Project Scotland

Moving times

HB Band sealing strip from ISO Chemie

Peter Thompson, general manager UK of sealant tape manufacturer ISO-CHEMIE, looks at differential movement in timber frame structures and technologies for bridging the gap

WITH timber frame expected to account for 27% of new housing by 2017 according to a report from the Structural Timber Association (STA), it’s increasingly important for those with specification responsibility to consider the requirements of differential movement surrounding timber frame installations.

In traditional brick and block dwellings there is no real movement between the internal and external walls, allowing the initial position of installations to be reasonably constant.

However, timber frame construction produces a set of unique requirements that are caused by the differential movement between the internal timber and external masonry walls.

The height of the internal timber wall will shrink due to a combination of load and moisture loss factors, while the external brick will increase in height in the face of swelling caused by moisture absorption. As the windows and doors are generally fixed to the internal timber wall this can lead to a large amount of downward movement within the aperture of the masonry wall – a common problem experienced on many building sites.

Indeed, the Structural Timber Association’s ‘Differential Movement Calculator’ indicates that even on a two storey dwelling the first floor windows may move by upwards of 14 mm while those on the second floor of a three storey house can shift upwards of 27 mm.

This clearly has implications for the fenestration industry. For manufacturers, any problems surrounding the quality of an installation are invariably laid at their door as an apparent product fault, while for installers a poor seal can damage their reputation with the housebuilder, occupant or even the door and window manufacturer.

Just as the requirements for producing an aperture in the external wall have changed for timber framed buildings compared to brick and block, then the same applies for window installations. Rather than installing the window centrally in the external aperture, windows in timber frame structures need to be set unevenly higher in the aperture to allow for the downward movement previously mentioned. The amount of movement at each floor should be readily available from the housebuilder or timber frame supplier, some of whom already supply standard drawings indicating this information.

It also means that using a traditional silicone seal, which gives a MAF (Movement Accommodation Factor) of between 15% – 50% to accommodate the timber frame movement, will largely be ineffective – unless ridiculously large gaps around the window are required to accommodate movement.

As a result, this is where foam sealing tapes are of benefit and add value to the quality of the final finish. The latest high performance products such as ISO-CHEMIE’s ISO-BLOCO T-Max tape can accommodate up to 36mm of movement (14mm to 50mm joint = MAF of 257%), with a single size tape, while remaining weather proof to Storm Force 10.

When sealing the windows in timber frame houses the gap at the head increases as settlement occurs, while the gap at the cill decreases with most of the movement occurring in the first 12 – 18 months, (movement can also continue afterwards for many years). The window should initially be located higher in the aperture than the expected final position, which means that the head tape will be in a more compressed state when initially installed, while the cill tape will initially be in a more expanded state. This will equalise or reverse following the settlement movement, but the tape will remain intact and weather tight.

Sealing the vertical sides also becomes problematical due to sheer movement which is equal to the vertical movement in size but again this can be resolved with tapes. The ISO-BLOCO 600, for instance, will adhere to the window frame only, allowing it to accommodate the vertical movement to the point of 45 degrees. After this point the non-adhesive side of the tape will simply creep down the brickwork during the settlement period, accommodating the substantial sheer movement that can tear a silicone seal apart. At all stages the tape remains intact and weatherproof with a constant flat surface finish.

The failure of the installer to allow for the unique movement requirements on timber frame structures will lead to serious problems. If insufficient movement allowance is made the window will over compress the seal under the cill and come up against an unmovable force as it rests on the external brick wall. This could lead to the window becoming deformed and not opening/closing properly, or could put sufficient strain on the fixings to the internal timber wall that it damages the internal wall finish. (We have heard stories of many such instances of this on various building sites.)

If a seal with insufficient MAF is used this will simply lead to the failure of the seal with the probable need to return to site to remove the defective material and re-seal the joints. This was common practice for some housebuilders in Scotland prior to the use of foam sealing tapes. Issues around moisture resistance, airtightness and thermal insulation for timber-framed buildings can also be addressed by a tape such as ISO-CONNECT HB-BAND, which provides an effective, durable and easy-to-install permanent horizontal seal to prevent moisture transfer (rising damp) between wall sole plates and foundation walls or floor slabs in timber framed buildings. At the same time the integral foam strips accommodate the unevenness between foundation and wall to give an airtight seal, without the need for additional materials.

Tapes can also be installed at very low temperatures and the installer doesn’t need to go over the seal with a tool to get an acceptable smooth finish, because tapes already have a flat finish when supplied. This advantage is provided by the open cell structure which ensures the surface remains smooth and flat, regardless of the expansion/compression cycle – the tape always expands and compresses with the same footprint/width as it was originally installed with.

Installation is quick and simple with sufficient compression to provide a weatherproof seal commensurate with the type of tape chosen, the average being 600 Pa  – at this level a joint can resist wind driven rain at violent Storm Force 11 (68 mph wind). Furthermore, because the tapes do not rely on adhesion to provide an effective seal they can be installed in the wet, saving time and money – a ‘breathable’ quality ensures that any trapped moisture in the joint will evaporate when the dry weather returns.

It’s clear that seals around fenestration and movement joints in timber frame as well as brickwork structures perform a vital function. However, the correct specification of jointing materials is essential in ensuring that the structure remains capable of accommodating changes and variations in gap size and achieving a high performance and long lasting installation.

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