip edges as a function of horizontal projection and the angle of the kick-out. As expected, edges with greater horizontal projection are more efficient at removing running water from a face. The paper showed that even minor horizontal projections are useful, with efficiency figures of around 50% for 10 mm horizontal projections.

BEP education

The Building Envelope Professional (BEP) qualification offered by the Institute of Architects British Columbia is a five-module course concentrating on thermal and moisture with passing reference to other areas, such as structure and acoustics. It has not set out to encompass all of the skill sets required in a cladding design but supplements the traditional engineering courses available in these areas with moisture and thermal. Many of these skills need developing in older architects and engineers, whereas acoustic and structural training has been well established for many years. The five-module course takes about 9 days. BEP-qualified designers are able to provide ‘letters of assurance’ that effectively sign-off on the heat and moisture performance of building envelopes. It seems that most designers active in this area are now qualified.

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Articles are correct at the time of publication but may have since become outdated.

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