External Wall Insulation in Timber frame – How to Detail Window & Door Reveals for rendering

Here, using some simple SketchUp models, we look at how to correctly detail our rendered wood fibre external wall insulation systems around window openings (which also applies to door openings) so that you can be sure you will not get wind-driven rain penetrating and causing issues or damage.

This video shows a quick run-through of the process and below are the more detailed instructions.

Step-by-step instructions

To correctly seal around an existing window you’ll need the following materials:-

Ampacoll SillSkin Self adhesive sill DPC

Ampacoll Primax primer

Ampacoll Komprimax Joint Sealing Tape

Ampacoll Fenax 40/60 window sealing tape

20mm Wood fibre reveal boards

Baumit StarContact White contact mortar for glueing the reveal board

In terms of installing the products, these are the steps to follow:-

Step 1

(click on the models to allow 3D viewing, full screen icon bottom right) In this model we show how the wood fibre boards are finished flush with the edges of the reveal and the Ampacoll SillSkin seal is applied to the sill. The SillSkin provides a waterproof barrier in case of any leakage around the sill and prevents water ingress into the masonry wall and the wood fibre insulation.

The surface should be cleaned of dust and loose material and the top edge of the wood fibre board should be cut to a fall towards the exterior, away from the window.

Cut the SillSkin so that it extends 50mm up each side of the reveal, 50mm down the front of the wood fibre boards and 50mm out either side of the opening. If the windows are already on site and ready to be installed there is no need to have the SillSkin layer extend right to the interior, only to the underside of the window frame. If there will be a delay in installing the windows the SillSkin should cover the entire sill area, as shown, to prevent rain penetration into the timber frame and wood fibre insulation.

Once ready, apply a coat of Ampacoll Primax Primer to all the surfaces that will be covered by the SillSkin and leave to dry. Once dry, remove the 50mm section of the backing paper on the SillSkin and, starting in the middle of the sill and working towards the corners, stick to the inner edge of the sill. Be careful – it sticks immediately and doesn’t let go!

Once this has been installed, remove the rest of the backing paper and, starting from the middle, stick the SillSkin down to the sill, stretching it out around the corners to ensure a continuous seal. 

For a detailed video on how to install this, please see this video:-

Step 2

(click on the models to allow 3D viewing, full screen icon bottom right)

Once the windows arrive on site they can be installed as shown. The Ampacoll Fenax DS 30/70 tape is applied to the edge of the frame, one strip on the outer edge and one on the inner edge. Apply the 30mm section to the frame with the 70mm section to be bonded to the reveals. In the corners of the window create a fold/flap in the tape that is roughly 1.5 times the width of the gap between the frame and the structural opening.

Apply a layer of Ampacoll Komprimax to the perimeter of the window to seal and insulate the void between the window frame and timber frame. Once the window is fixed in place the Ampacoll Fenax DS 30/70 can be applied to the surface of the reveal (the entire perimeter of the window frame), ready to be connected to the airtight/weathertight layers.

Prior to fitting sill, apply a strip of Ampacoll Komprimax 15mm sealing tape to the front edge and sides of the SillSkin to seal under the window sill once installed.

Step 3

(click on the models to allow 3D viewing, full screen icon bottom right)

With the sill installed and fixed, the Ampacoll Komprimax 15mm sealing tape is applied to the perimeter of the window (head and sides) and the top of the sill end cap, to seal between the window and reveal board.

Step 4

(click on the models to allow 3D viewing, full screen icon bottom right)

The 20mm wood fibre reveal boards are glued in place using the UdiSpecial Adhesive wood fibre glue. These should be cut so they fit firmly against the Ampacoll Komprimax foam tape, keeping it compressed, so that a good seal is formed. The board at the head of the window should be applied first, followed by the two sides. This can be held in place until the glue has dried with some timber supports.

Any Ampacoll SillSkin tape that remains visible should be covered with a layer of Ampacoll Fenax tape to ensure that the render bonds to the surface correctly. The SillSkin is not directly renderable.

Once the above is completed, the system is ready to receive the render.

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