SAP Calculations - Can you DIY these?

[WoOt]

Building a large back extension on my own, on a building notice and without too much background in construction. So I was always going to run in to things I didn't know yet... .

Had Building Control out the other day and he gave me a formula to calculate the ratio of windows and doors I would have in the new extension.

Extension in m²/4 + existing openings that are to be covered up in m²

Even though I'm working to a very rough plan, that can be changed as I go... I was planning on adding in 5.5m long bi-fold or sliding doors, and two 1.5m by 3.5m roof lanterns.

In short, I'm likely to go over my allowance. BCO said I might be able to get away with it if I change the boiler, which I'm planning to do anyway. Just trying to figure out how these calculations work and if I can do them myself?

I.e: changing the boiler to a new A-rated model will allow me to add an additional X square meters of glass.

 

[freddiemercurystwin]

edit

 

[tony1851]

Is it private building Control you are using, or the council?

 

[WoOt]

Using the council. They were quite good this far, giving me loads of advice on their first visit, such as:

• Put in wall ties every 750mm * 450mm (I assume this horizontal * vertical)
• If you have a 550mm return to the cavity on a corner, you won't require a windpost
• (width window 1 + width window 2)/6 = minimum pier width to avoid windposts
• and the heads up on this glazing ratio stuff...
  • Extension is 42.17m², so that give me a 10.54m² allowance
  • Existing glazing to be covered up is 9.38m²
    • For a total of 19.92m²

Unfortunately, my current plan has:

• Double door: 3.82m²
• 5.5m bifolds: 11.33m²
• Window: 2.12m²
  • For a total of 17.28m²
• + Two roof lanterns of 4.5m², which I may omit now...

 

[tony1851]

Using the council.

That figures.
I've found private inspectors are usually more relaxed when checking your calculations of the existing glazing areas if you get my drift. I mean....once the old glazing has been removed, who's to say it wasn't around... say....20m²???
Bit late now, though.

 

[WoOt]

Bit late now, though.

Or is it? He's not taken any measurements of the existing windows... . Risky game I guess.

Out of curiosity, I took down an old back porch which also had a large window and a door. Would I be allowed to include this in my allowance?

Any advice on these calculations @tony1851? Is there any way to DIY figure out what added allowance I'd have for things like changing the boiler?

 

[Swwils]

If you can't get within the allowance you need to demonstrate that the proposed extension is no less compliant than if an extension of the same size and shape was built according to the 25 per cent limit.

For example, you could increase the thermal resistance (U-value) of the walls, floor, roof, or glazing elements above the default values in order to increase the area of glass your going to put in.

If you are close to the allowance one of the first things that's easy to target is the notional glazing U-value of 1.6-1.8W/m2K. High-performance, gas-filled, low-e-coated glass could see the U-value tumble to 0.9 or better, effectively allowing you more than 40 per cent extra glazing...

If it very highly glazed (more than 50 per cent), you may need to use the Standard Assessment Procedure.

You can do them yourself, but there are accredited assessors for a reason. As far as I am aware some software is still "free" but the assesor cost is not too burdensome really and they will be knowledgeable.

The SAP will demonstrate that the calculated carbon dioxide (CO2) emission rate from the dwelling with its proposed extension would be no greater than the dwelling and a notional fully compliant extension of the same size and shape. This method allows you to keep almost 100 per cent glazing by upgrading existing elements and services within the original part of the house instead – for example, by installing a more efficient heating and hot water system, or increasing the insulation within the existing roof space.

Low hanging fruits tend to be energy efficient lighting, boilers, boiler controls, etc all can contribute to the SAP - that when done properly with the correct information is actually a really nice thing to have - as long as the actual build reflects what was calculated...

 

[WoOt]

Thanks Swils, I got all that. But still have no idea how to do these calculations. Is there a website, or an excel file or something for this?

 

[freddiemercurystwin]

https://www.charnwood.gov.uk/files/...e_calculator_2010/L1b_Calculator_1.3_2010.xls I've used this successfully in the past.

 

[Swwils]

It's available on the gov portal which links to BRE.

http://www.bre.co.uk/sap2012/page.jsp?id=2759

I'm not am expert but the usual route is to do through a qualified assessor, you may be able to find other things that might be acceptable for your application.

Did you not think you could switch to the low e glazing to demonstrate its compliance?

 

[WoOt]

https://www.charnwood.gov.uk/files/...e_calculator_2010/L1b_Calculator_1.3_2010.xls I've used this successfully in the past.

Assuming I didn't mess that up, I think we'd be OK even without doing anything fancy and potentially too expensive. Let's see if my BCO accepts my back of fag packet calculation.