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

[RobinBHM]

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

It is a key reason for failure, yes.

Double glazing is usually installed as a drained and vented system.

So an air gap is required all the way around and at the bottom, there should be drain holes drilled so water can escape.

If DG units are in contact with water constantly they break down.
No gaps means higher risk of premature failure.

Typically DG unit spacer bar sightline is 12mm, so a rebate of 16mm will allow a gap of 4mm

Double glazing can be fully encapsulated and that's how slimline glazing is done, but it's not recognised as optimal.


Double glazing is made by a having one sheet of glass on a bench, then spacer bar, which is about 6mm wide is laid on the glass its about 6mm in. Then the 2nd glass pane is laid on. Then hot melt glue goes in between the glass, sealing the glass and the spacer bar. Now that joint is waterproof, but if it's constantly wet and combined with temperature changes, slowly the water will work its way in.

I've seen DG units with 6" of water in the bottom!

 

[RobinBHM]

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.

Face putty is an option, although trad linseed putty is not suitable.

Acrylic putties like dryseal would be the correct product, but given where we are, I'd say that would adhere well.

 

[Jacob]

Face putty is an option, although trad linseed putty is not suitable.
....

Why not? (not arguing just wondering )

 

[Blackadder]

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.

Thanks Jacob

Is there any particular benefit of putty over a bead (or visa-versa)?

D

 

[Jos7000]

My suggestion would be, retain the units that haven't failed. Sand back to wood and glue extra h/w to increase the depth of the sash. Get some flush pins (flat things) then linseed putty to seal. Silicone doesn't last long on south facing windows, putty however can survive for decades.

I suppose it's all down to how much you want to spend and when.

 

[RobinBHM]

Why not? (not arguing just wondering )

It's because linseed face putty has oils which can soften the hot melt in DG units.

Also trad face putty has a tendency to dry out and crack....I know this because I did 20 windows with through bar Georgian and slimline glazing.....most of the units broke down within 18 months. Luckily for me I ordered from a glazing company that supplied and fitted them, so they replaced them. They used linseed face putty and it had cracked and shrunk away from the glass.

 

[Ollie78]

Make new sashes is the best answer. I have seen too many like this.

If you don't want to replace them, then seal around them with Dryseal or Hodgesons heritage putty (ie hybrid polymer sealant not putty) . You will need to remove the units, clean up the rebate, bed them on the sealant (do not use silicone) then pin them in with a sprig gun or flat pins. Then face them with the Dryseal or Hodgesons.
Double glazed units in this type of application should not be installed in a vented gap method. The sealant must be non reacting to the glue on the units and should encapsulate the unit entirely.

Ollie

 

[Jos7000]

Linseed putty has linseed oil in it hence the name. Linseed oil is a drying oil, so it won't affect anything for long, however I imagine if it is applied and subjected to extreme heat or damp too soon, it will be impaired.
I renovated one of my Victorian sash windows last year and the putty had only cracked at the bottom of the pane and that was after 120 years.

 

[Ollie78]

The problem with the linseed oil putty is that it can eat away at the glue in a DGU.
It also has no structural properties until very dry.
This is why if you really must use it you must seal all around the unit first with a silicone or hybrid polymer sealant. Also birds eat it and it takes ages to dry before you can paint it.

Ollie

 

[Jos7000]

Ollie 48 - 72 hours and you can paint it.
Modernisation is great, new dgu's that are weak in so many respects. There must be some way of treating them to prevent linseed from damaging them.
Such as an epoxy paint.

 

[RobinBHM]

Make new sashes is the best answer. I have seen too many like this.

If you don't want to replace them, then seal around them with Dryseal or Hodgesons heritage putty (ie hybrid polymer sealant not putty) . You will need to remove the units, clean up the rebate, bed them on the sealant (do not use silicone) then pin them in with a sprig gun or flat pins. Then face them with the Dryseal or Hodgesons.
Double glazed units in this type of application should not be installed in a vented gap method. The sealant must be non reacting to the glue on the units and should encapsulate the unit entirely.

Ollie

That's the problem with the existing windows....it seems the construction is a halfway house between drained and vented and fully encapsulated.

Weirdly it seems the rebate is 16mm so big enough for a drained system, but the space left for the bead is too small.


Fully encapsulated systems usually have the DG unit bedded on silicone, then silicone is used to fill the 1mm or so gap and a small bead on the outer face.
When dry, face up with hybrid polymer.

For a diyer, I would recommend fitting new beads with a rebate for silicone against the glass.

Trying to face up with hybrid polymer can be awfully messy, it's really tricky to do.

 

[Jacob]

Thanks Jacob

Is there any particular benefit of putty over a bead (or visa-versa)?

D

A putty bead can last 100 years or more as long as it is touched up with a bit of paint occasionally. How they work with DG I do not know. I gave up on DG in restoration projects after the first few failures, but have done acres of single glazing.
I see a suggestion of using metal pins above. In my experience these are doomed to fail - or cause the putty to fail at that point. If glass or putty cracks it's often the sprig which has caused it, maybe years after it was put in.
I have used glazing sprigs but only ever as a temporary holding whilst putty went off enough to hold the glass (if holding needed). Then to remove the sprigs and finish the putty.
Your photos show deteriorated paint on the bead so that must be primary cause of the failure. Blame lack of maintenance or blame the paint!
Basic prob with new ways of doing things is that it takes 20 years or more to discover whether or not they will last 20 years or more. So departing from known tried and tested methods and materials is a complete gamble.

 

[Ollie78]

RobinBHM

You are right about these windows being the worst of all options.
Hence my suggestion of replacements in this case.
I see no point bedding on silicone and then using a hybrid, my supplier of slim units has recommended hybrid for many years now and it works superbly well.
You are also right about facing with the sealant it is a knack.


Jacob

Not disputing that putty can last well but it is really not suitable for this application.
The sprigs are fine, they are just to hold it until the sealant dries. I suggest that they are more likely to cause a problem when using putty. With a modern sealant they are basically encased in rubber once it dries. I have been using slim DGU's and dryseal for at least 15 years, never had a unit go yet ( touch wood) and the system has been around much longer than that.


Ollie

 

[Blackadder]

Thanks everyone for your input - feel much wiser, but still a bit unsure on exactly what to do. I'll post questions to your various responses, but all very much appreciated.

 

[mr rusty]

I agree Ollie78 in post #33 has the solution - use 4-6-4 units and get some decent beading.

FWIW I made a load of sash windows about 2-3 years ago now. Mine were internally beaded. The DGU were dry-glazed using clips from reddiseals and the glazing had lipped EPDM tape on the outside, and ribbed EPDM (white) tape on the inside. No sealants used at all except for a smallest clear silicon on the lower outside seal mitre joints. Mine used 24mm DGU, but my sashes were 56mm thick instead of 45mm.

This method was copied/adapted from the CAD drawings one of the "big name" sash window manufacturers kindly put out on the internet for their internally beaded conservation range...