DIY Doctor UK DIY Forums

Hi,
We have just had a couple of our old wooden windows replaced with UPVC and were due to paint the new interior wooden cills when we noticed that there is frequently moisture (and now some mould) soaking into the interior wooden cill along the bottom of the window and also a few inches up the sides. This appears to happen when the outside temp is particularly cold (like much of this winter!!). When we looked outside we saw that there is a gap all along the under side of the frame, between the frame and the lead tray, big enough to just about push my fingers to the back of the frame. I can certainly see the underside of the frame. The exterior cill is the original cement tile cill and does slope quite steeply. We presume that the moisture was because cold air was coming under the frame directly to the turn up of the lead tray and the wooden cills and then meeting the warm air in the house. I spoke to the (FENSA registered) fitters about sealing this gap and they have said that they do not see a problem and that we should leave the windows ajar because it is the windows being too efficient?! The windows are opened on a regular basis, but obviously not when it is freezing outside! I would have thought that some expanding foam and a bead or some cement along the exterior would have done the job and prevented airflow under the frame? They say that this is difficult because of the expansion and contraction of the lead? Can anyone PLEASE advise?? Thanks
Simon

duffer2502,
Not sure about your reference to a lead tray - if a piece of lead has been fitted under the new window frame and turned up on its inner edge then you have cold bridging. This is almost impossible to overcome with this installation method.
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Hi Stoneyboy,
Thanks for your reply. Please excuse my reference to a lead tray, I had thought that was what it was called.... it is a piece of lead as you describe. My understanding from what you say is that the lead is transferring the cold rather than any breeze/wind under the frame? Should or could the windows have been installed differently in order to prevent 'cold bridging'? I was hoping it was going to be as simple as me filling under the frame with foam or cement? I really don't want the interior cills to rot! For info, there is no exterior pvc cill fitted (the frame finishes then there is the gap and then the lead) if that makes any difference? Should the windows be reinstalled from scratch? Sorry for so many questions! If you can advise I would be extremely grateful! Many thanks again!

duffer2502,
I assume you have upvc windows with no cill and the original masonry cill has been retained. Since the old cills "slope quite steeply" I do not see any reason for the lead flashing - the upvc windows should have been sealed to the original cills with mastic.
The question is why the window installer deemed it necessary to add the lead flashing.
You could try filling the gap under the windows with foam which would reduce the temperature gradient over the depth of the lead but lead is a very good conductor of heat so I doubt you will remove the condensation problems.
Really this is down to the installer to correct and if they say "they do not see a problem and that we should leave the windows ajar because it is the windows being too efficient?! I would suggest you mention male cattle and their poos.
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Hi again stoneyboy,
Thanks for your reply. yes they are upvc frames with no cills..... the cement cills are approx 20-30 degree slopes. I am thinking from what you say that the best option would have been to have a small upvc cill at the bottom of the frame and no lead on the cement cill. I think that the lead was down to my chippy/builder suggesting that because of the timber frame construction of the house I would need the lead to protect the cavity/timber? I conclude then that there is not really a diy solution in terms of filling the gap with foam etc due to the conducting qualities of the lead and therefore that I should have the windows removed and a upvc cill put in and the whole thing sealed directly to the cement as they have done to the sides and top of the window?Would you concur? Your input has been extremely valuable... thanks! Simon.

duffer2502,
I think you will have to be guided by the type of original timber window which was fitted. Did these have a projecting cill?
If yes then the window company have not replaced like for like.
If no then the new windows should have been sealed to the original masonry cill (as per the sides but back behind any drainage holes in the frame), no lead fitted and preferably a cavity closer fitted under the new wooden window cill.
Fitting cills to the upvc windows is now going to be difficult and will probably entail completely new windows.
If it is feasible I suggest that the windows are dropped onto a bead of sealant and a drip moulding fitted to the top to conceal the gap. Do ensure that any drainage holes in the bottom of the frame are not sealed off.
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