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Building Regs And Excessive Glazing


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#1 Rich1880

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Posted 26 August 2015 - 08:17 AM

Hi All

Looking for a bit of a steer and to see if anyone else has had the same issue as myself.

I've designed an extension which has in excess of 25% of the floor space as glazing. I am therefore required to improve the energy efficient elements of the rest of the extension to compensate. I'm happy to do this however to get building control to agree it I need to provide them with a report. To me this shouldn't be a too difficult task however two things would be useful. If anyone has had such a report completed in the past a copy would be great. Secondly I am developing a spreadsheet and wouldn't mind passing this to someone who may be able to comment on it etc.

Any help would be great.

Edited by joiner, 26 August 2015 - 09:15 AM.


#2 tony51

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Posted 26 August 2015 - 09:16 AM

The maximum amount of glazing normally permitted is 25% of the new floor area created PLUS the area(s) of any glazing or doors which are covered over or do not exist because of the new extension.

#3 jsharris

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Posted 26 August 2015 - 09:34 AM

Have a look at improving the U value of the glazing to significantly better than BR minimum fabric requirements, as well as increasing the U values of all the rest of the insulation and then all you technically need to do is provide to the BCO that the you have complied with the minimum fabric standard regs by comparison (I.e. If built to BRs with 25% glazing the heat loss would be XXX W/m².K, and with your improved glazing and insulation you have a fabric heat loss that is significantly less than this.

Shouldn't be at all hard to do, as the BR minimum fabric standards are very poor, so improving on them is dead easy (and arguably something worth doing any way, just to reduce energy consumption).

Edited by jsharris, 26 August 2015 - 09:34 AM.


#4 Rich1880

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Posted 26 August 2015 - 10:04 AM

Thanks for the replies. I'm comfortable that it is 25% of the floor area plus any existing openings.

I can improve the thermal properties of the walls, windows and roof. It's more how this is presented to Building Control.

For example if I have the following:

Scenario one. I will show the glazing at 25% plus openings and take that sap calculation.

Scenario two. I will then increase the glazing to how I want it which is in excess of the 25% and do a sap calculation but improve the walls and roof etc.

If scenario 2 is the same as or better than scenario one then I guess it passes.

Some issues are that when the builder buys thermalite blocks for example they never know the uvalue as generally they don't even know the make, the use generic terms like 'thermalite' and as such it's difficult to add the correct value.

Secondly I always want to achieve in excess of the Buikding regulations, therefore I specify 100mm full fill cavity insulation as standard, in doing so am I expected to use these assumptions for my scenario one or simply scale these down to just passing building regs and then upgrade through the scenario 2 calculation. E.g specifify 50mm air gap and 50mm insulation to walls which will get me to a sap rating of around 0.30, I then use this in scenario one. I then up this to 100mm full fill and this would give an improved sap rating of around 0.24.

I hope that makes some sense.

#5 temp

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Posted 26 August 2015 - 10:06 AM

As I recall you used to be able to show compliance by calculating an "area weighted" average u-value. If that was below certain value then you were ok. Just be aware that if you have a lot of glass you can be in a situation where no amount of insulation elsewhere will be sufficient to get the average low enough. I haven't looked to see if this is still one of the approved methods.

#6 tony51

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Posted 26 August 2015 - 05:13 PM

@OP; you're making heavy weather of this - you don't need to do a report.

A few simple figures will do if it's just a case of justifying a larger-than-normal area of glazing.

Give them the new floor area created; take 25% of that. add the area of existing glazing and if the total is less than
your proposed glazing area, you've done it.

This is nothing to do with area-weighted calcs, or SAP.

Edited by tony51, 26 August 2015 - 05:14 PM.


#7 jsharris

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Posted 26 August 2015 - 06:13 PM

View Posttony51, on 26 August 2015 - 05:13 PM, said:

@OP; you're making heavy weather of this - you don't need to do a report.

A few simple figures will do if it's just a case of justifying a larger-than-normal area of glazing.

Give them the new floor area created; take 25% of that. add the area of existing glazing and if the total is less than
your proposed glazing area, you've done it.

This is nothing to do with area-weighted calcs, or SAP.

Exactly what I meant, Tony.

Just work it out on the back of a fag packet for as it would be with BR min fabric standards, do the same for the improvement, show that the improved figures are better than the BR min figures if it only had 25% glazing and there;s nowt the building inspector can do but tick the box.

Given that the limiting fabric values in Part L1A are pretty damned poor, it isn't exactly hard to chuck better windows in and a bit more insulation everywhere else and improve things overall by a fair margin. If it were me I'd look at making as much improvement as I could afford, as it's relatively cheap to add a bit more insulation and better spec glazing, compared to the energy cost of having poorer stuff and the cost of replacing or improving it down the line.

#8 temp

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Posted 27 August 2015 - 09:01 AM

Quote

Give them the new floor area created; take 25% of that. add the area of existing glazing and if the total is less than your proposed glazing area, you've done it.

I assumed that the OP had already done that and found it over.

#9 Rich1880

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Posted 27 August 2015 - 10:11 AM

Yes I am well over the 25% plus existing opening allowance. I therefore need to imprve the thermal efficiency of the other elements of the extension and it is this that is required via a SAp calculation, there is no way around this as the SAP calculation provides the justification.

In the end I spent yesterday drafting a spreadsheet which now compares the notional build with the proposed and allows me to change each element as required until it passes. We initially stated to BC that we would improve certain elemetns, however this was not accepted as tehy wanted to see how much of an improvement it was etc.

#10 rcaines

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Posted 27 August 2015 - 03:35 PM

We had 30% and had a SAP report prepared to show that we would be more compliant with the proposed glazing than without. Ours is a large extension with extensive renovation.

The bloke who prepared the report said it was very complicated to prepare as he had to work out the figures based on the whole house rather than just the extension (otherwise it wouldn't have been compliant.

He is called Martin and the company was www.etophouse.com.
Might be worth giving him a call for his thoughts.

#11 temp

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Posted 27 August 2015 - 07:41 PM

See Part L (Existing Dwellings) Page 13 Extension of a Dwelling

Para 4.5 Area Weighted U-Value Method.

http://www.planningp...AD_L1B_2015.pdf

If the area weighted method failed to show compliance then you would need to go for a whole house SAP.

I see no reason why you can't calculate area weighted u-value yourself.

Edited by temp, 27 August 2015 - 07:43 PM.


#12 Rich1880

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Posted 23 November 2015 - 12:17 PM

Thanks everyone, we spent a little time on this over the last couple of months and devised a spreadsheet which now asssits in calculating and it has been approved by our building inspector.