What would you fit and why?

[Makeitstop]

Part of a job my daughter and son in law have underway is posing an either or type discussion..... This is..... 

 

The rear of the property is being extended and will be approx 70..ish sq metres once complete.

 

The aspect is pretty much dead west facing, so plenty of sun.

 

The glazed elements of this rear section are....

 

Slider of approx 4 metres by 2.4 metres

Single pane t&t window of approx 1.8 metres high by a metre wide.

Velux Rooflight of approx 2.5 metres long by 1 metre wide, split into two sections, one half fixed and other half electrically operated opener, fitted into small section of flatroof.

 

Now, given the fact that this is west facing, which means a fair solar gain, would people choose 2g with solar glass, or 3g with standard glass?

 

I ask, as the prices come in about the same for each.

 

To me anyhow, this is how they stack up....

 

Benefits of 2g and solar glass

 

1) less solar gain. Thats about it from what I can come up with.

 

Benefits of 3g std glass

 

1)  Less noise during heavy rain

2) Better ability to trap in warm air that rises within the room during colder weather

3) Lower risk of condensation

 

Given the fact that this rooflight has the ability to open during hot summer weather too, which allows warm air out, how much benefit does solar glass provide in reality?

 

Both will have some affect on light transmission I know, but I've not looked into what the "actual" diff is.

 

At the same cost, what would you pick and why?

 

Am I missing anything obvious here?

 

 

Edited Thursday at 19:21 by Makeitstop

[Sparrowhawk]

My understanding of solar control glass is that up close you can see the coating on it. And that coating also reduces the light transmittance to around 70%, similar to triple glazing.

 

So if the slider can take the weight then I'd go 3g

[Jonny]

Also, with solar glazing, you are losing the benefit of “free” solar heating in the winter months. 

[Makeitstop]

The question relates to the rooflight only guys.

 

 

[Mr Punter]

Probably 3g as you may get better Ug value.  External blind if solar gain needs eliminating.

[Makeitstop]

My view on any significant solar gain, is that it's going to be happening in the warmer months, when the external temp is elevated also. 

 

This is where the electrically operated opener comes into its own, as the warmer air is going to travel up into the ceiling void where the rooflight is positioned. If that is opened up, it allows that warmer air to escape.

 

If it were a fixed rooflight, then the need for control of any gain is more crucial yes?

Edited Friday at 16:26 by Makeitstop

[Mr Blobby]

All glass absorbs some solar gain so standard 3G is always going to have less solar gain than standard 2G.   So 2g with solar glazing makes no sense to me as the solar gain% on 2G with solar glass is probably about the same as 3G but just colder in the winter.

 

If you really want a roof light with low G then the velux passive has 5 panes (now mostly discontinued) of standard glass and so the G factor of just 35%.  .

For the slider, look at 3G with SKN 176 solar glass.  This is what we have on our big kitchen slider and it is amazing.   It looks exactly the same as standard glazing but is way cooler.

 

Solar gain factor, as percentage of the heat that gets through is typically:  Standard 2G = 65%,  Standard 3G = 50%  3G + SKN176 = 35%.  New velux with solar glass coating = 29%.

 

3G all the way, more panes the better, and SKN176/other solar coating where it is needed.

Edited Friday at 19:38 by Mr Blobby

[SteamyTea]

Reducing the size of the window would lower the energy input into the room.

 

[Makeitstop]

Like an utter moron, i forgot to make clear that the 2g solar glazed v 3g std glazed part of this question relates to the rooflight only.

 

The slider and window are going to be 2g only.

 

They have been leaning towards 2g opening Velux rooflight with solar glass, but i feel a standard triple glazed unit offers more for the same cost.

 

If the room gets hot during summer, and to some extent due to solar gain, they can open the roof window to allow warmer air out. The benefit of 3g during winter is surely a real advantage, as it has better ability to trap any rising warm air generated by UFH within the room.

 

To my thinking, 2g with solar control glass is moot if you can vent the bloody heat out.

[Nickfromwales]

32 minutes ago, Makeitstop said:

Like an utter moron, i forgot to make clear that the 2g solar glazed v 3g std glazed part of this question relates to the rooflight only.

Can you go and stand in the corner, face the wall, and we'll tell you when you can come and join the class again.  

 

 

 

On 17/09/2026 at 20:20, Makeitstop said:

Given the fact that this rooflight has the ability to open during hot summer weather too, which allows warm air out, how much benefit does solar glass provide in reality?

You are herby absolved of any guilt.  

[Makeitstop]

Ive been stood there like a naughty boy looking at the floor until the next post came up to gain my attention.

 

Thanks Nick. My legs were starting to ache a bit.

[Makeitstop]

Soz about looking at floor and not the wall.

 

I'll go back to the corner

Edited 44 minutes ago by Makeitstop

[Nickfromwales]

7 minutes ago, Makeitstop said:

Soz about looking at floor and not the wall.

 

I'll go back to the corner

We don't enjoy doing this, trust me, but it's for your own good......

 

😉😆

[Makeitstop]

Accepted