Area of Windows (AD L1B para 4.2)

[wessex101]

Just thought I would test your little grey cells with a conundrum that has always puzzled me.

The above rule only allows you to take the floor area of the extension. However often the extension totally encloses an "inner" room and the separating wall is removed to make one large space.

So for example you have a 10m² extension on a 15m² room. The rules only allow 2.5m² (25% of 10m²) of windows plus any existing windows so lets say another 2.0m² giving us 4.5m² in total.

With the current trend for these huge sliding folding doors and rooflights the window area nearly always exceeds the 25% of the extension. If you took the room as a whole, 25m², then 25% of the total floor area would allow a total window area of 6.25m². If you have a small extension on a large room the anomaly is even worse. So the 25% rule contradicts the 20% rule for minimum window area. I usually get round it with an area weighted calculation but it always bothered me that the client has the extra expense of added insulation and thicker walls etc. for no good reason.

Has anyone ever come across a scenario where Building Control has allowed you to take the floor area of the entire room not just the extension? That strikes me as the logical way to calculate the maximum window area.

 

[wessex101]

Oh, just me then. I've got one at the moment where I just cannot get it to comply unless I have triple glazing in the sliding folding doors to get the U value down to 1.5 and I have already reduced the size of the rooflights against the owners wishes. I think I will try and put together a case for the building inspector to take the floor area of the entire room and see what happens. Watch this space.

 

[freddiemercurystwin]

Are you getting SAP's done?

 

[wessex101]

Thanks Freddy, no SAP. I have always done it on an area weighted calculation before as I felt the cost of the extra insulation was probably lower than the cost of a SAP but this one is proving more of a challenge. The extension is quite small but the owner wants maximum glazing as it is enclosing a larger semi-basement room that is very dark.

I might suggest to the client that we get a SAP done if the building inspector wont play ball as that will probably be cheaper than the triple glazing on top of all the other extra insulation I have had to squeeze in.

It is really the wording of the guidance that annoys me, it just doesn't make sense.

 

[freddiemercurystwin]

Have to say, I just resort to getting a SAP done, add another 20mm insulation in the roof or whatever, jobs a goodun.

 

[wessex101]

I think I've been making life difficult for myself. The last time I looked at the cost of a SAP calculation (which was a long time ago) it was about £150 - £200 but I can do a quick area weighted calc in about 10-20 minutes and I get to choose how and where I put the extra insulation. I shouldn't be trying so hard to save the clients money.

 

[freddiemercurystwin]

I dictate to my SAP eng as to where I can live with the increase. If you opt for a cheapy online calc you get a pants service (I expect) but I've been using my SAP engineer for years and he's a good chap, last job he did for me was a new barn conversion @ £180+VAT and the EPC was another £45.

 

[wessex101]

Interesting. Thanks for that.

 

[tony1851]

I shouldn't be trying so hard to save the clients money.

Why bust your gut to save the client a few £? Quite often, they don't appreciate it because they don't understand the technicalities.

Secondly, get on board with a local firm of private inspectors. Once they get to know you, my experience is that they will often wave these things through
on the basis of a few random figures. Conversly, LABC will give you the runaround.

 

[wessex101]

I shouldn't be trying so hard to save the clients money.

Why bust your gut to save the client a few £? Quite often, they don't appreciate it because they don't understand the technicalities.

Secondly, get on board with a local firm of private inspectors. Once they get to know you, my experience is that they will often wave these things through
on the basis of a few random figures. Conversly, LABC will give you the runaround.

Quite true, I really have got to toughen up. I'm rather old school and always go the extra mile. And no, they probably don't appreciate it, in fact it probably counts against me as I'm too thorough and therefore not the quickest.

As for using a private inspector. I've looked into it a while ago and we weren't really covered down here for small residential jobs, those that did were quite expensive, again trying to save clients money. For my sins when I started out about a million years ago I was a trainee building control officer so I do have a bit of a soft spot for them but some of the wallies I have to deal with these days, it isn't up to the standard of the good old days. It is situations like this where you would appreciate a bit of common sense when you get the pedantic jobsworth but we will see how it turns out, I've jiggled the figures and got my area weighted calc within a fraction of complying even without the triple glazing.

 

[Nakajo]

There are good BCOs and bad BCOs, and good AIs and bad AIs. As a gross generalisation, when dealing with fairly run-of-the-mill stuff, my sense is that AIs are more useful and accommodating at the design stage and BCOs are more rigorous during the execution (which is what I want). That said, I think my practice is gradually moving over to using AIs pretty much exclusively.