Fixed vs openable rooflights

[Adsibob]

Slightly overwhelmed with choice. Architect has specified 1 fixed rooflight and 4 openable ones, all 2G, without specifying any further details other than dimensions. With the fixed, it isn't too much more to go for 3G, so I think I will do that (although it's a non-standard size, 3700mm by 100mm so need to actually check 3G is affordable... it probably isn't!) My question is, for the other four roof lights, do I really need them to be openable? I describe the size and location of each, they are all being fitted in what will be a brand new tiled roof (probably concrete tiles), so insulated to current building regs. We are not building to passive house standards, just doing up a 1930s house as best we can without going too crazy with the budget.

 

1: 1000mm by 2000mm In the master bedroom, part of a flat roof, not directly above the bed, but pretty close. It's towards the side of the room near the room's rear window (which are going to be 2G with extra solar control). The room is west facing and my main concern is to avoid overheating (we sleep on a tempur matress which can get quite warm), so the better U value with 3G would obviously help with that, but if it is to be openable, that gets very pricy. The ceiling is not too high, so wouldn't necessarily need automation to open it, although that would be nice, but I'm wondering whether if we go for 3G fixed, we will miss being able to open it? It's so close to the window anyway that if both that and the window were open in summer, I'm not sure we'd get a throughbreeze anywhere in the room other than between the two windows; does anybody have experience? The two external walls in this room are: (i) an upgraded 1930s solid single brick wall, containing no windows; and (ii) a rear cavity wall, west facing, built this year to building regs with the 2G windows.

 

2: Two windows close towards the top of the house, either side of the roof's ridge. One is West facing the other is East facing. Both are above a central stairwell and are intended to bring lots of light into the middle of the house. The East facing one is 1500mm by 1500mm, whereas the West facing one is slightly smaller at 1500mm by 1000mm. These are so high up we would definitely need remote motorisation to open them. But should both open or is that overkill? Will I get too much overheating with 2G?

 

3: A window on the East facing side of my loft conversion (converted to current building regs), but on the pitched part of the roof, also quite close to the roof's ridge. 1000mm by 800mm, though should possibly be bigger given this is at one end of quite a long room, with west facing windows at the other end. I will definitely go 3G for this one for the extra solar control as this will be my office and I work from home 2-3 days a week (even in normal times).

 

In terms of other sources of ventilation in the house... we are looking at possibly installing a MHRV, although to get a really quiet one we've been told we need to spend £3k - £3.5k plus on a Zhender Q450 (and that doesn't include the installation of it!) which isn't really something we'd budgeted for. If the answer to my queries above is: if you have a good MHRV you don't need openable rooflights, then that would be an easy solution as the savings on the windows will pay towards a big chunk of the MHRV. But I'm not sure that is the answer, because the MHRV doesn't cool a room as far as I'm aware.

 

Any help much appreciated. I need roof windows delivered by 16 March, so don't have heaps of time.

 

[MikeGrahamT21]

I'm not 100% sure what you are aiming for, but heres a couple of points which sprung into my head whilst reading your post:

 

• 1m x 2m flat roof window almost directly above the bed - Assuming you are going to have come kind of blind on this to stop light coming through at night?
• Solar control and 3G are two totally seperate things. Yes a 3G panel on a pitched roof window will heat up less than a 2G one, but I also had to install exterior awnings on mine to really bring down the imported heat on a summers day. You can get solar control glass in both 2G and 3G which tends to be tinted, and this is totally seperate to how many panes there is. You can also have solar control film applied to the outside which is reflective from the outside, and almost transparent from the inside.
• Whether you go openers or not depends on how much you like a breeze through the house. The MVHR will provide all the ventilation required to meet BC standards, but there may be other things to think about such as fire escapes.
• Whilst I appreciate a very expensive Zhender MVHR would be super quiet, there will be other units which will come pretty close. Take a look at some other units and their sound output at the same amount of airflow as the one you've been recommended. I bought a £400 Vortice unit off ebay brand new and i'd say its pretty quiet, so I wouldn't say there is a need to spend a fortune, just compare the specs, and perhaps speak to BPC Ventilation about this, they will be able to recommend a unit which meets your needs.

Hope this helps with choosing and maybe saves you a few ££

[Gone West]

We have a triple glazed roof window in our bathroom with a g-value of 0.32. It doesn't cause overheating in the bathroom and is only opened, when the weather is hot, to purge heat in the evening. Roof windows are very good for purging heat in conjunction with an open door downstairs. Double glazing can also have low g-value glass.

[TonyT]

I installed 3 velux windows in my roof, went for 3G for extra heat/noise insulation purposes.

 

2 years in and never opened them.

I suppose it depends on the strategy you are planning for heating and ventilation 

[AliG]

It sounds like some of these are roof lights on a flat roof and some are Velux type windows on sloping roofs.

 

If you look on this website you can price up various options -

https://roof-maker.co.uk/?_quoting_tool_version=2&utm_expid=.VxwcUnfvQKuUhOW5HQUrpA.1&utm_referrer=https%3A%2F%2Froof-maker.co.uk%2Frooflights%2Fflat-rooflights%2Ffixed-flat-rooflights%2F

 

Suffice to say that an opening roof light is roughly 2x the price of a fixed one.

 

I presume that you mean 3700x1000mm for the fixed light. That is enormous and will be very expensive. The company above will only quote up to 3m, I would check that 3.7m is not a massive increase in price.

 

Similarly the sloping roof lights would likely be Velux windows. these come in numerous standard sizes and a sensible architect would have drawn the windows to one of these sizes not a much ore expensive bespoke size.

 

We have a few Veluxes and are about to buy a couple of the flat 3G lights for a new project. For me the main reason was that roof lights are noisy when it rains and the 3G was quieter.

 

They do offer useful ventilation, but if the room has other openings in it then that should suffice. Normal windows are barely any more expensive if they open versus being fixed. An electric ceiling mounted window will be very expensive by the time you include extra electrics etc. If there is a window you can just open easily that will likely provide all the ventilation that you need. So I would only pay extra for opening windows where you need them.

 

As to the MVHR that is nonsense, the suggestion you need Zhender for it to be quiet. We have a much cheaper Dantherm system and cannot hear it. Nevertheless it is not cheap and it will not provide enough ventilation to cool your house. I doubt it is worthwhile in a 1930s house, especially if you still have windows with vents them in the rest of the house.

 

 

[AliG]

Sorry I was a bit rushed answering this. In fairness Roof Maker can make sloping roof rooflights which you could use instead of Velus windows, perhaps that is what your architect had in mind.

 

So-

 

3700x100/0 - I would check the cost as it is an unusual size. If the cost is prohibitive can the size be changed? 3G for noise, insulation and less solar gain, but if it has to be the very large size it may have to be 2G for weight issues.

 

1. 3G for noise and insulation. No need to open as you can open the normal window. Opening is very expensive on a flat roof window.

 

2. These would probably be Roof Maker style sloping roof lights. They come 3G as standard I think. Opening is an extra £295+ extra cost for electrics etc. If there is no other opening window in the room I would make one of them opening otherwise I would not bother.

 

3. Seems like this should be a Velux window perhaps as you could reach it and open it manually.

 

Edited January 12, 2021 by AliG

[Adsibob]

This is all very helpful. Thank you. I will investigate these options and report back.

[Adsibob]

3 hours ago, AliG said:

If you look on this website you can price up various options -

https://roof-maker.co.uk/?_quoting_tool_version=2&utm_expid=.VxwcUnfvQKuUhOW5HQUrpA.1&utm_referrer=https%3A%2F%2Froof-maker.co.uk%2Frooflights%2Fflat-rooflights%2Ffixed-flat-rooflights%2F

Amongst the dizzying array of options on roofmaker is the tint I want for my roof lights: none, subtle blue, subtle grey or privacy.

 

Privacy is out because I want to see the sky.

 

The advantages of the coloured tints, according to roofmaker are just that they protect your furniture from fading and take away some of the glare. What tints do people have experience of and are you happy you went for a tint?

 

Thanks!

[Gone West]

10 hours ago, Adsibob said:

Amongst the dizzying array of options on roofmaker is the tint I want for my roof lights: none, subtle blue, subtle grey or privacy.

 

Privacy is out because I want to see the sky.

Has your architect calculated the level of solar gain through all these roof lights. The g-value of the glazing could be critical for comfort levels in the rooms affected.

[Adsibob]

On 13/01/2021 at 09:24, PeterStarck said:

Has your architect calculated the level of solar gain through all these roof lights. The g-value of the glazing could be critical for comfort levels in the rooms affected.

No, he hasn't. I asked him to and he said that normally one would get a specialist consultant to do this because mapping out the seasonally changing arc of the sun vis-a-vis the obstacles that cast shade on the house is quite technical and would require some modelling. Not sure if he is overcomplicating things or not.

 

Can anyone refer me to some (hopefully free) online calculators that can take inputs such as room size, window size, glass spec, window orientation, latitude coordinate of property  etc. so that I can try and work this out for myself?

[PeterW]

Just now, Adsibob said:

Can anyone refer me to some (hopefully free) online calculators that can take inputs such as room size, window size, glass spec, window orientation, latitude coordinate of property  etc. so that I can try and work this out for myself?

Sketchup does this for you if you take the map and the house in 3D and put it into a model. 

[Adsibob]

5 minutes ago, PeterW said:

Sketchup does this for you if you take the map and the house in 3D and put it into a model. 

Thanks @PeterWAccording to this only the paid for SketchUp studio does "thermal comfort analysis" and "daylight analysis". Or have you found an equivalent feature in their free version?

[the_r_sole]

I was on the Beta programme to try that sketchup advancement - it was absolutely awful imo, pretty much determined that no where in the uk would have overheating issues but maybe they improved it when bringing it into use

[Gone West]

23 minutes ago, Adsibob said:

No, he hasn't. I asked him to and he said that normally one would get a specialist consultant to do this because mapping out the seasonally changing arc of the sun vis-a-vis the obstacles that cast shade on the house is quite technical and would require some modelling. Not sure if he is overcomplicating things or not.

I've played around with FindMyShadow which is quite good for basic, when the sun would be hitting the glazing. I wanted minimal solar gain through my bathroom roof window so asked for the best glazing for reducing solar gain without cutting out too much light. The g-value of 0.32 with triple glazing seems a good compromise.

https://findmyshadow.com/

Edited January 18, 2021 by PeterStarck

[Dan F]

1 hour ago, Adsibob said:

Can anyone refer me to some (hopefully free) online calculators that can take inputs such as room size, window size, glass spec, window orientation, latitude coordinate of property  etc. so that I can try and work this out for myself?

 

Sketchup will give you an idea of how much light enters via rooflights, but won't take into account glass spec,  insulation or ventilation and tell you how much your house will/won't overheat. 

 

How highly insulated is the house going to be?  If its going to be pasive standard and have numerous rooflights then the best approach would be to model in PHPP, else I think you'd just be guessing as to what solar glass and/or shading you do/don't need.  If house is not quite so highly insulated, airtight or you don't have MVHR then its maybe less critical and you could go with fairly minimal solar glass probably and not worry.

Edited January 18, 2021 by Dan F

[Dan F]

On 12/01/2021 at 09:01, PeterStarck said:

We have a triple glazed roof window in our bathroom with a g-value of 0.32. It doesn't cause overheating in the bathroom and is only opened, when the weather is hot, to purge heat in the evening. Roof windows are very good for purging heat in conjunction with an open door downstairs. Double glazing can also have low g-value glass.

 

What brand did you use? We need to order one 1000x1000mm. Debating whether to make it openable though..

[Adsibob]

3 minutes ago, Dan F said:

 

Sketchup will give you an idea of how much light enters via rooflights, but won't take into account glass spec,  insulation or ventilation and tell you how much your house will/won't overheat. 

 

How highly insulated is the house going to be?  If its going to be pasive standard and have numerous rooflights then the best approach would be to model in PHPP, else I think you'd just be guessing as to what solar glass and/or shading you do/don't need.

It's very far from a passive house. We are:

• taking a 1930s semi that has had zero work done to it since 1930, apart from some first generation uPVC double glazing installed (which converted the first floor rear bedrooms into saunas during last summer - which was itself a pretty hot summer for English standards!); 
• completely gutting it extending the rear with a full width ground floor extension and a partial width first floor extension as well as converting the loft;
• replacing all the glazing as well as introducing 5 sky lights; and
• upgrading the insulation of the house with a new concrete or limecrete slab and 100mm of insultation in the floor, and then all old walls of the house (that are not being knocked down to make way for the extensions) are being upgraded with 52.5mm of Cellotex insulated plasterboard on the inside and 25mm of  this insulating lime based render
• obviously all of the ground floor rear wall and the extended portion of the first floor rear wall will be a cavity wall to current  building regs. All other walls in the house are NOT cavity walls.    

Appreciate that one can always put in thicker amounts of insulation, but we don't have the space for it.

 

In terms of solar gain, my main concerns are:

1. the rooflight in the flat roof of the first floor rear extension which will be the roof over part of our master bedroom - the architect has specified as 2000mm wide by 1000mm deep, to match the 2000mm width of the window this sits by. I think this is too big, so will probably keep the 2000mm width for aesthetic reasons but reduce the other dimension to 750mm or 600mm
2. the aluminium 2G sliding doors we will install in the new ground floor rear extension which will look out onto our west facing garden. These are pretty big, aproximately 3.7m wide by 2.15m high, which together with a large window that is also on that elevation (also 2G) mean the west facing wall of that ground floor extension is almost 65% glass.
3. the rooflights and veluxes in the loft which are west facing (i'm not too worried about solar gain from the east facing ones, as I think that will help warm up the space in winter more than it will bother us in summer).

On the sliding doors for instance, I asked the supplier about adding solar control glass to reduce the G value and the only options available are tinted. Is that always the case? As you will see from the attached picture, the tints look pretty dramatic! 

 

Doc1.pdf

[Gone West]

46 minutes ago, Dan F said:

 

What brand did you use? We need to order one 1000x1000mm. Debating whether to make it openable though..

It's a Fakro which can be opened.

[Adsibob]

12 minutes ago, PeterStarck said:

It's a Fakro which can be opened.

Was this for a flat roof or a pitched roof? 

[Gone West]

Just now, Adsibob said:

Was this for a flat roof or a pitched roof? 

It's a pitched roof.

[Dan F]

1 hour ago, Adsibob said:

On the sliding doors for instance, I asked the supplier about adding solar control glass to reduce the G value and the only options available are tinted. Is that always the case? As you will see from the attached picture, the tints look pretty dramatic!

 

You might need to find a different supplier!  A good solar-glass is near enough transparent and will let almost as much light through as standard glass while reducing the solar gain.

 

There may be other manufactuers available also, but this is the moslty commonly used stuff AFAIK: https://www.saint-gobain-glass.com/products/cool-lite-skn

 

Looking at the spec (when made up in double-glazing) this is what you are looking at:

- The 0.39 g version give 65% solar gain for 96% of light.

- The 0.36 g version give 60% solar gain for 84% of light.

- The 0.32 g version give 53% solar gain for 74% of light.

 

[Adsibob]

The thought of trying to find another aluminium sliding door supplier that can meet all my other requirements is now making me wonder about the solar control for the west facing sliding doors: in winter the solar gain will help keep the room warm. In summer, the whole point of the sliding doors is to open up the back of the house to the garden. Can anyone think of why in these circumstances I might regret not getting solar control for these sliding windows?

[MJNewton]

4 hours ago, Adsibob said:

As you will see from the attached picture, the tints look pretty dramatic!

 

Be very careful judging glass tinting/colour up against a white background - they will look very different in real life, up against the sky etc. We got samples for the differently-tinted glass for our roof lantern and soon discovered judging them against a white background was meaningless compared to holding them up to the sky or, even better, seeing a real life installation. 

Edited January 18, 2021 by MJNewton

[Gone West]

10 hours ago, Adsibob said:

Can anyone think of why in these circumstances I might regret not getting solar control for these sliding windows?

We have south facing triple glazed sliding doors and decided not to have solar control glazing. We are happy with them as they do provide solar gain in the winter and the sun doesn't penetrate very far into the room in the summer. We could easily build a brise soleil if it caused a problem.

[Adsibob]

2 hours ago, PeterStarck said:

We have south facing triple glazed sliding doors and decided not to have solar control glazing. We are happy with them as they do provide solar gain in the winter and the sun doesn't penetrate very far into the room in the summer. We could easily build a brise soleil if it caused a problem.

Thanks @PeterStarck. Where are your 3G sliding doors from?