Window u-values

brdboard

I have receIved a quote for tg windows, each with different gaps, ranging from 2 8mm gaps to a 10mm and 12mm gaps.
I have a cert for 1 of these build ups, and am waiting for he other ones.
Should there be much of a difference between u-value for a 4-8-3-8-4 window and a 4-10-4-12 window?

BryanF

brdboard wrote: »

I have receIved a quote for tg windows, each with different gaps, ranging from 2 8mm gaps to a 10mm and 12mm gaps.
I have a cert for 1 of these build ups, and am waiting for he other ones.
Should there be much of a difference between u-value for a 4-8-3-8-4 window and a 4-10-4-12 window?

if these windows are both the same price & U-vlaue with similar Sg values, I would go with the 8mm air gap - my logic: less of an air gap (presuming the same gas etc) means less thermal resistance (poorer U-value for the glass). so this by default would suggest that the frames are thermal superior. at the moment its the frames that are often the most expensive elements related to the difficulties in stopping heat transfer. some manufacturers are selling 'great triple glazing but not so good 'overall window U-value'.

brdboard

BryanF wrote: »

if these windows are both the same price & U-vlaue with similar Sg values, I would go with the 8mm air gap - my logic: less of an air gap (presuming the same gas etc) means less thermal resistance (poorer U-value for the glass). so this by default would suggest that the frames are thermal superior. at the moment its the frames that are often the most expensive elements related to the difficulties in stopping heat transfer. some manufacturers are selling 'great triple glazing but not so good 'overall window U-value'.

Thanks Brian,
It is actually 1 quote with different buildups on different windows.
I'm not sure why there are different buildups and am trying to figure it out with the suppliers.
As far as I can gather, each window has a 68mm frame, so I don't think the thermal resistance of the frame changes.

If the frames are the same, will the bigger gaps have much of an effect on u-value?

BryanF

brdboard wrote: »

Thanks Brian,
It is actually 1 quote with different buildups on different windows.
I'm not sure why there are different buildups and am trying to figure it out with the suppliers.
As far as I can gather, each window has a 68mm frame, so I don't think the thermal resistance of the frame changes.

If the frames are the same, will the bigger gaps have much of an effect on u-value?

more air gap better U-value generally.. but there are loads of variables to be considered, is this a new build? what is your architect suggesting?

fclauson

read this http://www.boards.ie/vbulletin/showthread.php?t=2056530851

windows are complicated - don;t get miss sold just on U value

brdboard

fclauson wrote: »

read this http://www.boards.ie/vbulletin/showthread.php?t=2056530851

windows are complicated - don;t get miss sold just on U value

Thanks FC,

Tg 4-10-4-10-4. ug 0.5, g 54%, overall u-value 0.9
Tg 4-8-4-8-4, for glass doors, ug 0.7, g 57%, overall u-value 1.5.

Have to input these into my deap model to see what effect they have.

BryanF

seems to me we have a long way to go with the quality (more to do with reducing the cost) of doors in this country.

brdboard

BryanF wrote: »

seems to me we have a long way to go with the quality (more to do with reducing the cost) of doors in this country.

It seems like a big difference between windows and doors alright.
What I don't fully understand is if the windows have a ug of .5 and uvalue of 0.9, why does the door ug of .7 increase to a uvalue of 1.5? It seems like a much bigger increase.

BryanF

brdboard wrote: »

It seems like a big difference between windows and doors alright.
What I don't fully understand is if the windows have a ug of .5 and uvalue of 0.9, why does the door ug of .7 increase to a uvalue of 1.5? It seems like a much bigger increase.

BryanF wrote: »

its the frames that are often the most expensive elements related to the difficulties in stopping heat transfer. some manufacturers are selling 'great triple glazing but not so good 'overall window U-value'.

its the overall value that is important.
as fclauson suggests a slight reduction in the 'glass' to improve the amount of light getting in, maybe worth while, presuming an increase of frame u-value - hence the importance of the 'overall' U- value.

but this brings us to the next issue - why spend all that money on better frames, if your going to install them against a thermal bridge

so that's why i wondered

there are loads of variables to be considered, is this a new build? what is your architect suggesting?

brdboard

I've done a lot of reading up on cold bridges, airtightness, etc ( a lot of it on this forum!) so I am satisfied I won't have a problem here. I also have a construction background, so I'm not coming at this blind.
It is a new build, and I've put a lot of thought into each element.

What has surprised me is how quickly things are moving on at the moment. I have specified a full fill 200mm cavity, and now I hear of people increasing this to 250 and 300mm!

Regarding the door u-values, I rechecked the spec, and the door frames are 58mm thick, while the windows are 68mm thick, which would explain the higher uvalue for the door.

braftery

The biggest issue with doors in relation to overall U-Value is in the threshold.

The threshold must be load resistant, made of a material that will take shoes, boots, etc hitting it over and over, day after day. It must be designed to assist inthe resistance of water and finally after all of this it should be thermally broken.

Due to the above requirements they are usually made from Aluminium and have a high U-Value as a component of the door, even when thermally broken.