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Replacing single glazing in timber sash and case windows

Discussion in 'Windows and Doors' started by wilco88, 1 Nov 2020.

  1. wilco88

    wilco88

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    I want to install double glazing to my single glazed timber sash and case windows.
    I realize that is possible to do this by simply adding a new sash with a double glazed unit, however my question is:

    Is it possible to install double glazed units of glass in between the existing muntins?
    My style of window is a half georgian (see attached example).
    I realize that for the bottom sash i could take out the existing single glazing, router out the channel around the sash to allow a thicker double glazed unit to fit in (as long as the thickness of the sash allow this).

    However, what about for the top sash? Do i have individual double glazed units made to fit in between each muntin and router out around each muntin? Or do i remove the muntins and fit a double glazed unit to the sash and then add new muntins on top?
     

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  3. endecotp

    endecotp

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    Is it a listed building / conservation area?
     
  4. wilco88

    wilco88

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    Yes, but ignore that for the purposes of the question please.
     
  5. endecotp

    endecotp

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    Well I could waste my time describing the different options, but it would be more efficient to find out what your local authority would accept by consulting their guidance documents and then we could concentrate on that.
     
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  6. wilco88

    wilco88

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    Put it this way, it's in a conservation area but all three other properties in the row (including the directly attached property) have had their timber windows replaced with uPVC and not in a similar style to the original windows.
    I am not interested in uPVC windows so my original question still stands.
    If you have time to describe the different options that would be much appreciated. Thank you.
     
  7. endecotp

    endecotp

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  8. wilco88

    wilco88

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    Thanks!

    Any further info from the community on what has worked best for them would be great. If anyone has any experience of adding double glazing to existing windows themselves (as in, actually doing the physical work) i'd be interested to hear from you.
     
  9. Notch7

    Notch7

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    The short answer is: yes you can

    You need slimline units, typically 4-4-4 i.e. 12mm thick overall with a spec of soft coat low e and krypton gas fill. It needs special small margin of about 7mm

    The glass is bedded on a small bead of low modulus silicone, then once set a bead is run around the outside to fill the gap between glass and rebate. You can't use linseed face putty it needs an acrylic putty like dry seal MP.

    You will need to change the original weights for lead and rebate the sashes to accommodate the increase in glass thickness - which is possible as there will be a flat of 13mm or 16mm before the moulding.

    Reddiseals sell lead weights.

    Be aware slimline units are £25 each for those small Georgian panes
     
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  11. wilco88

    wilco88

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    Thanks Notch7.
    I'm really impressed with how useful this forum is and how quick people have been to respond. Thanks!

    Notch7
    Why Krypton and not argon? Or are they similar?

    Any why not linseed putty?

    Thanks again! (y)
     
  12. Notch7

    Notch7

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    Krypton lowers u value, it can get you down to buildings compliant u values but if you aren't bothered that's a money saver, just use argon

    Here's the issue......double glazing technology is designed for drained and vented application, i.e. A 5mm gap all around allows water getting in to run down to the bottom rebate and drain out through holes.

    Slimline units have to be fitted as fully encapsulated - i.e. no gaps. S the small gap must be filled with low mod silicone and then face puttied. Traditional face putty shrinks and cracks, moisture will get in.

    The acrylic putty sets a bit like hardish silicone, it has much higher adhesion to the glass and timber than putty.

    The other issue with slimline is that despite what you may read on the makers sites, it's nothing magic, it's simply a double glazed unit with the spacer bar set close to the edge than normal to create a small margin and then sealed with normal hot melt, but the gluing area is narrower, from say 9mm down to 3mm - making it more vulnerable to premature misting up. That said if it's done as per above and quality units are used, you will get a fair number of years before the units break down.

    I used to own a joinery company hence knowing the construction details
     
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  13. wilco88

    wilco88

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    Again, such useful info. Thank you!

    So it sounds to me like the hot melt that is used is prone to shrinking or cracking in some way? Surely there is a product out there that they can seal the units with that create an almost lifelong seal!?

    Clearly there isn't otherwise it would be in use......right?
     
  14. wilco88

    wilco88

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    Think i answered my own question with this video.

     
  15. Notch7

    Notch7

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    Yes there are 2 pack polyurethanes, special silicones, polysulfide options. I don't know if they are used much.....I believe it's the cure time that is the reason they aren't used, but I don't know. certainly almost the whole industry in UK uses hot melt for standard DG units.

    Hot melt doesn't crack the seal to the glass fails allowing moisture in. In drained and vented systems it's often caused by unit sitting without shim down on rebate and drain holes blocked so unit sits in water.
     
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  16. wilco88

    wilco88

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    So when you say "seal to the glass fails" what are you referring to if not the hot melt? I assumed the hot melt was the seal?
     
  17. endecotp

    endecotp

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    Some years ago I was told to budget £60 per unit!
     
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