reader’s request: more information on window fitting

I received a request for further information about how we fitted the windows to minimise thermal bridges. So here goes:

Window fitting stage 1

Each window unit has been fitted into an external structural timber frame, or sub-frame. The sub-frame is standard C16 grade timber and doesn’t need to be tanalised as it shouldn’t become wet within the insulation layer. The size of the timbers in this case were chosen to be 60mm deep for reasons outlined below. The first timber fitted is the sill piece to act as a ledger to support the window during positioning and alignment. The window unit is fixed back to the masonry using nail plates, but most of the weight of the window is on this ledger, so it must be a good fix. The other three sides of the sub-frame can then be fitted snugly around the window:

Window fitting stage 2 Window fitting stage 3

In our case, the surrounding masonry was treated with a slurry (sand/cement) render base coat, to act as the primary air barrier. So, once the windows were fitted and the render had dried, we could deal with the airtightness sealing – sealing the window to the sub-frame and the sub-frame to the wall, using proprietary air sealing tapes. In hindsight we made an minor error here. We should have taped the sub-frame to the masonry and then rendered over the top of the render layer tape (black tape).

We chose 60mm deep sub-frame because we were using 2 x 60mm layers of insulation. The thickness of the window frame is 78mm. So the idea was to have the first layer of insulation run into the side of the sub-frame and the second layer would fly across the face of the window frame by around 40mm as shown:

window detail

Again, with hindsight, we should have used sightly deeper timber for the sub-frame (say 70-75mm) to account for the 10-15mm adhesive layer for the first insulation sheet. This would have saved a bit of  ‘shaving’ on the second layer, which, of course is a slight thermal compromise too.

The picture below shows the first layer of insulation running into the sub-frame.

window insulation

I believe the principle we have adopted is one of the best ways of minimising thermal bridges between the window frame and the surrounding masonry. We could have gone one step further and used a structural insulation material for the sub-frame, such as CompaCFoam, manufactured in Austria, but I was put off a bit by the costs (around 10x cost of timber), and, as we haven’t gone for ultra-high spec, thermal break window frames (£££), I didn’t think it was a worthwhile investment in this case (investment greater than energy saving). But, it’s worth a look.

Hope this helps and would be interested to hear if others have adopted a different approach.

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week 23: a sealed envelope

bi-folding doors installed

The last of the doors and windows are fitted to the extension area. The last of these units to be fitted was the bi-folding doors on the lower ground floor. With some of the scaffolding out of the way temporarily (to allow the balcony to be built), we are able to see what the house will look like.

More importantly, we are now able to start proper drying-out. A de-humidifier and a heater are now set-up and the space is now heated!

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week 18: new window installation begins

Living room window removed  Bedroom window removed

At last, the new triple-glazed windows are being fitted. The old windows are removed, temporarily exposing the house to the elements. A timber frame surround is constructed around the structural opening as we are moving the new windows forward of the existing elevation. This is to allow them to be installed within the external insulation layer, thereby reducing thermal bridges.

Bedroom window fitting  window detail

The windows are fixed into the new surrounds and then taped to form the air seal using Tescon tape. The timber surround is taped to the masonry wall using Contega EXO tape and Orcon F sealant. However, because a lot of this work has happened in between wet and cold weather, adhesion has not been optimum. We have therefore taken the additional precaution of applying a second render base coat on top of the Contega tape.

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week 14: windows and doors arrive

 

The new triple-glazed windows arrived this week. Made in Lithuania, the EcoContract windows, sourced via the Green Building Store, arrived in-tact. They are still mostly in their packaging, so have only really checked the colour match is correct. But they look good, really good. The bi-folding doors (top left photo) weighs some 400kg and we will need to dismantle this before we can move it into place – more to follow.

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