On 22 Nov, 22:40, geraldthehamster wrote:
I have some quite largetimber-framed casement windows in the front of
my house. As the frames are in serviceable condition, I had wondered
about replacing the glass panes withdouble-glazed units, in the same
frames, as a less expensive alternative to having the entire windows
replaced. I think the frames are deep enough. Has anyone done this?

Yes. several problems but chief one is depth of glass rebates in frame
which used to be less than now. Easier for fixed (non-opening) panes
which effectively have deeper glass rebates, but more difficult for
opening sashes. Mostly I made new opening sashes.

Problem you will encounter in (no particular order).

Single glazed glass held in place by hardened putty makes the whole
frame rigid. That helps a softwood frame (which of course is normally
weaker than a hardwood one) stay square. But DG panels should not be
fixed in rigidly otherwise leaks will occur and contamination of the
gap will occur (condensation etc). That means you need to take care
that softwood sash frames are well protected from damp (ie well
painted and primed) and corner joints are rigid.

There is a conflict between optimum spacing for heat insulation and
sound insulation. Regret I can't recall the exact figures, but IIRC
the nos are around 15mm for heat and 20mm for sound.. Wider gaps
encourage convection currents in the trapped air which aid heat
dissipation. In any case few single glazed sashes will be thick
enough to allow more than 4-6-4 DG (ie 4mm glass+6mm gap+4mm glass).
IIRC the norm is now 4-14-4 and new timber frames have been thickened
to allow for that. I changed SG to 4-6-4 DG in the smallest room
(typical 600 wide window with horizontal opening storm vent at top) a
while ago in winter and the increase in comfort level was noticeable
immediately and substantial. The room was no longer draughty and the
radiator was turned down a notch. Possibly there wouldn't have been
much further improvement with 4-14-4DG?

Some posters here question the economic justification of DG. That's
is very possibly correct, prospective life of the units possibly being
short maybe only 10 to 30 years? And I have little doubt that energy
used in production is a goodly proportion of potential energy saved.
However IMHO the increase in comfort levels from draughts caused by SG
more than makes up. Further DG is paid for at today's prices and all
I can see is energy prices escalating in the years ahead at a far
greater pace than inflation, so future heating bills have been capped
to some extent.

DG units are easy to obtain, just ring round glaziers in the yellow
pages. Get several quotes: I found prices can differ widely. You
will find some will take your order but contract out the work to a
different firm with the fabrication machinery. Avoid them - ask if
the units are made on their premises. Also because of FENSA and the
building regs monopoly they have, ask for DG units without date
stamping (usually fabricators print the date made in dot matrix style
on the separating plastic inside the gap. That will hinder future
home inspectors demanding a FENSA certificate.

Fixing the units: Sideways (in the plane of the glass) there must be a
3mm gap between unit and the timber frame. My glaziier gave me a
bundle of hard black wedges about 3mm x 8 x 20 for this (use c. 400mm
apart). The unit must NOT be set in hard putty. SFAIUI the corrrect
stuff for a timber frame is a non-setting butyl material. The only
make I know of is Flexistrip made by Hodgson Sealants, Beverly (tel no
in 2001 was 01482-868321) [I've just checked they now have a web
site http://www.hodgsonsealants.com/produ...ips_tapes.php]
but they only sell in boxfuls of c 20 reels - ie gbp70 up. You may be
able to find local DG suppliers have stocks of this or an alternative
- you could try asking your DG panel fabricator for a fixing &
supplier recommendation. Flexistrip comes in reels of double sided
"tape" style, about 1.5mm thick and 12mm wide and is incredibly sticky
(IIRC different thicknesses and widths are available). Just unroll it
into the rebate then carefully drop the DG unit in. Once the DG unit
and flexistrip make contact you will not be able to separate them
cleanly. If the fit is not accurate & you have to redo the job then
the Flexistrip is wasted. You can't fudge an adjustment or retry as
mucked up flexistrip seal shows through at DG edges & looks awful.
You must take out the panel and reapply the Flexistrip.

Finish by sealing the DG panel in with glazing bead NOT putty.
Finally plug any small gaps with a mastik sealant such as a bathroom
sealant then prime and paint thoroughly.

If fitting an obscured unit in a bathroom etc then fit with the normal
plain glass side of the DG outwards. That makes for more reflections,
improving the obscuration effect.

HTH

Good luck