Comparing u-values in windows

Muahahaha

Just looking for some pointers here on the relativity of u values for windows. At the moment Im looking at one product that is 1.1 and another 0.36. What exactly does this mean in comparison, is the 0.36 three times more thermally efficient? Or is the difference between the two products negligible? Im not going for a passive standard or anything so just curious if there is a big difference between 0.36 and 1.1 as there is a fair difference between the prices quoted

It would be good if someone could quantify it in terms of energy loss to give me an idea if the more thermally efficient window is worth the extra expense in terms of saving on energy bills. And just for comparison on what I currently have what might be a ball park u-value of a 20yr old double glazed window?

Also when companies quote u values are they quoting on the glazing only or is it an overall value for glazing plus frame.

sydthebeat

It more likely that the 0.36 figure is for the glazing alone... So you need to find out what the over all window u value is.

Alternatively the 0.36 could be a North American u factor figure, which would equate to about a 2.0 u value.

Some more homework for you to do

BryanF

Muahahaha wrote: »

Just looking for some pointers here on the relativity of u values for windows. At the moment Im looking at one product that is 1.1 and another 0.36.
1. What exactly does this mean in comparison, is the 0.36 three times more thermally efficient?
2. Or is the difference between the two products negligible?
3. Im not going for a passive standard or anything so just curious if there is a big difference between 0.36 and 1.1 as there is a fair difference between the prices quoted

4. It would be good if someone could quantify it in terms of energy loss to give me an idea if the more thermally efficient window is worth the extra expense in terms of saving on energy bills.
5. And just for comparison on what I currently have what might be a ball park u-value of a 20yr old double glazed window?

6. Also when companies quote u values are they quoting on the glazing only or is it an overall value for glazing plus frame.

6. They should ideally all quote overall unit u-value. But many don’t
5. (Following u-values not calculated) 3w/m2k maybe no equivalent wall insulation
4. 1.1w/m2k maybe equivalent to around 20mm wall insulation ;
0.33w/m2k maybe 50mm equivalent
3. Yes big difference. But I’d say, one is quoting overall and the other glazing only.
2. Definitely not negligible
1. Yes. At the levels you’re talking about.

Muahahaha

Thanks bryan and syd. Yeah I think ye are likely right about the 0.36 being glazing only as it is a lot lower than other products I've seen. Its frustrating the way they sell it like this when it is the overall u-value of frame, window, glazing bars and seals is what you really want to know about. Something to keep an eye on for sure. Anyway I'll go back and clarify as the difference between the quotes is almost 30% so Im not yet sure if the 0.36 guy is way overpriced or if he genuinely has a very thermally efficient window and frame. He came in at 6,500 for 6 windows and french patio doors while the 1.1 guy came in at 4,800 for the same. Another nationally known company came in at 6,800 for the 1.1 windows and today I discovered they are using the exact same windows which are manufactured by Specta, its the Elite 70mm both vendors were quoting for. So I've two vendors stating 1.1 Specta Elite 70mm with one quoting 4,800 and the other 6,800 which is quite a hike even if the company are well known, I suppose their national radio ads have to get paid for some way! Any feedback on Specta Elite windows if someone have them installed?

Also just wondering can you put u-values in laymans terms? I understand it is a calculation of thermal loss with for example a u value of 1.0 meaning the window will lose 1 watt of heat per sqm. But how is this quantified, a watt per hour/minute? And am I right to say the sqm refers to the sqm of the actual glazing or is it a measurement of the entire room?

BryanF

mod note comments on specific companies by PM only.

OP
Is this a new dwelling your self-building or windows your retro-fitting ?

Calahonda52

My understanding is as follows:

U value is stated in in units of W/m²·K where K is degrees and measures the delta T ,DT, across the element in question, so say outside is -3 and inside is 21 then DT is 24 [21 minus minus 3]
so suppose the U value of a wall is 1.0 and the wall is 100 sqm, and the DT is 23 then the heat loss of that wall is 2.3kW, for 1 hr will be 2.3Kwh

It is done by component, so if whole window area you will need whole window U value and the design DT
HT

Muahahaha

BryanF wrote: »

mod note comments on specific companies by PM only.

OP
Is this a new dwelling your self-building or windows your retro-fitting ?

It is a retrofit. Im also installing a wood burning stove with the windows and composite door. Then I'll give it next winter to see how heat loss is then and make a decision on either internal or external insulation or hopefully no need
for insulation at all if the windows help improve things significantly but that remains to be seen.

Muahahaha

Thanks for the explanation Calahonda. Just to check I've got this right, in my case I've got 9.5sqm of glazing across the building. So assuming the new windows are a U value of 1.0 and the temperature outside is 0c and inside is set to 21c then I have a DT of 21 minus 0=21. How do you then arrive at a per hour kwh heat loss of that 9.5sqm of windows?

Calahonda52

Muahahaha wrote: »

It is a retrofit. Im also installing a wood burning stove with the windows and composite door. Then I'll give it next winter to see how heat loss is then and make a decision on either internal or external insulation.

The wrong approach, especially if considering ewi, which when done like SAS did here maybe 10 years ago, when he mounted the windows on the outside of the wall before putting on the ewi, is a really good design.
New windows and then considering insulation is a mad idea.

Muahahaha

calahonda52 I should clarify that the likelihood of me getting EWI is fairly low, I'd be 90% sure it is going to be IWI. I get that installing new windows and EWI should be done simultaneously so the windows can be placed further forward flush with the EWI. But why would doing windows first before IWI be the wrong approach? A tradesman had advised to do the windows first then IWI later as if you do it the other way around then the window installers are going to be disturbing the newly installed insulation at the window reveals, it kinda made sense to me but is there something I'm missing here? Also a neighbour a few doors down got their windows replaced last year and reports a big improvement in heat loss, he said to me that he had planned IWI after the windows but having just been through the last winter he now isn't proceeding as he feels the house is now warm enough and feels that the 10k+ cost of IWI would have too long a payback time.

I don't have the budget to do both at the same time and as the windows are 25+ years old and in need of replacement anyway I'd thought it made sense to begin there with the IWI following later.

Calahonda52

With IWI, the thermal bridge between window frame and wall needs to be addressed.

Muahahaha

Calahonda52 wrote: »

With IWI, the thermal bridge between window frame and wall needs to be addressed.

So in my own scenario of replacing windows and frames now but possibly getting IWI later is there a way of then addressing the thermal bridge? Should the IWI contractors be filling that gap between window frame and the insulation with some sort of air tight membrane/tape?