Part O building regs glazing limitations

[Jane W]

We have been planning a 150sqm house with lots of fairly large windows including glazed gables facing north and East, and 2 or 3 sets of bifold doors covering almost all of the ground floor west facing elevation. Looking at how others have had to reduce the size of windows etc I'm guessing this is no more than a fantasy now. Not done any calcs as only just secured land and playing with possibilities of what we could fit on the plot . But thinking lots of light, lots of big windows is impossible. 

Could anyone who is further on, with designs or calculations in place give any advice or comment on how part O has affected their design,  especially if your plans have large expanses of glazing. Thanks 

[Temp]

I've not studied part O but perhaps overhangs and or external shutters can be used to help meet the regs?

[PNAmble]

Design the house you want, then run a Part O model and add mitigations.  But if you are doing something complicated you’ll need to pay someone to model it, (cost us less than £400) as the simplified spreadsheet doesn’t cover external blinds, MVHR etc.   or use PHPP if going Passiv.  

[AppleDown]

3 hours ago, PNAmble said:

Design the house you want, then run a Part O model and add mitigations.  But if you are doing something complicated you’ll need to pay someone to model it, (cost us less than £400) as the simplified spreadsheet doesn’t cover external blinds, MVHR etc.   or use PHPP if going Passiv.  

Could you PM who you used please - sounds a good price!

[Canski]

I had Part O calcs done for £200. I will DM you details.

[PNAmble]

1 hour ago, AppleDown said:

Could you PM who you used please - sounds a good price!

PMed you. 

[Alan Ambrose]

I have similar objectives to you i.e. a good amount of glazing - so, I'm planning on putting in the max amount of glazing while squeaking through the regs. My strategy (which I won't know completely works until I have planning and BC approval) is as follows:

 

+ read and understand the Part O simple calc. Bear in mind these opaque calcs were written by and for the big developers and originally drafted to prevent overheating of city centre flats FFS. Why single self-build dwellings should be landed with them you'll have to put down to a lack of political nous on behalf of the self-build community.

Consequently these calcs don't take account for external blinds, brise soleil, low G glass etc - which is all stuff the big developers won't do - they would rather give their customers smaller windows. 

+ do an initial house design with the amount of glazing you prefer. Think about the house orientation re the sun. You may want to do a check against Part O Simple Method and PHPP simple % guidelines to see if you're anywhere in the ballpark. Note that restrictions on N glazing are fairly light, W & E glazing a bit more and max restriction on S facing.

+ send in your design for planning approval, knowing that you will probably reduce and/or amend the glazing later. That should be no problem, it's adding glazing later that will exercise the planners.

+ find a BC that will accept PHPP solar gain calcs, my LPA BC, for instance, will only accept Part O (or presumably TM59).

+ model well for solar gain with PHPP and tweak the design to get under the PHPP bar. You may need any or all of complete shading modelling / brise soleil / external blinds / lower G glass.

+ if you can't get what you want and you have an OK budget, try modelling with TM59 and see if that comes up with a better result than PHPP.

+ check that BC will sign off your calcs and run the glazing amendments past planning for approval.

 

Depending on your design and your proclivity for running calcs yourself you may need a consultant for PHPP and even for part O if you prefer to spend cash rather than exercise your Excel skills. (Actually I believe there's an online calculator now.)

You will almost certainly need a consultant for TP59 if you want to use that instead.

 

 

[Alan Ambrose]

As a follow on (as is common here with the post-and-dash brigade, we have not heard again from the OP)...

 

I'm finding this strategy is working with the PHPP calcs - so I can try out the variations of brise soleil, external blinds, low-G glass etc ensuring that I limber under the PHPP overheating limit.

 

I'm aware though that this is a somewhat fragile strategy (the PHPP overheating calc itself is a bit fragile anyway) and, in practice, reducing the glazing a bit might make the chosen solution a bit less brittle.

 

This does also force you to start selecting particular frames / glass / blinds for the modelling, which I guess is good.

[Jane W]

On 16/04/2024 at 12:20, Alan Ambrose said:

As a follow on (as is common here with the post-and-dash brigade, we have not heard again from the OP)...

 

 Apologies...not so much a post and dash, more of a busy-life-read-replies-slow-responder🤦🏼‍♀️

I like the idea of using the PHPP although having briefly looked, it seems quite a task. Think I'll definitely try to use it for modelling once all of my fairly random and unorganised ideas come together to create a more fully assembled and organised design. Being able to model and adapt the vast number of variables in all adpects of the build will be very useful if not slightly overwhelming. My rookie thinking in terms of part O is that if glazing choice and mvhr is not enough to prevent overheating then some type of canopy or ac cassette would be the backup plan. From what I've read so far it seems that the more complicated calculations required to take account of things like mvhr would need a professional to help. 

[Jane W]

On 31/03/2024 at 19:20, Alan Ambrose said:

I have similar objectives to you i.e. a good amount of glazing - so, I'm planning on putting in the max amount of glazing while squeaking through the regs. My strategy (which I won't know completely works until I have planning and BC approval) is as follows:

 

+ read and understand the Part O simple calc. Bear in mind these opaque calcs were written by and for the big developers and originally drafted to prevent overheating of city centre flats FFS. Why single self-build dwellings should be landed with them you'll have to put down to a lack of political nous on behalf of the self-build community.

Consequently these calcs don't take account for external blinds, brise soleil, low G glass etc - which is all stuff the big developers won't do - they would rather give their customers smaller windows. 

+ do an initial house design with the amount of glazing you prefer. Think about the house orientation re the sun. You may want to do a check against Part O Simple Method and PHPP simple % guidelines to see if you're anywhere in the ballpark. Note that restrictions on N glazing are fairly light, W & E glazing a bit more and max restriction on S facing.

+ send in your design for planning approval, knowing that you will probably reduce and/or amend the glazing later. That should be no problem, it's adding glazing later that will exercise the planners.

+ find a BC that will accept PHPP solar gain calcs, my LPA BC, for instance, will only accept Part O (or presumably TM59).

+ model well for solar gain with PHPP and tweak the design to get under the PHPP bar. You may need any or all of complete shading modelling / brise soleil / external blinds / lower G glass.

+ if you can't get what you want and you have an OK budget, try modelling with TM59 and see if that comes up with a better result than PHPP.

+ check that BC will sign off your calcs and run the glazing amendments past planning for approval.

 

Depending on your design and your proclivity for running calcs yourself you may need a consultant for PHPP and even for part O if you prefer to spend cash rather than exercise your Excel skills. (Actually I believe there's an online calculator now.)

You will almost certainly need a consultant for TP59 if you want to use that instead.

 

 

I think you hit the nail on the head- start with what you want and compromise just as much as necessary to get through. We have no glazing North or South, but would like double height glazing in the entrance to the East, all other east facing rooms have standard sized windows, and we're hoping to have almost wall to wall windows on the West facing elevation ground floor( living kitchen and sitting room) and standard windows upstairs. It is the new room by room calculation that make it more difficult to pass. The standard calculation also doesn't account for mvhr which we really hope to have. Not sure if you or anyone else knows how much difference that makes? I'm thinking that to reduce the compromise I'd rather go a/c cassette...

[Alan Ambrose]

>>> would need a professional to help

 

It depends how much you want to push the limit. If you start with the PH glazing suggestions:

 

...design for good, but not excessive daylight, with a maximum of about 25% of the internal south facade as glazing. All other orientations
should ideally have only as much glazing as is needed for views, daylight and ventilation. As a rule of thumb total glazing area (excluding frame)
of around 15-20% of treated floor area (TFA) is a good starting point for design.

 

https://www.passivhaustrust.org.uk/UserFiles/File/Technical Papers/How to Build a Passivhaus - Rules of Thumb - compressed.pdf

 

Or you can start with the Part O limits and work from there.

 

If that doesn't look like enough glazing for you and you want to push the limits then solar glass / brise soleil / external blinds plus PHPP or TM59 or both. (If both, you can then take the best result if your BC will allow it.)

 

At that point, you're working quite hard (and quite expensively) to get extra glass.