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Wooden Window Frame External Beading Double Glazing and Correct Method of Refurbishment

Postby NickDB » Wed Jun 08, 2016 6:43 pm

I have a large non-listed building in Wales that at some point has had timber framed double glazing windows installed. The place was badly mantained before I bought it and I'm now in the position of stabilizing it as is before converting it into a live/work unit for my business.

The windows are all timber frames, with external timber beading holding in the double glazing units. There are a mix of windows, but the main ones are 4' x 8' (1.2x2.4m) with six panes. Al the beading is in very bad condition and the windows need painting although there is no rot in the wood (yet!)

I need to refurbish/replace the wooden beading and paint the windows before they start to rot (too much!)

My initial research was that butyl glazing putty should be used between the double glazed units and the window rebates and (probably) the beading.

Subsequent research seems to indicate that I should use a low modulus neutral cure silicon sealant between the external beading and the double glaze units. I will pin the beading with 25mm stainless steel 18g brad nails with a pneumatic nail gun (I have an air compressor in the building).

Should I fill the void behind the beading with the silicone sealant? (ie the 3mm gap around the double glazed unit as it sits on its spacers and positioners.) Some people say yes, others say it will trap water and cause rot. I'm not sure if it is a ventilated drained design.
Should I put silicon on the bottom part of the bead (ie the wood to wood boundary between the beading and the window rebate.)


The window frames are wedged in place. The building dates from 1910 and lime mortar was used in the construction, and there it is a 2" (50mm) cavity wall of brick. There is a large airgap between the window frame and the brick of the wall (from 5-30mm), and you can see light through the sealant the previous owners used to cover the gap. This lack of airtightness seriously compromises the building's soundproofing and thermal efficiency. I need to fill the gap - is foam an acceptable solution for this or will it cause condensation to form on the window frames and cause rot?

Any other advice/comments welcome. Thanks!
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Simply Build It

Postby welsh brickie » Sun Jun 12, 2016 8:23 pm

remove all the beading and use, as you say a low module clear silicone, fill the void around the units, and bed the new beads in aswell.
Yes use expanding foam for the gaps this will insulate and seal the frame.
As for condensation occurring, usually a trickle vent is installed, if this is not possible, then a vent in the wall may have to be installed
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