Timber sealing solutions, special perimeter seals for timber frame construction

The proportion of UK new-build properties currently built using timber frame construction is increasing as the benefits of this method become more widely appreciated. Key factors are thermal efficiencies, speed of build and reduced requirements for skilled site labour.

Confidence in the timber construction process has grown, helping many of the earlier reservations of this type of construction to be overcome. As a result, rather than simply residential developments, a wide range of commercial buildings including hotels, offices and various types of accommodation are now constructed in timber frame. Furthermore, these buildings are being constructed to higher levels – no longer is the typical timber construction restricted to single or two-storey properties.

Differential movement

One of the main drawbacks which continues to require careful consideration and appropriate solutions however is the differential movement which occurs during the build, essentially between the timber frame structure and external masonry. This is caused by vertical shortening of the frame and affects various elements. However perhaps the most significant is the sealing of windows, which requires a much more considered technical approach than letting someone loose with a silicone gun.

Timber frame construction is now confidently erected up to six storeys, classed as medium rise, following invaluable research feedback from the TF2000 project supported by several industry partners. It is these taller timber structures and buildings that now require more considered and focused sealing solutions.

There are four main contributing factors to the differential movement occurring. These are shrinkage, settlement, elastic shortening and movement of the external skin.

Shrinkage, caused by a reduction in moisture content, can be reduced by using engineered timber or less cross-sectional timber. Settlement occurs as a result of loading of the structure with internal walls, floors and the weight of the roof. It is therefore advantageous to delay construction of the external skin until more of the loading is achieved. Once the external brick or blockwork is erected, this will absorb moisture and expand (as much as 1mm per metre height of building).

Industry bodies such as TRADA, BRE and NHBC offer guidance on the movement allowances which should be designed in, however there are variations in the figures quoted. On 5-6 storey buildings, differential movement of over 50mm can occur.

If the sealing of windows is undertaken later, then the joints will be subject to less movement

Expected movement

Windows, usually comprising PVC-U frames, are installed within the timber frame prior to erection of the external brick or block skin. As the courses are laid gradually up the building, consideration must be given to the expected subsequent differential movement of the frame – with the windows effectively dropping in relation to the masonry.

As the window frame moves downwards the joint width at the bottom, under the sill, will decrease. At the head of the frame the joint space will be increased.

As a result, if a sufficient gap is not left between the underside of the windowsill and the external brick/stone sill, within a few months, the two components will meet and significant damage will result.

Similarly at the window head, an increasing gap will present a potential weather and insulation problem in addition to the changing aesthetic.

Contractors experienced in timber frame construction have become aware that if the sealing of windows is undertaken later, then the joints will be subject to less movement and lower dimensional tolerance. It is clearly not good practice to perimeter seal as soon as the brick or blockwork is in place as costly remedial work will be necessary due to probable failure of the sealant. Silicones or any other ‘wet’ sealants are not capable of accommodating the potential movement that can occur and it is important to consider the jambs in addition to the head and sill locations.

A much more appropriate solution is to use impregnated foam tapes with the required expansion and compression properties capable of withstanding the often high movement criteria outlined above and providing sustained weather performance over 25 years.

With higher buildings, it may be necessary to delay application until some of the movement has taken place

The thick, impregnated foam strips are used under compression with the cellular nature of the foam carrier acting like a spring. When inserted into the joints between the window frames and the neighbouring masonry, the foam strips expand to fill the gap and conform to any irregularities in the surfaces, exerting a continuous outward pressure on the adjoining building components, whilst yielding to further compression to provide a tough, rubber like seal.

Earlier installation

It is also possible to install the tape earlier in the programme and still avoid remedial visits. Typically the differential movement encountered on up to four storey buildings can be resolved with tapes applied to the window heads and cills. With higher buildings, it may be necessary to delay application until some of the movement has taken place, but this can still be carried out whilst the building remains scaffolded. Solutions are also available to seal the window jambs, which present a different movement problem. In this case, any sealant around the sides of the frame will be subject to shear forces trying to pull the material downwards as the window moves vertically in relation to the brick/blockwork. Encapsulated impregnated tapes enable location to the window jamb prior to bricklaying with subsequent activation of the seal and movement against the external skin. It is therefore possible to provide a four-sided sealing solution using two different impregnated foam tapes.

Whilst the foam can compress at the window sill (and expand at the head), it must be remembered that there will always be a minimum tape thickness that must be allowed as the tape cannot simply compress to nothing. Some contractors are using a 90 degree angle trim, fixed to the underside of the lintel that masks the increasing sight line at the head of the window during differential movement. The impregnated foam, whilst protected from the weather in this case, is still providing the benefit of thermal insulation and a barrier to draughts and the potential ingress of insects.

Despite the problems identified above with window sealing, solutions exist which provide user - friendly, highly durable seals and eliminate the need to carry out costly remedial work.

A range of specialist impregnated foam tapes with the high movement capability required for timber frame construction is available from illbruck Sealant Systems UK Ltd.