illbruck sealing products achieve astonishing airtightness for oak-framed building

Green oak-framed buildings are notoriously difficult to seal due to the movement that occurs naturally as the oak dries and then from season to season. However, thanks to a variety of air- and vapour-tight barriers, sealants and gap-fillers from tremco illbruck, a new-build oak-framed office building has achieved an astonishing airtightness of 1.5m3/h/m2 rather than the expected 4.5-5.0m3/h/m2. This is on a par with a passive house, even though the office had not been designed as a low-energy building.

The single-storey office was constructed at the same time as a series of equine barns that were being erected at a prestigious stud farm in Surrey. The architect was very familiar with designing oak-framed barns and stabling but was less familiar with the building regulations relating to occupied buildings. Once the office framework had been built, with brick infill at the lower levels and the roof covering in place, the architect sought advice before the windows and doors were installed.

By the time the expert from tremco illbruck became involved, the oak frame had been standing for almost six months and had, to a large extent, stabilised – though it was fully expected to continue moving for some time. The oak frame had numerous gaps and hollows between the structural elements, and a complicated cavernous roof void. Fortunately the range of illbruck products is extensive, so the optimum could be selected for each area of application around the stud office.

Rob Halliwell, Area Sales Manager (WF&S) for tremco illbruck Ltd, recommended a combination of three products to provide an air- and vapour-tight barrier in the roof void and extending down into the upper elevation of the ground floor. illbruck ME003 Vapour Barrier is a three-ply co-extruded polyethylene (PE) membrane. Although this is flexible and lightweight at only 110µm thick, it is manufactured from high-grade polymer granulate so it is therefore both highly vapour-retardant (sd >107m) and tear-resistant. To complement the vapour barrier, Rob Halliwell proposed illbruck ME317 Internal Membrane Tape, a single-sided, reinforced kraft paper tape with solvent-free, modified acrylic adhesive for bonding the vapour barrier overlaps. Easy to tear by hand, and with a release paper that can be removed as the tape is applied long the joint, this product is very user-friendly. The third product was illbruck OT301 Endurance Adhesive, for bonding and forming an air-tight seal between the ME003 vapour barrier and the structural elements. This quick-drying, high-adhesion, high-elasticity adhesive suited this project well because it was easy to apply and remains permanently flexible to cope with any further movement in the oak frame.

One of the challenges with the office project was to seal around the windows, which were installed between stud partition infills that were hand-built in situ and were therefore vulnerable to unplanned air leakage and vapour migration through the fabric layers. For sealing between the under cill of the timber stud infill and the brick coping, Rob Halliwell recommended illbruck FM330 Pro Foam Air Seal and illbruck SP925 Air Seal. FM330 is a high-performance polyurethane (PU) foam that is often used for filling and sealing construction joints, with its high movement accommodation factor (35 per cent) ensuring that the seal is maintained even in the event of movement. SP925 is a versatile multi-purpose product for ‘hard-to-seal’ areas that is applied the same way as other sealants – manually or by spraying – and brushed out to ensure the area has been sealed fully. Capable of filling gaps up to 6mm and 6mm deep with no backing material (or wider, deeper gaps if a backing is applied first), SP925 dries to form a fully elastic air- and weather-tight barrier that is excellent for areas that are likely to be subject to movement.

For sealing between the window perimeter and the studwork formed reveals, and also between the stud partitions, the recommendation was to use illbruck TP651 Trio impregnated pre-compressed sealing tape and ME500-W Duo Flexible Window Membrane. T651 provides a weather-tight and air-tight seal and, at the same time, thermal insulation such that the energy rating of the window can be maintained for the installation. The soft open-cell PU foam is pre-impregnated from one side using a flame-retardant synthetic resin, with the one-sided impregnation resulting in different vapour diffusion properties for the internal and external sides. Being pre-compressed, the tape is easy to apply, yet it expands to fill and seal the gap extremely effectively, typically taking a few hours to complete this process. The ME500-W intelligent membrane achieves an air-tight and vapour-tight perimeter seal around the window frame, with the membrane being applied around the frame prior to installation (alternatively, ME500-E can be applied post-installation). Manufactured from polyethylene copolymer film, ME500-W also features a non-woven fleece fabric. Bonding to the window frame is achieved with an acrylic self-adhesive strip, while bonding to the wall is with a butyl self-adhesive strip. An integral movement pleat caters for variable joint sizes and accommodates for any post-installation movement.

Elsewhere on project, FM330 PU foam was used to provide thermal and acoustic insulation as required.

When tested for airtightness, it was expected that the building would achieve in the order of 4.5-5.0m3/h/m2 but in fact the measurements showed a remarkable figure of 1.5m3/h/m2. The experts commented that this was the best result they had ever assessed and recorded; the figure of 1.5m3/h/m2 might typically be measured when constructing a passive building, yet the architect had not set out with this as a goal. Credit for this exceptional airtightness is due to the careful selection of the optimum sealing and insulating products for the various areas around the building, the ease with which they can be installed correctly, and the technical performance of those products.

When Rob Halliwell first visited the site he effectively had a ‘blank sheet of paper’ from which to work, as the architect had not specified any sealing or gap-filling. He therefore marked-up the architect’s drawings, supplemented by hand-drawn sketches, to show his recommendations and make technical annotations. Additional support was provided via telephone calls and email exchanges, and the contractor watched the relevant ‘how to’ videos on illbruck’s YouTube channel. Once all parties were satisfied that the recommended illbruck products were right for the project, the order was placed with and fulfilled by Grant Haze, one of tremco illbruck’s distribution partners.

Mike Williams of MWA Consultants, the firm of quantity surveyors involved with the project, comments:

“The airtightness of the finished building was amazing and a real testament to the quality of the materials. I was very impressed and, without a shadow of a doubt, I would definitely use illbruck products again.”

For further information on tremco illbruck, please call 01942 251400 or visit