Deejay_2

Combating Solar Gain

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We are still in the process of finalising our plans for a self build house. I have been researching solar gain and have come across many posts on this forum. We completed a house build on this site in 2016 and no consideration whatsoever was given to solar gain and the house has many large rooflights. We mistakenly purchased a brand that does not provide external roller shutters/blinds. The internal blackout blinds do not do much to keep the heat out. Many of the rooflights are on the south south west elevation and the house gets very hot during summer hot spells. In order to prevent this overheating on the new house we need to be more genned up at this stage.  Apologies for the length of the post!

 

What I think I have established from this forum so far is:

 

  1. Eventually 3g windows will become the norm, which will have the effect of prospective purchasers being put off any house that has 2g glazing. However, 3g is good in the winter months when it helps to keep in the heat, but in the summer actually generates more solar gain than 2g. To counteract this solar gain, shading of some sort would need to be added.

  2. To counteract this, one could apply external film – which appears to work really well in reducing the solar gain. However, this will act as a barrier to the sun in the winter months and therefore could increase heating costs.

  3. Solar reflective glazing will have the same effects as the film, making the house cooler in the summer, therefore less air conditioning, but also cooler in the winter, requiring more heating – and once it’s in, it’s in.

  4. It is suggested that an overhang on the roof would help to shade rooms during the summer months when the sun is high.

  5. If there are habitable rooms in the roof, then rooflights become a problem as regards solar gain. After mistakes on our previous build, we realise we should have chosen a top brand, ie Velux, who provide all the gadgetry ie electric/solar operation, solar heat reducing blinds, roller shutter blinds.

     

    My thoughts on our house design relative to solar gain are now as follows

     

  6. Install 3g windows but some additional shading will probably be required such as external shutters, external roller/venetian blinds. These would need to be electric/solar operated so research is required now to check on what needs to be installed with the build, even if the shutter/blind is added later.

  7. Add an overhang to the roof – say 18” - does anyone have any thoughts on this. Pre-planning want kneelers and copings (Conservation Area), but I would prefer to omit these (and the chimney), and spend the money on a suitably sized roof overhang.

  8. All of the veluxes on the proposed house are on the eastern elevation. None on the SSW elevation – for reasons already stated. Would it be better to fit 3g and add external shutters or external blinds to enable heat from the sun in the winter to enter, and to shut it out in the summer. As an aside, I purchased some solar film from a company called Purlfrost to put on the inside of the rooflights at the previously built house but this is rented out and we’ve not been able to install it yet, so nothing to report on its effectiveness. However, I now think putting film on the inside is not such a good idea from what I have read on here. Better to stop the heat getting past the external glass. This company do all types of film and were the only firm I could find that sell the film on a DIY basis – we got ours on a 1m wide roll and so will have to cut it, but I think the company may now cut it to size. Anyone wanting to try out the usefulness of film might find this DIY option useful for a trial. In our case, the rooflights in the built house are mainly very accessible and therefore relatively easy to install the film internally (or externally). It is possible to remove the film for the winter months. With the new house we would look at the external film for the rooflights if things really got too hot. The external shutters/blinds that are electrically/solar operated would be the most sensible option but they are very expensive. However, once fitted they would just be used when needed. Has anyone had any experience of the Velux external roller shutters.

 

Extreme heat really knocks me out and this aspect is therefore very important to me. The 1970’s bungalow we currently live in, which will be demolished is very comfortable. We have blackout vertical blinds on all of our windows and they work very well, although the large ssw facing lounge window still lets in quite a bit of heat, even with the blinds closed all day. We have no rooflights – I now believe these cause major over heating problems. The bungalow does not take a lot to heat it up in the winter, via a gas combi boiler and our heating bills are quite low – considering there is no wall insulation. In fact, if we installed an external blind on the lounge window for really hot days, I would say the temperatures of our living environment are near perfect. Strange how all the new stuff being introduced seems to cause nothing but problems, in that concentrating on the winter months, has brought serious overheating problems in the summer, when in fact we have known for a long time that the winters are getting milder and the summers are getting hotter. I have just read in the newspaper that the National Grid has had to fire up two coal fired power stations this week (heatwave Sept 2021) to increase supplies of electricity because of the increased use of air conditioning. You couldn’t make it up!

I would be so grateful if anyone has experiences to share of any of the above and also point out where I have got things wrong.  Many thanks.

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A few thoughts on this.

 

Triple glazing does not increase solar gain.  Fit triple glazed windows from a good manufacture.

 

Design the new house with an Air Source Heat Pump as primary heating.  Fit solar PV to the roof and then if you need summer cooling, the ASHP will do your cooling powered mostly by your free self generated electricity.

 

Good insulation will keep the heat out in the summer as well as keeping the heat in in the winter.

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Make your veluxes as small as possible. Ours are only 600x900 and max two per large room... Let more than enough light in.

 

Deep reveals and large overhangs make a huge difference.

 

Get somebody to do a proper solar gain analysis in PHPP and work it from there.

 

If you get these details right, you may not need to spend a fortune on external shading.

 

Our modelling says we need 6m X 2m bride soleil over the southern windows... As you can guess the quotes have been quite large. We've extended the roof overhang and are going to see how it works next summer before spending more money on shading.

Edited by Conor

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I can’t speak from experience but I would have thought triple glazing would reduce solar gain rather than increase.

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We had our PHPP calcs done, and found we needed to take account of the solar gain. So this is what we did -  picture taken from the kitchen where I am typing this post...

 

overhang.thumb.jpg.49151587f278cdcb2cf1cb485b3af22b.jpg

 

Yes, its a large overhang - its called a Winter Garden - fancy name for overhang. Kevin WhatsHisFace would call it a Breeze Corridor. Anyway, whatever the name, without it, we'd cook.

You are looking through six sheets of glass on the left hand side (one slider moved to the left handside), and no glass to the right of the vertical : that's 2 and a bit meters of open window - 27 outside and 22 inside out of the wind.  

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I have a slightly different take on solar gain, my south facing large conservatory I was told would overheat badly, but apart from a short time when it gets as hot as it has done these last few days, the rest of the year the solar gain is very welcome, even in cold weather with sun in your skies the temp out there is lovely, in fact we use it to heat the house a lot of the year by opening the bifolds into the kitchen and lounge. 

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An 18" overhang is probably unlikely to be anything like enough.

 

Use appropriate software / website to model where the shade ends.

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There are only two aspects in the way that windows affect solar gain.

There light transmittance

The reflectivity.

 

All the rest is geometry.

 

The easiest way to limit solar gain is to  fit a reflective film.  This can be retrofitted to the windows that are giving you a problem.

If you are concerned that a film will reduce the light levels in the house, then this is only what shades do.  So no difference in reality.

 

If you want to get really high tech and expensive, then self dimming glass or clear PV glass.  Cheaper to fit and run aircon.

 

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3g reduces solar gain a little - good in summer , not so in winter. We have very large 3g south facing windows. Its get very very hot if we don't open and let the breeze blow through. It sounds like externally you cannot do much in terms of planning as solar shading from outside is the best and cheapest option by far. It's super common on the mediterain. Google images "brise soleil" for examples.

 

You need to go with small /limit your south facing windows - this will help with summer heating and keep you warm in the winter

 

Edited by gc100

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Thank you for all the very useful responses from which I have gleaned the following:

  1. 3g glazing is the way to go.
  2. We had already decided on ASHP for heating and cooling.
  3. Further research to be given to Solar PV and I'll ask questions in the appropriate section.
  4. Will research the insulation aspect.
  5. Already have reduced the size of the veluxes which are all on the eastern elevation (roof).  We can add external shades retrospectively if required.  
  6. Very interested in the overhangs but will need to discuss size with architect.
  7. We have removed all rooflights on the Ssw elevation - roof now clear for Solar PV).  The sse (I think) elevation has only 3 small windows so not a problem.
  8. We may reduce width of sliders in kitchen/breakfast area but will investigate installation of electrical connection to fit external blinds retrospectively.  
  9. We are keen to take advantage of the winter sun so hope these measures will help with solar gain in heatwaves, but at least we can fit electrically operated shades/blinds retrospectively if required.  

Many many thanks for all the input - it is so great to have support and I am learning so much.  

 

 

 

 

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On 08/09/2021 at 11:38, Deejay_2 said:

Extreme heat really knocks me out and this aspect is therefore very important to me.


As it is very Important to you don't leave it to guesswork, get your house design thermally modelled. PHPP would really help with this aspect, even if not considering achieving PassivHaus levels of energy loss.
 

15 minutes ago, Deejay_2 said:

but at least we can fit electrically operated shades/blinds retrospectively if required.  


External venetians and roller blinds have large head boxes. They are best "designed in" to the building fabric, they're not such a good look as a retrofit.

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Thank you for the advice.  Have you any suggestions as to who I would go to to have the house thermally modelled.  We are building the house in stone and are not looking at PassivHaus.    We are restricted on design and orientation.  We like everything about the house we completed in 2016 except for all the massive rooflights and the excessive solar gain in the summer.  

 

I'll do further research on the blinds as I hadn't taken the size of the headbox into account.  

 

Any pointers re thermal modelling would be much appreciated.

 

Many thanks again.  

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The most elegant solution is, IMHO, SageGlass. Expensive but it works brilliantly.

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9 hours ago, IanR said:

External venetians and roller blinds have large head boxes. They are best "designed in" to the building fabric, they're not such a good look as a retrofit.

 

Also, if you can get them supplied with, and fitted with, your windows it derisks things and ensures they look great and not an add-on too.

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Thank you again.  I'm now researching external blinds (to be fitted in the future if required) and have a few questions.  Is it complicated to create an external reveal (?) on sliding doors over which to fit the headbox.  Would we be better off having a different style of patio door ie two side panels and two opening doors - all triple glazed (although not preferred).  The doors we had fitted in the previous build are approx 4.5 m wide and upvc double glazed sliders and were very  heavy.  Are aluminium clad doors "thinner" and lighter.    I am thinking we may go for a smaller opening ie 3.5 m but we shall not be having any rooflights on this elevation so I am also thinking of the light aspect.  It's a largish kitchen/breakfast/family room but does have a further 2 x 1200 x 1200 windows on the southern aspect.  It would be good to have large doors but I feel we may live to regret it.  Also - someone mentioned ordering their blinds with their windows - the company's name began with a G but I can't remember the name - does anyone know the name of the company.  My architect is RIBA but mainly retired and does not seem to offer advice re anything really - he is not expensive and certainly not out to fleece me - so I'm not complaining.  However, I feel I need to do as much research  as I can before the plans are submitted to avoid changes later.  The advice I have received on this site has been amazing and invaluable and I am really grateful.  

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We used Greengauge Building Energy Company for our overheating modelling using the PHPP spreadsheet. Highly recommend - I was very pleased with the report they produced which helped us to make some design decisions about shading our east and south facing windows. We will have dark roller shutters on our East facing windows, and a retractable canopy over the south facing bifolds. I also plan to grow a wisteria or grape vine on the sunniest sides of the building to provide some natural shading in a decade or so. 

 

One piece of advice the engineer, Mitch, gave us was that external blinds should be dark to prevent heat coming into the building, and internal blinds/curtains should be white to reflect heat/light back out again.

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1 hour ago, Omnibuswoman said:

One piece of advice the engineer, Mitch, gave us was that external blinds should be dark to prevent heat coming into the building, and internal blinds/curtains should be white to reflect heat/light back out again.

I am a bit dubious about that. Reflectivity is the important part. Or in Cornish, "is it shiny or no".

Colour is more to do with what wavelengths of light are absorbed.

Not all wavelengths will give a problem, but that is getting into the Ultra Violet Catastrophe, which quickly leads into the quantum nature of photons.

 

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Out of interest how was the roof insulated in your previous home?

 

Do you know what materials were used as different types of insulations have very different behaviours in heat even with the same u value. 

 

Do you have any plans of the house you're proposing to share?

 

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11 hours ago, Deejay_2 said:

Also - someone mentioned ordering their blinds with their windows - the company's name began with a G but I can't remember the name - does anyone know the name of the company


That would have been @Bitpipe who used Gaulhofer external venetian blinds supplied by myself.

 

Lift and slide doors are very easy to operate and slide very easy.

Edited by craig

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10 hours ago, Iceverge said:

Out of interest how was the roof insulated in your previous home?

 

Do you know what materials were used as different types of insulations have very different behaviours in heat even with the same u value. 

 

Do you have any plans of the house you're proposing to share?

 

Hi - my spreadsheet indicates 

SIG Roof Rectocel Rigid Board PIR 130mm
SIG Speedline Thermal Laminate 37.5

I am hopefully attaching photos from the house we built previously.  I have photos of every stage which is handy as I can now see what could be regarded by the experts as "not very well done".    The architect has tried to send the plans for proposed dwelling over to my PC but we have failed so I can't upload them. We are communicating by Royal Mail at the moment - he sends the amended plans (misses stuff off), I re-amend them and post them back - works well to be honest.   I'll upload the plans for the previous house as they are similar.  However there is a projection on the rear of the proposed house and the garage is a detached triple with granny flat above and we have an additional 4.5 m single storey "gym" to adhere to the 25 degree rule re daylight.   We never intended building on the site ourselves so were real amateurs.  However having Premier Warranty was an additional back up and well worth the money.   Don't laugh but I sort of project managed it myself - ordering all the stuff and then organising trades when the builder had left at the watertight stage.  We had no hiccups and the build ran smoothly and stress free.  You may say, that's because I let them get away with murder - who knows!!!  I'm interested to hear your views on the insulation.  Struggling to upload pictures so will do another post.

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On 08/09/2021 at 12:26, Conor said:

Make your veluxes as small as possible. Ours are only 600x900 and max two per large room... Let more than enough light in.

 

I would not necessarily agree - we have two very large Velux (PK10) and a 4 MK04 but all east / south facing ones have their external motorised blinds which work really well.

 

We have a mix of 240v Integra ones in inaccessible areas and standard manual ones. The blinds take a 24v feed so plug directly into the Integra units and work of their touchscreen controller. For the manual windows you get a KUX100 controller and use either the supplied remote or pair with the Integra touchscreen.

 

The blinds do allow a degree of opening for the window, I normally put the blind down first and then open the window.

 

14 hours ago, craig said:


That would have been @Bitpipe who used Gaulhofer external venetian blinds supplied by myself.

 

Lift and slide doors are very easy to operate and slide very easy.

 

Yup, I have the Roma venetian style blinds that work really well  - they're powder coated to match the window alu frames and run off 240v, just wired into rocker switches in each room.

 

They are never up, just tilted various angles during the day and mean no curtains are needed in those rooms also. 

 

Highly recommended. Having them come ready fitted to the windows meant install was a breeze, just needed to make allowance for the recessed blind box in the frame. I know @jack had a lot of grief with his blinds as they were fitted separately.

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2 hours ago, Bitpipe said:

I know @jack had a lot of grief with his blinds as they were fitted separately.

 

Yes, the window installation was easily the most stressful part of the build, largely due to the incompetence of the installers. Despite being resellers of the very product they were installing, they'd never installed windows with this particular blind system before, and didn't look at the drawings before they started.


Edited to add: All of that said, the blinds themselves are one of the best parts of the house. If anything, I wish I'd included them on a couple more windows. They are remarkably effective at keeping out the worst of the sun. I also quite like how they look when they're down and tilted 70-80% to horizontal - the ones at the front of the house are very rarely retracted all the way.

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I guess your first buildup was something like this.

 

I calculated a U value of 0.188 and a phase shift (decrement delay) of 6.7hrs.  This isn't a great number and may have added to your overheating. 

 

image.png.4e62a4b69a6be74728a8e94242af33d9.png

 

A better option for heat protection. Similar U value but a phase shift of 12.8hrs. Notice the much less extreme swing for internal surface temperature in the summer. This makes overheating easier to control.

 

On a cold winters day both roofs will perform the same against the cold.  

image.thumb.png.3ecb2372f049a6d29e5da6925d47178f.png

The website I used was. It's worth having a play. 

https://www.ubakus.com/en/r-value-calculator/?

 

It includes this importance sentence in the blurb there too. 

 

"For a proper summer heat protection, materials with a high heat storage capacity should be used, ideally in combination with an outer insulation. In addition, the direct sun exposure through windows must be reduced to an acceptable level by shading means, e.g. outer blinds, roller shutters or sun sails"

 

 

 

 

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