Draughty new double glazing

a1b2c3

I've just had new double glazed units installed as part of an extension build. There is a noticeable draught at the base of the windows, and the internal windowsill is very cold. Looking at the outside, I can see a clear gap between the upstand on the sill, and the base of the window unit (in fact you can actually see light from the room coming through small gaps around the internal windowsill if you look into the gap from outside). It appears that cold air is just blowing straight through here and into the house. Unfortunately I am unable to post a picture link here.

I discussed this with the builder, who advised me that the work is not finished, and he will be adding sealant to fill the gap. I want to make sure this is fitted correctly, so can someone advise what the correct installation method is for a double glazed unit like this? Am I correct to assume that the unit should butt up against the sill, rather than having such a large gap? Is it correct to just be filling the gap with sealant? The double glazing manufacturer has not been very helpful with installation advice, and says it is up to the builder!

Thanks in advance for any advice.

Furts

A typical detail is the window is made 3-5mm smaller all round than the opening. It is fitted into the opening maintaining this gap. This allows for expansion and movement. The gaps are sealed with silicone from the outside and made meticulously water and weather proof. The gaps are also sealed from the inside to prevent heat loss from your room. Silicone is best here before your window is plastered or dry lined. This sealing is an area where scimping can occur so make sure it is done properly.

Hope this helps.

phil24_7

The window frame should not be sealed to the window sill. It should rest on it only. This is to allow any condensation from inside the window frame to escape out of the bottom. If this is sealed then any condensation could potentially pool up inside the frame until it finds a way out...probably into the cavity! Around the frame and under the sill should be sealed with external frame sealant.

Regards
Phil

a1b2c3

The bit I don't understand is how you prevent heat loss around the glazing unit if it if not sealed to the sill? I can see a clear cavity under the glazing unit, behind the sill. In fact, looking from outside, on the far side of that cavity, I can see internal room light through small gaps around the edges of the internal windowsill woodwork.
So air and heat can flow straight through the cavity from inside to outside, under the glazing unit.
What is the correct way to seal this? My builder seems to be talking about putting sealant in the external gained between glazed unit and sill.

phil24_7

The window should sit in a small channel on the sill. This will prevent air leaking in as it will but uo agains 2 surfaces. It sounds as though this has not been done properly.

Can you put a picture on a hosting site and then post a broken link (put some spaces in it) so that we can see what you are talking about?

Regards
Phil

phil24_7

https://www.angelplastics.co.uk/application/images/Product_category/1406640619_cill-pic-for-brochure.jpg

The window should sit flush against the flat bit in this picture, butted right up against the lip at the back.

a1b2c3

Thanks for the replies. Here is a broken link to a picture that illustrates my question (replace pluses with dots and dashes with slashes!)

ultimatehandyman+co+uk-forum1-download-file+php?id=39340

Hopefully you can see the clear gap between the glazed unit frame and the sill upstand if you zoom in.

Ectophile

http://ultimatehandyman.co.uk/forum1/download/file.php?id=39340

a1b2c3

Thanks for fixing the link!

phil24_7

Can you put another up showing the whole window so I can get an idea of the size of the gap in relation to the window? Maybe one showing the inside of the window at the bottom too?

There doesn't look to much wrong to be honest.

Regards
Phil

a1b2c3

There's a clear gap between the bottom of the unit and the top of the upstand of approximately 3mm (and obviously a greater distance between the unit and the surface of the sill).

There is a clear air path from the outside, under the window unit and in behind the internal window sill. If you put your hand at the bottom of the window trim inside, you can feel the wind blowing through, and the whole internal windowsill is noticeably very cold; heat is just soaking straight out of it. That's what's wrong!

When I asked about this, the builder told me that the bottom of the window was not sealed yet because of wet weather (but didn't point this out when the window was initially fitted). That gap doesn't look like it could be sealed effectively with silicon, and I would have assumed that a void under the unit like that should be filled with something, or the window should be sitting on top of the cill?