I once installed a couiple of UPVC windows, in my house and the ony fixing
I used was expanding foam from a can. I never had any problems.
Now i am about to install another on the first floor of slightly wind-prone
gable end. Is expanding foam alone sufficient for fixng frame in place?
I don't think people appreciate the difference between strength in
tension and strength in shear. Glue is often only stressed in
shear..e.g. the classic glued tenon and mortice joint, which is fine
until there is 'rocking' when it fails in tension, not shear.
Which is why gap filling with glue requires a glue strong in tension as
Expanding foam is no particularly strong in tension, but a typical
window frame is not able to rock sufficiently to rip the foam apart,
especially in a rebated frame. And its quite a narrow gap as well.
So its the shear strength that counts. And here the foam elasticity is
an advantage, spreading the stress evenly down the foam line, so there
is no stress concentration ..
I've seen more screwed frames come awry than foamed ones.
Its standard industry practice to as has been said, wedge to get
straight, then foam, then cut back and plaster the inside and mastic the
In massive trauma situations - explosions - frames do not fly out, they
Strong winds will break the glass before they break the foam.
Define "slighly wind-prone", how much the glass in the curent window
flex in reponse to gusts? Our old single glazed and over 3' square
windows would flex about 1/2", t'was rather alarming watching it. The
new 22mm DG units only about 1/4" but they are a different shape
about 1'6" high and 3' wide.
For our windows they have frame fixings. Foam alone in less exposed
locations is probably OK but it will flex and move thus may reduce
the life of any silicone to building joint.
I've never noticed the 3-ft square pane in the old window flexing, but then
I never really looked. I know one thing: the frame cannot blow inwards. So
it would have to get sucked outwards, which I is difficult to imagine! I
think the roof tiles would blow away first. I think I'll quit worrying.
Thanks to all for the input,
You probably don't get the winds we do at 1400' and exposed. Gale
Force 8 (above 40mph sustained) happens several times a year gusts
with those are into the low 50's mph. Every so often it'll really
blow F10 55mph sustained gusts above 60mph.
F8 isn't pleasant, rain is painful, hard to stand up and move
sensibly about. F10 with gusts is down right dangerous, you have to
lean so far into the wind that you are unstable, sudden drop in wind
and you bite the dirt. Wind picks back up and you are quite likely to
get bowled over. Needless to say we don't go out unless we have to in
anything much above F7.
The all trees by us the nearby have the windswept curved look...
I've been at the helm of a 34-ft sloop, exposed to the elements, in a force
9 gale. The waves were about 15ft high. It was exhilerating - especially
since I was only a trainee crew member! The only thing that stopped me
losing my nerve was that the skipper was down below, apprently unconcerned.
I always refer back to that experience when I need to guesstimate strong
Builders frequently do fix windows with foam (a full circumference bond
of polyurethane adhesive in effect - so very solid fix).
However a few traditional frame fixings helps achieve and maintain the
desired position prior to foaming.
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