Tuesday, January 23, 2007

Window Energy Ratings

If you want a good example of just how complex low-energy housebuilding can be, take a peek at window ratings. Glazing is unique in this environment in that heat can flow both ways through it. Whilst at night, heat radiates out through the glazing, during the day and particularly when it’s sunny, substantial amounts of heat passes into the house. Over the course of a year, a south-facing window is often a net energy contributor. Because of this, the traditional way of measuring energy efficiency, the U-value, becomes just a little shabby when dealing with glazed areas, because it only deals with heat loss.

Now a few years back, some bright spark had the idea of introducing an energy-rating scheme for windows, just like the ones you see on fridges and washing machines, ranging from A (excellent) to G (not so excellent). They have been running schemes like this in North America, Australia and New Zealand but Europe has been slower to adopt. However, at long last it has got underway and there is now a body known as the British Fenestration Registration Council (BFRC) that has got the ball rolling in the UK.

The window rating system looks at three aspects of performance.
• The U value (or heat loss) of the window assembly
• The g value, or solar gain
• The L value, or air leakage of the assembly

Now the g value bit is interesting because the amount of heat admitted into the house obviously depends on the orientation of the window. A south facing window will absorb far more energy than a north facing one. How can you reduce this to a single value? I think they fudge this and assume it’s east or west facing, because they have to come up with a constant in order to make a comparison. Interestingly, the very low U values windows (which are sometimes triple glazed) tend to score rather poorly on the g values and it is felt there may come a point where an ultra low U value fails to make any appreciable difference in heat loss terms. Hell, I warned you it was complex.

Anyway, by hook or by crook, the BFRC have come up with a system that supposedly takes all of this into account (you can read more about it on their website if you are so inclined) and reduces all this conflicting data into just seven categories.

The very best windows, the A rated ones, are reckoned to be net heat contributors over the course of a heating season without having to be mounted in the south face of the house. When the BFRC originally set up the scheme, they didn’t anticipate there being many A rated windows, at least not for many years but the scheme has only been running 18 months and already there are loads of As, so much so that it’s beginning to resemble the A level results from Jade Goody’s comprehensive. The question is now arising as to whether the standards have been set too low.

As of time of writing, the accredited casement windows rate as follows:
• 24 A rated (of which just one is timber, all the rest being uPVC)
• 41 B rated (2 timber, the rest uPVC)
• 67 C rates (2 timber, 1 aluminium…)
• 22 D rated (3 timber, 1 aluminium…)
• 12 E rated (1 timber, 3 aluminium…)
• 0 F rated
• 0 G rated

Sliding sash windows are dealt with in a separate category and thus far only one firm, Masterframe, have bothered to submit their products for testing and their best rating is a C. It is not thought that it will ever be possible for a sliding sash to get an A rating, because of the design constraints.

Now whilst this haul represents a fair number of windows that have been submitted to this entirely voluntary scheme, it does make you wonder whether there is much point having an F or a G rating. This is hardly going to be a selling point for a window, especially as the building regulations, revised in 2006, call for minimum rating of D on new work or E on replacement windows. If an F rating is a fail, what is the purpose of having a G rating?

Looks like the scheme itself will have to be re-rated. Anyone for an A* or even an A***?

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67 Comments:

Blogger adr0ck said...

off topic

hope you don't mind

question regarding safety glass which is really racking my head

if a double glazed window is in a critical location i.e. less than 800 mm high etc on one side but more than 800 mm high on the other side, does just the one side need to be safety glass to BS whatever or do both sides need to be safety glass?

having difficulty finding answer to this

6:09 pm  
Blogger adr0ck said...

another question i cannot find an answer for

if you have a double glazed unit does it matter which piece of glass the low e coating goes on

i.e. the outside of the inside piece
or the inside of the outside piece
(so its still in the spacer in both circumstances)

i've had conflicting advice from different glazing companies

6:26 pm  
Anonymous Anonymous said...

to the properties of the sealed unit it makes no difference where the low e is placed

2:44 pm  
Blogger adr0ck said...

could you please advise where i could possibly find info to confirm that the position of the low e coating doesn't affect the U value of thr glazed unit

5:02 pm  
Blogger adr0ck said...

If an insulating glass unit is installed in a critical location where there is pedestrian access to both sides of
the unit, then both panes of the unit should meet the recommendations laid down in this British Standard.
However, in situations where pedestrian access is restricted to one side of the unit, then only the accessible
side has to conform to the recommendations. An example would be where there is low level glazing in the
façade of a building in storeys above the ground floor, but with no pedestrian access to the external faces
of the insulating glass units.

anyone know what this means for ground floors?????

to me it implies that if one side is in a critical location i.e. safety glass then both sides need to be safety glass...otherwise it should read:

If an insulating glass unit is installed in a critical location where it is a critical location to both sides of
the unit, then both panes of the unit should meet the recommendations laid down in this British Standard.

anyone any thoughts on this?

3:35 pm  
Blogger Mark Brinkley said...

Arr0ck

I don't know the answer to your query about safety glass. But I do believe the low-e coating can go on any face. Convntionally, it's put on the outside-facing side of the inner glass sheet.

9:04 am  
Anonymous Alan Burgess MD said...

Mark,

Many thanks for the mention; being the first company of any kind to get any rating we are chuffed, indeed we are still the only sash window company with any kind of rating.

Can I add a little later in this reply, once I've answered the two questioners needing guidance.

Safety glass and which side Low E.

Safety glass must be inline with approved Doc N of the building Regs (old BS6262), a free download is available from the DCLG web site.

In short, if a location requires Safety glass, then its that surface that MUST have, by is still best practise that BOTH sides have (after all it should be a fractional extra over cost to add the second sheet of safety glass, and is safer for all concerned).

Low E, can be applied on either surface two or three where 1 is the outside and 4 the room side surface. For best Optimal performance Surface 3 offers a small fractional improvement.

Mark if your readers need help I suggest you contact Ray our technical director @ www.masterframe.co.uk who’ll be pleased to assist

Back to Energy Ratings, don’t forget there is a "standard size" supposedly to simplify comparisons between window designs.

What’s not published is A performance IS often possible in practise as the window size increases, AND the thing we get hot about is that in comparison to casement windows, sash windows are often 100mm wider as they are located behind the outer brick skin, in other words there's extra glass, that’s extra free solar gain, and extra daylight, but standardisation doesn’t take account for traditional building methods!!

This extra 100mm of glass would increase WER significantly.

Finally we have some HOT news, tests have shown we can achieve a B rating, we just await confirmation before issuing certification.....

10:22 am  
Anonymous Ian B said...

If a home is built to maximise its ‘g’ value, finishes up facing north would it lose more heat than a building just focusing on its ‘u’ value?

8:32 am  
Anonymous James Wilson said...

if u maximise the g value of the house it will let as much heat into the property as available from the way it faces, with out a low u value you would only retain a small amount of the heat. Also any heating used in the house you would lose buy not having a low enough u value so u shoud take both factors into account

2:16 pm  
Anonymous Anonymous said...

Hmmm.... let me see now , the solar gain seems to have an over inflated influence on the ratings ?.
I don't suppose that there is any correlation between that and pilks inability to produce a Low E that gets near some of their competition. I don't suppose pilks had any influence on the calculations. but I bet with all the solar gain in june to august (whoopee) will have you switching on your air con and cooling fans whilst replacing your sun bleached furnishings. sorry but I'll have a "less efficient" , better insulated window thanks

11:10 pm  
Anonymous Anonymous said...

With regard to the question about safety glass and 800mm critical location. As long as you only fall through one pane you should be ok?
Know what I mean.

9:07 am  
Anonymous Anonymous said...

There are a few schemes around for window energy efficiency. I've read about Kitemark Window Energy Rating scheme operated by the British Standards Institution (BSI). Does it matter if its kitemark or a BFRC label or another?

10:47 am  
Anonymous Double Glazing said...

Double glazing A Rated energy efficient windows is the answer.It will make your home or office comfortable thus,saving bucks of your money from monthly electricity consumption.

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