solar control glass – worth it?/calculations

we’re having a large amount of (westsouthwest facing) glass put in our house a part of an extension – total door area is around 14m2, so glazed area slighly less (sliders with narrow frames). The room behind the glass will have a footprint of ~7m x 8m, normal height ceilings so a total volume of ~135m3

One question is do we want solar reflective glass. It adds a reasonable chunk to the cost (£500 or so). The room today is half the size, with half the glazing, and gets hot.

My wife is keen, I’d like to understand how much of a difference it really makes, both from experience and from stats.

I guess first off – has anyone had windows replaced with it and found it made a big difference? Are light levels significantly lower?

I found a calculator https://susdesign.com/windowheatgain/index.php

that works out the kwh gain per month of different windows. I’ve used their “G value” for normal alu/double glazing (0.65) and a mid-range reflective G value (0.4) to compare the results.

This seems to give a peak hour (mid-afternoon in July/August) of around 0.44 kwh/m2 vs 0.27 kwh/m2 of solar gain for normal vs coated windows, which multiplied by 13m2 of glass gives 5.8 vs 3.5 kw coming into the house at that time – a saving of 2.3kw

is there a way to calculate how much heat gain that is on a day when its warm outside? The house is well insulated (10 year old new build), lets say the day outside starts at 20C and peaks at 30C in the afternoon – how can I figure out what the difference in interior temps is from the solar gain? There’s almost no direct sun on the window at all until after lunch, so I imagine the differences are minimal until then.

Definitely go for solar control glass – it is money well spent, especially if your current room gets hot.

We fairly recently installed some south-east facing 4.5m wide x 2.6m high sliding doors in our extension also has a 2.5 x 2m roof lantern – both have solar control glass.

We thought that the main room (6 x 6m) might end up feeling like a greenhouse in the summer, but the glass keeps it at a decent temp & you wouldn’t realise it was slightly grey tinted unless you directly compared it to standard glass – there are different types available though which are more visible / coloured.

In direct sunlight you can really feel the sun on your skin but move behind the glass & you can barely feel the rays.

The calculator seems to spell it out pretty well giving you the ability to specify location, direction and size and returning kW/h of thermal generation.  On any given day, you can see the thermal glass is reducing the flux by almost 40%.  That’s a ton of blocked heat on a summers day, even just the afternoon, you looking at 12-18kwh.  That’s like running a 2kW fan heater for 6 hours.  Even in a large room, that would be uncomfortably warm.

Overheating is really common with the fashion for tons of full height glazing. Consider what you can do to shade the glass externally as well as solar control glass. You could also consider reducing the glazed area  framing a view rather than full height. Low level glazing increases overheating but does not help with increasing useful daylight.

Things are only going to get hotter.

It adds a reasonable chunk to the cost (£500 or so).

It’ll cost you a chunk more in a couple of years to replace every pane. You’ll also give Mrs 5lab the opportunity to remind you of this for the rest of your days…it’s worth £500 just to avoid that 🙂

How much of a chunk in the grand scheme, rather than the cost of glass?

thanks for all the advice.

How much of a chunk in the grand scheme, rather than the cost of glass?

in terms of the extension, not much. What I’m trying to figure out is how much (in terms of temperatue) difference it will make, but I don’t know how to run the maths. If it only saves us 1C, probably not worth it, if it saves us 5C, now we’re talking.

I’ve 135m3 of air with an average of 3.5kw or 5.5kw of energy going into it for 5 hours. The exterior air temperature on these hot days is probably close enough that once insulation is taken into consideration, heat loss through the walls is minimal.

I think 1m3 of air weighs 1.29kg – so 135 is ~175kg of air that can be heated up.

this calculator suggests that to raise air from 25 to 35 degrees requires about 0.5kwh of heat, to raise it to 100C is 3.5kwh (ie 1 hour with the solar glass) so I’m obviously doing something wrong 🙂

Heat capacity calculator

It is always far more expensive to solve an overheating issue than to design it out in the first place. £500 sounds like a bargain. But as I mentioned before, external shading from brise soleil/trees etc or less glass is the best option

the reason we have a lot of glass is because the room is deep (8 meters in places) and the glass along one edge of it is the only natural light source – due to neighbours/other rooms at the side and above we can’t get light in any othe rway. We’ve been in a bunch of houses where the rear is gloomy all winter long, and don’t want that for our main living space.

There might have been another way to design light in earlier, but right now we have a 6 x 2.4m hole in our wall, so need some solution for glazing it