In 1998 a pilot study project was coordinated by BRE colleagues working with the British Woodworking Federation (BWF) to begin to understand linking performance requirements for window joinery into the Timber Window Accreditation Scheme. More than 18 companies were part of the trial, which exposed 40 casement windows in the south facing test building at BRE.

After 18 months the manufacturers were brought back to see their windows and, in general, there was a feeling of shock and disappointment. Many of the windows were performing rather poorly, with coating starting to flake, resin exuding through from knots and joints opening, allowing moisture movement into the window. This shock was the catalyst for them to do better and, inspired by the BWF and BRE, we showed the importance of design for performance as well as the quality of the substrate and the coating.

In 2002 a new batch of fully factory finished timber windows were installed in a test building (the BWF/BRE Showcase project) next to the old pilot project. In total 38 windows from 18 companies (coatings, joinery and substrate) took part, with mostly casement but also a few sash windows. It was clear to see the design messages had been noted, with some windows being internally glazed so presenting a very simple and robust exterior face to the weather and minimising risk of moisture ingress.

The windows are all still in place after 15 years of facing south, the most challenging aspect in the UK. Assessment can be conducted at anytime and other windows have been added alongside them to build product performance data. We’d be keen to hear from manufacturers and the BWF about how we present this good news data.

Of the windows installed in 2002 that have now had over 15 years exposure it is astonishing to see how well many of them are performing. They have had no maintenance (except an occasional wash).

Through a combination of a good detail design and factory finishing, using a defect free substrate and a high performance coating, it is possible to get long life low maintenance timber windows. This could have tremendous implications for reducing even further the lifecycle impacts of a timber window for example.

BRE is also involved in long-term on-going tests of coatings on ThermoWood cladding.

The study is sponsored by the International ThermoWood Association so is specifically for its members and for the ThermoWood brand.

The aims of the ThermoWood cladding project at BRE are:

  • to promote best practice guidance for the use of ThermoWood;
  • to research coating and ThermoWood interaction and performance;
  • and to inform on coating selection for use with ThermoWood.

In 2004 a total of 28 cladding rigs were put up, of which 22 were made from ThermoWood-D, both spruce and pine, benchmarked against six cladding rigs made from western red cedar, untreated spruce, and preservative-treated pine. There were four different board profiles – plain feather-edged, rebated feather-edged, tongued and grooved, and overlapping. The surface finish to boards was planed, fine sawn, or mechanically brushed (to deliberately raise the grain).

Cladding boards were coated before assembly into rigs with one of the following:

  • brush painted with opaque white paint;
  • brush painted with semi-transparent exterior wood stain system;
  • factory finished with semi-transparent exterior wood stain system.

The cladding rigs were installed on April 26, 2004 and have been monitored since then.

The monitoring of the rigs was twofold, comprising subjective visual assessment criteria and data-logged moisture content of the boards. The moisture content data was collected from probes inserted into the back of the cladding boards to within 5mm of the exposed surface.

Visual inspection of the cladding rigs was conducted and moisture content data reviewed. An assessment scheme, based on subjective criteria (surface dirt pick-up, cupping of boards, splits and checking, raised grain, resin exudation, surface mould growth, and erosion of the coating), has been used to rate the performance of the various substrates and the coatings.

The data shows that ThermoWood offers advantages in reduced cupping of the boards, reduced raised grain, and a lack of resin exudation. The fine sawn and brushed surfaces reveal reduced checking and an improved overall appearance, especially when the brown wood stain is used. The moisture monitoring reveals the low moisture content of the ThermoWood boards compared with other substrates and how they respond to moisture.

November 2011 saw the redecoration and restoration of 12 of the cladding rigs and a continuation of exposure alongside four uncoated cladding rigs was completed.

These 16 cladding rigs are now 13 years old and continue on exposure. The coated cladding rigs show the value of keeping the aesthetic appeal of the warm brown colour of the original substrate using a brown wood stain.