External Gutex insulation
July 18, 2014
This week we almost completed the 100mm thick layer of external Gutex Multitherm insulation, which has several key functions in the construction. Firstly it forms a continuous layer of insulation around the timber frame and therefore helps to eliminate any thermal bridging. Secondly it protects the Spano Durelis board and timber construction below from the weather as it is water resistant and forms the weather-proof layer below the open rain-screen cladding. Thirdly it forms the wind-tight layer on the outside of the construction, which will help to minimise heat loss over the building as a whole: to minimise heat loss the construction must be both airtight on the inner face of the insulation zone and wind-tight on the outer. (A good analogy I have read for this is that it is similar to wearing a wind-breaker jacket over a wooly jumper!)
We chose Gutex Multitherm as it is made in a single-ply homogenous construction which means that it is resistant to de-laminating, although this is slightly less significant in a thinner layer.
The Gutex is held in place via the vertical cladding battens, which in turn are fixed all the way back to the primary structure using 220mm or 260mm long Heco-Topix screws. The shorter of these screws go in to the construction horizontally to resist suction on the facade and the longer ones are fixed at an upwards angle of 30 degrees to resist the shear loads.
The first stage in the process is to fit a plastic angle, which sits on top of the Isoquick insulating formwork and the DPM, and which forms a drip at the base of the wall. We just used a standard rendering bead and removed the mesh tape. This in turn protects the base of the Gutex from any water which may enter the ventilated cavity behind the cladding and run down the face of the Gutex, as it sits proud of the Isoquick which allows the water to drip off the base of the wall to the ground below. Later we will tape this angle to the Gutex to help hold it in place and to complete the wind-tight detail at the base of the wall.
The boards are tongue and grooved on 4 sides, which means that they do not require taping over the joints to create a wind-tight layer. The only places we will need to tape are where the boards have a butt joint, for instance at the external corners, or where we have had to use a butt joint to minimise wasteage, for instance around door and window reveals.
The Gutex over-sails past the edge of the window frames by 30mm in order to create a fully insulated reveal over the timber window frame - I will come back to this in more detail in a few weeks time when describing the window and door installation....
At the very top of the parapet the top row of Gutex boards are slid up underneath the felt roof flashing, which protects the top of the insulation from the weather. The top of the battens extend slightly above the felt roof drip, and are notched over the thickness of the felt. Once they have the last screw fixed through they firmly clamp the felt in place, which in turn helps to resist any slumping which may occur over time due to the length of the flashing piece of felt that is required over the top of the parapet. The very top of each batten is cut back on a 45 degree angle to allow any rainwater falling here to run-back onto the inward slope of the parapet.