Anticipating solar gain / overheating in a new extension

[Brix]

Hi,

 

My architects are proposing a relatively large area of glass roof on a new kitchen extension (something between 35-45%). In fact more accurately it is a glass box on one end of the new extension. They specified solar control glass to manage heat gain. Gain from the glazed vertical elements should not be much of an issue due to orientation. The architect and the glazing supplier are confident that they can make the extension meet building regs on paper.

 

I love the design but am concerned that even if it passes building regs the built reality could still be uncomfortable. The extension is not a huge volume and in summer the roof will be exposed to full sun (weather permitting!) between roughly 9.30-4. Both seem to be of the opinion that solar control glass will effectively manage heat gain.

 

Can the energy calcs for building regs be relied upon to expose any issues? Or are there additional checks / modeling that might help?

 

Thanks.

[Iceverge]

What direction is it facing? 

 

North, no problem.

 

Anything else will get too hot. Solar glass is a bit of a half solution. It can compromise the view and then not work perfectly well either. 

 

Overhangs work well for South facing windows but not so much for East West. 

 

External blinds work well but block the view. 

 

If it is outside the thermal envelope you can simply shut the heat/cold out from the main house like @joe90 did on his house. 

 

 

My personal opinion is that very large glazed areas only work when you have a very large heating and cooling budget or a very scientifically skilled design. 

 

 

 

 

 

Edited April 2, 2023 by Iceverge

[Brix]

Thanks.

 

The glazed roof is almost flat, so not really orientated. Slight pitch facing west.

Vertical glazed elements are east and north facing.

It's a kitchen, so not outside the thermal envelope. But in any case, it's the space itself I want to be comfortable rather than exclude it from any calcs.

 

The purpose of the glass is to minimise visual impact as it is a listed building so screens might be an issue.

 

Just not sure whether I'm being cynical or realistic about the design! 

[Simon R]

2 hours ago, Brix said:

The purpose of the glass is to minimise visual impact as it is a listed building so screens might be an issue.

We fitted internal electric blinds on our roof lights, they do a pretty good job of stopping unwanted thermal gain. We also ended up with an A2A split unit run of PV to keep everything comfortable.

[Russdl]

We probably need a bit more info, but if you’re being assured it’s not going to overheat in the summer are you also being assured it’s not going to lose heat massively in the winter?

 

3 hours ago, Brix said:

so not outside the thermal envelope.

because as Iceverge points out

 

3 hours ago, Iceverge said:

very large glazed areas only work when you have a very large heating and cooling budget

 

Edited April 2, 2023 by Russdl

[ADLIan]

Appr Doc L limits the glazed area of an extension, go slightly over and you will have to use high performance glazing/improved fabric insulation and use the area weighted average u-value method to show compliance. Go a lot over and you’ll be using full SAP calculations to show compliance. This may give an idea of overheating but SAP is not a very sophisticated tool in this respect

[Brix]

29 minutes ago, ADLIan said:

Appr Doc L limits the glazed area of an extension, go slightly over and you will have to use high performance glazing/improved fabric insulation and use the area weighted average u-value method to show compliance. Go a lot over and you’ll be using full SAP calculations to show compliance. This may give an idea of overheating but SAP is not a very sophisticated tool in this respect

I think this is my concern - that SAP is not very sophisticated. Certainly not an assurance that it's going to be comfortable.

[Alan Ambrose]

Rather than SAP which is a bit simple - you could check the calcs using Part O Simple method / TM 59 / PHPP / Part L criterion 3 if you want to get a feel for the likely overheating risk.

 

Alternatively, you could specify enough 'mitigation measures' by gut feel (solar glass, external blinds, shading etc) to cope with any likely overheating. Ideally your architect would do all that so that their client does end up disgruntled.

 

[Iceverge]

5 hours ago, Brix said:

Thanks.

 

The glazed roof is almost flat, so not really orientated. Slight pitch facing west.

Vertical glazed elements are east and north facing.

It's a kitchen, so not outside the thermal envelope. But in any case, it's the space itself I want to be comfortable rather than exclude it from any calcs.

 

The purpose of the glass is to minimise visual impact as it is a listed building so screens might be an issue.

 

Just not sure whether I'm being cynical or realistic about the design! 

 

Ah, a complete and total glass box . 

 

Install a high efficiency A2A mini split unit then and just pay the running costs I would say. The amount of external shading and internal insulating blinds you will need to make it work like a normal room will ruin the architectural merit. 

 

Even an excellent window is 5 times worse than an average wall these days in terms of heat loss and gain. 

 

 

 

 

[Brix]

3 hours ago, Alan Ambrose said:

Rather than SAP which is a bit simple - you could check the calcs using Part O Simple method / TM 59 / PHPP / Part L criterion 3 if you want to get a feel for the likely overheating risk.

 

Alternatively, you could specify enough 'mitigation measures' by gut feel (solar glass, external blinds, shading etc) to cope with any likely overheating. Ideally your architect would do all that so that their client does end up disgruntled.

 

Thanks. I'll ask about these calcs.

[Brix]

3 hours ago, Iceverge said:

 

Ah, a complete and total glass box . 

 

Install a high efficiency A2A mini split unit then and just pay the running costs I would say. The amount of external shading and internal insulating blinds you will need to make it work like a normal room will ruin the architectural merit. 

 

Even an excellent window is 5 times worse than an average wall these days in terms of heat loss and gain. 

 

 

 

 

Today is the first I've heard of the A2A mini split unit. Not quite sure what it is but will look it up. Some kind of air con?

[SteamyTea]

7 hours ago, Brix said:

Some kind of air con?

https://www.appliancesdirect.co.uk/p/eiq-12wminv/electriq-eiq12wminv-wall-split-air-conditioner

[Brix]

Thanks for the suggestions. I have to admit I feel that if the design of the new extension relies on an air conditioner, it is probably not the right design.

Edited April 3, 2023 by Brix

[Gill]

8 hours ago, Brix said:

Today is the first I've heard of the A2A mini split unit. Not quite sure what it is but will look it up. Some kind of air con?

 

Air to Air system heat pump (outdoor) and indoor air conditioning unit(s).

 

We're in the process of getting quotes to use a2a for home heating with the added benifit of ability to cool in summer. Our sun room overheats even in winter direct sunlight

 

The models we're looking at are stating COP of 5. Tied into PV that could be very cost efficient without having to change your designs. 

 

 

 

[SteamyTea]

44 minutes ago, Gill said:

added benifit of ability to cool in summer.

Your in Glasgow!

[Gill]

32 minutes ago, SteamyTea said:

Your in Glasgow!

Where the sun is cracking the slabs today! I expect a sun room temperature of 26 degrees by 11 am. 

[SteamyTea]

12 minutes ago, Gill said:

Where the sun is cracking the slabs today! I expect a sun room temperature of 26 degrees by 11 am. 

My mate up there, who is from NI, so knows about miserable weather, says that he appreciates it when the sun reached it peak altitude, around 15° isn't it.

 

This sunroom, how many days does it heat up.

That is quite a serious question, as well as being flippant.

Edited April 3, 2023 by SteamyTea

[saveasteading]

Skylights provide a huge amount of light compared to windows. Perhaps one or 2 small ones will provide enough light and be very much easier and much cheaper to build. 

You can get opening ones too, which would let the heat out quickly but you have to remember to shut it. With smaller area it wouldn't be a problem though.

 

Watching 'Your Home Made Perfect' is fun.

Without fail:

1. the architects show huge or entire roof glazing, with no structure.

2 on screen script says check the regulations for overheating and consult SE.

3. Finished product has a sensible compromise. They don't comment, on whether this was bco, SE, builder , budget, or all of these....or at all.

4. Closing credits say the Architects are only providing inspiration, and to check reality.

[Iceverge]

10 hours ago, Brix said:

Today is the first I've heard of the A2A mini split unit. Not quite sure what it is but will look it up. Some kind of air con?

 

Air-conditioning = Air to Air heat pump (A2A).  It's an air source heat pump (ASHP) but only heats air, not water (A2W). 

 

Some models only cool efficiently. Others heat with a very good efficiency too. This is commonly called coefficient of performance (COP) . For example if you put 1kWh of energy in at a COP of 3 you'll get 3kWh of heating. 

 

The manafactuers must produce a SCOP (Seasonal COP) figure for your climate zone.  This gives a good impression of what real life performance will be over a year. I've just bought this model with a SCOP of 5.1 for heating. Not installed yet. 

 

 

 

1 hour ago, Brix said:

have to admit I feel that if the design of the new extension relies on an air conditioner, it is probably not the right design

 

The trouble is it's not suited for colder climates. Cooling can be overcome by simply opening all the doors and windows (security and insects allowing) and letting the wind blow through or pairing it to PV and Aircon. 

 

 

However at night or in winter it'll require a lot of heating to stay hot. 

 

Let's assume you have an outdoor temp of 5 Deg and want 20 deg inside. You spec really really top class windows with an installed U value of 1w/m2K. 

 

Now let's say the extension has a glass area of 50m2 and a floor of say 16m2 at a U value of 0.15 w/m2K. 

 

Heat loss floor is

 

16m2 X 0.1w/m2K X (20deg-5deg) = 36w

 

Heat loss glass box is

 

50m2 X 1W/m2K X (20deg-5deg) = 750w.  

 

For context this about half the heat load of our entire 185m² passive house for your 16m². 

 

 

Ok in California or the Mediterranean with warm temps all year around. Elsewhere its costly to run. 

 

 

 

 

 

 

 

 

 

 

 

 

[saveasteading]

I've looked at spanish units as they seem so cheap. They are made for cooling and not efficient for heating. Noisy generally too. Electricity was cheap until recently too, but no longer.

[Brix]

2 hours ago, saveasteading said:

 

4. Closing credits say the Architects are only providing inspiration, and to check reality.

 

This is where I am coming from!

[Gill]

2 hours ago, SteamyTea said:

This sunroom, how many days does it heat up.

Last month we probably had at least 7 days where the temperature in there was over 20. 

Doesn't take much sun for the temperature to climb quickly. It will get any sun unobstructed morning through to early evening. 

 

[SteamyTea]

13 minutes ago, Gill said:

Last month we probably had at least 7 days where the temperature in there was over 20. 

Doesn't take much sun for the temperature to climb quickly. It will get any sun unobstructed morning through to early evening. 

 

Pretty good really.  Makes me wonder why we don't fit a glass panel over 'daylight' walls and pump the thermal energy around the house.  A posh trombe wall, but not using the wall as storage so much.

[joth]

58 minutes ago, SteamyTea said:

Pretty good really.  Makes me wonder why we don't fit a glass panel over 'daylight' walls and pump the thermal energy around the house.  A posh trombe wall, but not using the wall as storage so much.

Cycling through Essex a few months ago I saw a modern new build rural mansion with a three story hallway staircase "bolted onto" the side of the house entirely surrounded in glass. I couldn't help but grudgingly acknowledged how impressive it looked.

I'm sure it's total hell for heating/cooling, but if designed well (LOL) i.e. to be outside the main building thermal envelope it could act a bit like this. A giant greenhouse. Still be stinking hot in summer though. 

There was clearly an ungrounded swimming pool going in too, so odds are the whole place designed to hemorrhage money.

 

[SteamyTea]

1 minute ago, joth said:

whole place designed to hemorrhage money.

That is Essex for you, did they have primroses in the garden.

 

(I spend some of my early years in Essex and still have the accent when I am tired).