External wall insulation & airtightness elements started!
August 22, 2014
After a 2 week break from works on site for summer holidays, the last 3 weeks in August have seen us move on to progress the external wall insulation from the inside of the building, and begin some of the internal racking board, which is our airtightness layer.
We are using Steico Flex insulation, which is a flexible wood fibre board. As the insulation is installed in 3 layers of varying thicknesses, the first task was to complete the 100mm thick layer that fully fills the outer-most frame.
Here is the double height space with the first layer of 100mm thick Steico Flex insulation in place throughout...
From this layer inwards we are working on a room by room basis as the wood fibre insulation tends to swell when it is humid and if it is unrestrained, and it has been very wet and damp here recently! It is best practice if it is fitted in winter to install the internal vapour barrier immediately upon fitting, which is to prevent the insulation taking up too much moisture.
Below is the process we are following, using our future kitchen as an example, as this is the first room we have completed to the inner racking / airtight board level!
So, the first step is the 100mm thick layer of Steico flex in the external frame... In the second photo you can see the reverse of an external corner which we had to insulate from the outside of the construction, and which I described in the previous post!
The next step is to fit the 60mm thick layer which also has to be inserted between the gussets to make sure there are no gaps. As this layer is our second 'continuous' layer of insulation in the twin-wall construction, it has to be built up in several strips: the first is cut to fit between the inner and outer frame studs, and cut to fit for height in the spaces between the gussets, and then an additional shorter piece is added that just fits neatly between each pair of gussets, and so on up the height of the wall. We still have to make sure that the pieces are cut slightly over-sized both in width and height so that the layers are well packed together with no gaps that air could potentially move around in. This layer is definitely the most time consuming as you can imagine!
Then the final layer or 2 (depending on the wall depth) is fitted over the top. This then completes the external wall insulation, with the exception of a final layer that goes in the service cavity zone. This means that at this stage we have a total of 400mm thick insulation installed on the thinner walls, and 500mm on the thicker walls!
The next task is to fit the Spano Durelis Vapour Block boards which will keep the slightly over-sized insulation in place. These boards are similar in composition to the external racking board 'Populair' that we used on the outside of the frame, however this time they are factory fitted with a transparent vapour control and airtightness layer on one side, which faces in towards the building. We decided to use this board as our artightness layer as it is more reliable than OSB in terms of airtightness and eliminates the need for a separate vapour control layer. It is also a structural racking board, which we needed two of due to the design of the structure.
We are fixing this using a combination of screws and nails (screws to pull the boards into place and nails for the remaining fixings for speed), however it can also be stapled to the frame behind.
Finally, we then taped the joints with Tescon Vana tape, which is an airtight multi-purpose tape. We also taped over the nail and screw holes to make sure that the airtightness layer isn't compromised! The top edge will be taped to the Intello membrane we left hanging down over our intermediate floor junction later, the edge at the window reveals will be connected to the window / door frames using some more Intello membrane, and the base will be taped to the concrete floor using DA-S roll. More to come on these details in a later post about airtightness specifically!