New(ish) sash windows - maintenance / refurb / repair.

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Blackadder

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Hi all.
I have lots of Sash windows installed about 10 years ago - hardwood frames / sashes with double glazed 4/16/4 units in a 30mm rebate with a 6 x 16mm external bead. Seems like the design was badly flawed because the sealing between the bead and the glass has failed on most windows, allowing water to sit on the bottom rail causing algae and mold to grow on the glass and soak around the glass onto the inner of the rail and also distort it. What is especially interesting is that there are no spacers between the rail and the glass unit - it sits tight in the rebate on all edges.
So 1st question - is the lack of a 5mm spacer at the bottom (and gaps at the sides / tops) the root cause of the sealing failing? If so, I'll need new units (~5mm smaller each edge) and may well have problems with it affecting the sight line (cost and complications). If not then maybe it's a case of cleaning and drying it all up and rebuilding and resealing. Any suggestions on how to seal? What I suspect makes this tricky is that the bead is so small (only 6mm wide) and the front of the house is VERY exposed south facing - so lots of baking sun and rain like its from a fire hose (although same issue - just not as bad - on other elevations.
Any advice appreciated.
D
 

Jacob

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10 years is not good - modern DG sashes usually last 15 to 20.
I don't think there is a solution other than make do and mend.
 

RobinBHM

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unfortunately you have a window that is technically poor.

I imagine these windows were made by a small local joinery shop.

what they have done is squeezed a 24mm building regs compliant double glazed unit into sashes that areabout 45mm thick -its been done to keep the box frame depth down.

what size is the rebate? if the bead is 16mm is the rebate 16mm?

if so then the units could have been at least 4mm smaller all round.

how are the beads fixed in? -I cant see any pins being able to go into a 6mm x 16mm bead?
and what material was used for sealing -was it silicone?
 

RobinBHM

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You have a few options

you could replace with 4-12-4 planitherm total+ with warm edge spacer and argon gas fill -from memory that will get you a pretty similar centre pane value to your existing glass.

that would allow you to use a glazing bead of say 12mm x 16mm.

if you then got a joiner to make you some glazing bead -get a rebate of say 5 x 5mm on the corner where it meets the glass. You could then glaze the windows with neutral cure silicone -using a bead of silicon around the glass before putting in the beading.

Then once thats cured, go around with a mastic gun and fill the 5mm x 5mm rebate which will seal between the face of the glass and the timber bead.

alternatively you could go for a Q-wood bead and fit with silicon on the back of the bead where it meets the glass -not as secure as the 5mm x 5mm rebate though.

 

Doug71

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A photo would be useful.

This is a page from my glass suppliers website, if you scroll down it shows the different U-values for different cavities etc.


An expensive option is the Slimlite/Heritage style units which are Krypton filled.
 

Blackadder

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unfortunately you have a window that is technically poor.

I imagine these windows were made by a small local joinery shop.

what they have done is squeezed a 24mm building regs compliant double glazed unit into sashes that areabout 45mm thick -its been done to keep the box frame depth down.

what size is the rebate? if the bead is 16mm is the rebate 16mm?

if so then the units could have been at least 4mm smaller all round.

how are the beads fixed in? -I cant see any pins being able to go into a 6mm x 16mm bead?
and what material was used for sealing -was it silicone?

Thanks - Robin' - I suspect your opening two opinions are only too true. The parting and staff beads all appear to be s/w, and finished poorly - maybe just primed / u/c - certainly not to the same finish to the h/w shop built and sprayed frames (so corners cut at install?). And yes - small local shop that went bust a few years after the final delivery- Aargh!!

The box frame (outer face to outer face) is shown on some drawings I have as 140mm with 45mm deep sashes, as you say.

The rebate is shown on the drawing as 15mm, so I may have mis-measured the bead (or it may be 1mm bigger on the outside).

When you say the units could have been 4mm smaller all round, is this limited by the DGU spacer becoming visible? Are these of standard 'height', i.e. distance form the edge of the glass? Looking at the windows, I think there is ~4mm of the bead visible, so this would suggest you are correct.

The beads are fixed with VERY fine pins (0.5mm), at ~5 per meter. I doubt they ever did much. There is a rebate in the back of the bead which appears to be filled with a hand applied sealant (quite a few voids / air pockets where slightly insufficient sealant used). Not sure if it is silicone - it feels very hard (you can scratch it off with a fingernail).

But the key question is - how critical is a gap between the glass and the sash? Is this the primary reason for the failure of the sealing? I am concerned that is I clean it all up and rebuild with the same DGUs the problem will just reoccur.

But thank you!

D
 

Blackadder

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And some pics.....

Bottom rail, external view(!) with and without bead.
 

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Blackadder

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...and the inside of the bead. you can make out the rebate (where little sealant) and the air pockets. (and the algae). 😩
 

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Blackadder

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You have a few options

you could replace with 4-12-4 planitherm total+ with warm edge spacer and argon gas fill -from memory that will get you a pretty similar centre pane value to your existing glass.

that would allow you to use a glazing bead of say 12mm x 16mm.

if you then got a joiner to make you some glazing bead -get a rebate of say 5 x 5mm on the corner where it meets the glass. You could then glaze the windows with neutral cure silicone -using a bead of silicon around the glass before putting in the beading.

Then once thats cured, go around with a mastic gun and fill the 5mm x 5mm rebate which will seal between the face of the glass and the timber bead.

alternatively you could go for a Q-wood bead and fit with silicon on the back of the bead where it meets the glass -not as secure as the 5mm x 5mm rebate though.

....and thank you also for your second post.

Understood re thinner DGU and bigger bead.

However, don't understand the idea of the 5 x 5 rebate. Is this in the right angle corner of the bead (where bead, glass and frame all meet)? Or maybe at the thin part of the bead - giving a visible 5x5 silicone sight line? If the latter - how easy is that to get it neat?

Thanks again!!

D
 

Blackadder

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A photo would be useful.

This is a page from my glass suppliers website, if you scroll down it shows the different U-values for different cavities etc.


An expensive option is the Slimlite/Heritage style units which are Krypton filled.
Thanks Doug

Pics now posted. Very ugly. :-(

Website very useful - great reading, cheers.

D
 

Doug71

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DG units fail when water gets in, it normally gets in behind the bottom glazing bead which looks like is your problem.

The units want a gap around them, with a 15mm rebate you will get a 3mm gap around without spoiling the site line.

Some people recommend drained/vented glazing which in theory lets any water that gets in out again but yours look fully sealed. Fully sealed is fine as long as they stay fully sealed which means keeping on top of maintenance.
 

Silfi

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Very unusual to have no clearance around glazing fitted to a wood window. Sides and top have a small gap (2mm) but bottom can be bedded on frame.
I am very surprised the DG units have not failed with the rot. Cannot see how it is possible to get a satisfactory repair without removing the units---is that possible?
 

Blackadder

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DG units fail when water gets in, it normally gets in behind the bottom glazing bead which looks like is your problem.

The units want a gap around them, with a 15mm rebate you will get a 3mm gap around without spoiling the site line.

Some people recommend drained/vented glazing which in theory lets any water that gets in out again but yours look fully sealed. Fully sealed is fine as long as they stay fully sealed which means keeping on top of maintenance.
Thanks Doug. Yes, my windows (rebate) is definitely the sealed type. My critical question is whether the sealing of the bead to the glass failed because:
1) No gap between DGU and frame - expansion issues(?)
2) Bead too small - unsupported(?), lack of meaningful pin(?)
3) Wrong sealant - although don't know what was used
4) Lack of maintenance (although I believe that the seal was compromised early in the life of the units)
5) Operating environment - sun and rain (although I doubt that this is unique to me - albeit we are in an exposed spot)

Maybe a combination of 2 or more (all?!?) but trying to figure this out is important to improve the chances of making as cost effective repair as possible that is fully effective.

Thanks again
D
 

Blackadder

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Very unusual to have no clearance around glazing fitted to a wood window. Sides and top have a small gap (2mm) but bottom can be bedded on frame.
I am very surprised the DG units have not failed with the rot. Cannot see how it is possible to get a satisfactory repair without removing the units---is that possible?
Thanks Silfi

Yes - the 'how to repair' (when I have established what repair to make) is a good question!

At present, I have bought and fitted polycarbonate sheets over the outside of two of the windows, and am ventilating the gap between. This is proving very effective at drying the sashes, whilst maintaining some level of insulation / security - but I will need to remove the sashes to effect the actual repair (remove DGU, clean, paint, replace DGU, rebead etc etc according to whatever solution I end up with) at some point, before moving onto the next windows. With 19 windows to do, this is going to take a while!! For sanity's sake, I'll probably end up with at least 2 more poly carb sheets - which will help speed things up.

But any other suggestions very welcome!!!

D
 

RobinBHM

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....and thank you also for your second post.

Understood re thinner DGU and bigger bead.

However, don't understand the idea of the 5 x 5 rebate. Is this in the right angle corner of the bead (where bead, glass and frame all meet)? Or maybe at the thin part of the bead - giving a visible 5x5 silicone sight line? If the latter - how easy is that to get it neat?

Thanks again!!

D
If rain was running down the glass, where it gets to the glazing bead, the first thing it would hit is the 5mm x 5mm silicone.

That is exactly the place where your existing glazed windows are leaking.
 

Jacob

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If you clean it all up when it's dry in June (if we are lucky!) couldn't you just put a putty fillet where glass meets bottom rails, in place of bead? I'd prime the wood with shellac first and then do a proper paint job when the putty has gone off a bit.
I wouldn't bother with sprigs or anything if the other 3 sides are well held with beads - it's unlikely to budge with all that weight.
 

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