L1B without SAP

[cheradenine]

Extension with over-sized windows.

When I've done this previously, BCO allowed area-weighted calculation based on the floor area of the final room (25% floor area of room + extension). Seemed reasonable to me as the spirit of 25% is clearly for the actual living space.

This time round, computer says no, can't take into account thermal upgrades in the (final) same room, whole-house SAP pls. Maybe that's right. It's certainly different experience from last time. What if the room was already under-lit?

This is a 1938 house. As it happens, we're also replacing ~25 square meters of single-glazed crittall windows. This alone will save roughly 95W/degC. The extension vs the 'notional' one is over by 18.3W/degC. I.E: the energy savings on windows alone will be *five times* that of the loss difference from the new extension.

It seems perverse to have to go through the palaver (and expense) of a SAP assessment when the equation we're trying to prove is "loss(Actual) <= loss(Notional)", and 90% of that equation cancels out, and what remains isn't even a close-call - without even trying to faff with solar gain (big windows will be south-facing!).

Is there a simplistic Watt to CO2 loss conversion that I could use, based on an average south-east england climate?

 

[tomfe]

It's 25% plus any glazing you take out. So if you are getting rid of the 25sqm of glazing you can have that plus 25%.
If BC has asked for a sap then doubt you can get around it, if you supply all the details to an EA you can get them for around £100.

 

[cheradenine]

The 25m^2 is elsewhere in the house, sadly.

I may work backwards through the SAP worksheet. Since the values for the majority of the sections will be the same, they'll just cancel out so I don't actually need to be accurate.

I'm reluctant to spend money proving something that seems so obviously an improvement

 

[cheradenine]

In case anyone else walks this road : there is some software (downloadable from rusfa.com) which will do the calculations if you feed it the data.

I don't know whether to laugh or cry at the fact that on its sums, I don't even need to replace all the single glazed windows - the improvements from solar gain and reduced lighting more than compensate for the losses.

 

[wessex101]

Thanks for that, I haven't come across that rusfa site before. The 25% rule for extensions where you are not allowed to include the existing floor area of the room has always struck me as a nonsense. Like you I have always used the area weighted calculation (not having the benefit of a reasonable BCO) but if you have a small extension tacked on to a large existing room it can be very troublesome.

 

[cheradenine]

but if you have a small extension tacked on to a large existing room it can be very troublesome.

This.

In my case, the "extension" is really just rationalising a structure that's already there. You get credited with the glazing area it covers - but not for the fact it's currently really bad glazing. I.E: if I built the same extension attached to a different part of the room, I could area-weight improvements on that element and probably pass!

It seems the spirit of the guidance (and the 25% *plus* what was covered up) was to give more flexibility, not less (that it'd be "no worse"). Interpreted the way it is, if you knock a kitchen into a large, windowless utility room as a part of adding a small extension, you could end up seriously under-lit! If you applied to add a window under building regulations, it'd pass.

Daft.

 

[wessex101]

Yep I had one last year, a lower ground floor/semi basement adding a small extension.

The existing basement room was dark and dingy so we wanted to put lots of glazing in the extension. We got the "computer says no" response when asked about taking the 25% of the combined floor area even though it would be one room. Bonkers.

In the end just managed to scrape through with the area weighted calculation adding insulation to the walls, floor and roof. I've got a spreadsheet to do the calculation but I cannot remember where I found it or whether it is totally legit. Just found this on Google which is the same...http://www.charnwood.gov.uk/pages/useful_links. Also comes in handy with the current trend of massive sliding folding doors and lots of rooflights, usually exceeds the 25% rule.