Replacing windows - u values

[Supercell]

We are going to replace the windows at the back of our house as they really dont insulate very well and are slick with moisture in the winter all the time.
Looking at various glazing websites it seems most places are offering windows with u values of 1.4 , 1.2 and 0.8.
Is it worth spending the extra for say 0.8 and is there much of a difference between 1.2 and 1.4 in reality?

[kadman]

Roof and wall spec have a far higher impact on energy conservation.
When you mention your windows are slick with moisture in the winter. Could be because they are single glazed, low performing double glazed,
or high moisture environment, and low performing windows.

Which do you think it is. Whats the makeup and condition of your existing. You can get retrograde high spec glazing fitted into existing windows, depending on what you have

[dawg]

We are going to replace the windows at the back of our house as they really dont insulate very well and are slick with moisture in the winter all the time...

This is an indication of excessive humidity and may be an indication of poor ventilation.
You may need to do more than just improve the u of the windows

[Supercell]

They are old double glazed windows, the newer ones we put in when we bought the house (10 years ago) are bone dry. You can feel the cold standing beside them in winter nights.

[kadman]

What heat source, if any do you have in the cold rooms where you feel the cold by the window. Are the cold rooms north or south facing.
Unless you establish a few things like heat source, ventilation, facing direction, then its hard to know why water is streaming down the windows.
And until you do that I personally would not purchase new windows thinking thats the fix, it may not be.

Example.
If you have a kitchen or bathroom with lots of cooking/steam next to a room with no heat source and no ventilation, then water will flow down that pane of glass whether its a new double glazed unit, or 20 year old dg unit.

[dawg]

WRT condensation on windows, do you have any sort of gizmo that measures temperature and humidity ? If not then would suggest that you get one ( usual suspects : ebay, Amazon, specials at lidl & aldi etc ). There are many different types and they are inexpensive ( no need for super accuracy ). ( Its hard to know what the extent of your problem is without being able to measure it )

[Supercell]

I have a Vent-Axia Sentinel Kinetic whole house ventilation system here which i put in when I bought it as i have asthma, its not excessive air moisture in the house, its utterly crap glazing the previous owner put in god knows when.
The windows are south west facing. The house (bungalow) was built in the 80's and is single leaf cavity block. So lots of heat loss though that, eventually when funds permit we'll get it wrapped. We had about 30cm of fibre glass put into the attic space when we bought it which did greatly help, just havent had the funds until now to do anything about the back windows (we replaced the others when we bought it about 10 years ago now).

[kadman]

Older double glazed units were separated by air as opposed to gas as in modern units. So they would be poorer in performance than newer ones.
Single leaf cavity is gonna be a major heat loss as you have pointed out. The blocks are going to pull any heat out of the building, and draw any cold into it.
Does it have any dry lining on the inside of the building at all. And replacing the windows will not necessarily stop moisture on the inner panes, with the single leaf cavity block. That type of construction is only suitable for a shed in reality.

You could insulate from inside with studded construction, and insulation. But this method causes loss of internal room due to stud thickness.

[dawg]

Not about anything in particular but might be useful for context :
1. I have to look after a house double leaf, single glaze. Last winter condensation started to form on inside of windows when outside temp approx 2C
2. If you are thinking of adding insulation then probably best to get some prof input ( you probably have to anyway if its grant eligible ) as there is a bit of science when it comes to working out where condensation actually happens in the sandwich when dew point is reached

Back to your OP :

Will you be replacing the frame(s) ?
If yes : head into triple glaze unless there is a very good reason not to
If no : then what you do will be dictated by what the frames can take

We are going to replace the windows at the back of our house as they really dont insulate very well and are slick with moisture in the winter all the time.
Looking at various glazing websites it seems most places are offering windows with u values of 1.4 , 1.2 and 0.8.
Is it worth spending the extra for say 0.8 and is there much of a difference between 1.2 and 1.4 in reality?

( Calahonda and others knocking around here know all about this stuff, I'm only busking on the topic )

[Supercell]

Older double glazed units were separated by air as opposed to gas as in modern units. So they would be poorer in performance than newer ones.
Single leaf cavity is gonna be a major heat loss as you have pointed out. The blocks are going to pull any heat out of the building, and draw any cold into it.
Does it have any dry lining on the inside of the building at all. And replacing the windows will not necessarily stop moisture on the inner panes, with the single leaf cavity block. That type of construction is only suitable for a shed in reality.

You could insulate from inside with studded construction, and insulation. But this method causes loss of internal room due to stud thickness.

Theres fibre glass between the studs under the plaster but thats about it.
Dont want to go the internal route as i've read that damp and mould can be a problem where the thermal plaster (dont know what the correct term) meets the cold wall and as i've asthama thats a risk that i'm not going to take, plus its a small bungalow anyhow.
Its only these frames that are so wet, all the new ones we replaced elsewhere in the house are dry, the old ones might as well be single pane for all the insulation they are giving, they dont seem to be air tight either.
Wrapped will be the next thing but thats going to cost at least 15k (or thats what we were quoted when we bought the place so likely a good bit more now), the windows should only be about 5k'ish i think which is money we have saved so thats what its going to be first.
From the sounds of it 1.4 vs 1.2 isnt going to make much of a diference now, assuming the house was wrapped in a few years that may not be so i guess?

[Supercell]

Back to your OP :

Will you be replacing the frame(s) ?
If yes : head into triple glaze unless there is a very good reason not to
If no : then what you do will be dictated by what the frames can take

The whole lot, the frames are not fitted very well and drafts are noticeable in windy weather, the aluminium frame itself is icy cold to touch in cold weather too so seems to be acting as a thermal bridge, going upvc this time for cost and (i presume) better insulating , though I assume modern metal frames are better constructed.

[kadman]

Well if you have air leaks at the window then yes you will definitely get moisture on the glass for sure, thats a given.

And the suggestion by the other poster who to contact, is a good un. Cal knows his onions

[dawg]

...the aluminium frame itself is icy cold to touch in cold weather too so seems to be acting as a thermal bridge, going upvc this time for cost and (i presume) better insulating , though I assume modern metal frames are better constructed.

Good call on changing the frames.
From what I hear triple not much more than double - get both priced up ?
Any recent metal frames that I've seen seem very, very good but I believe a spendy option compared to PVC.

For background, how many SqMtr of window opening are involved in this project ?

[kadman]

Are you going modern metal frames, or PVC or wood. If you go metal make sure they have a thermal break in the construction,
not all of them do.