Too much solar gain!

[2104GJ]

Hello.

We live in Hampshire and have just been granted PP after 18 months! I have been reading many building blogs and have found the information invaluable.  We are thinking of building using MBC and 2/3 of the roof area will be flat. At an Environmental Workshop, the aim of which was to ensure that the house does not overheat, it was suggested that we would need to install a green roof to prevent overheating. The thinking being that the green roof will provide shading and the aggregate an amount of thermal mass.  Does any body out there have experience of controlling heat gain through a flat roof?  Is a green roof the only option to control temp gain? 

Many thanks  

[ProDave]

Are you worried about excessive heat gain through the roof?

 

If so then the key to combatting that is the amount of insulation and importantly the type of insulation. And that is no different between a flat roof or a pitched roof.

 

Solar gain through too many windows is a different question.

 

MBC should know about fitting enough of the right sort of insulation to ensure you don't have a problem.
 

[Nickfromwales]

Hi, and welcome to the forum. 

Try reading that from the first post and you may be slightly better informed . 

 

[IanR]

As ProDave said!

 

1 hour ago, 2104GJ said:

it was suggested that we would need to install a green roof to prevent overheating.

 

Why do you think you will have an overheating problem? Has your house design been thermally modelled? ie. PHPP or the like?

 

If so, it will guide you on what to do to reduce the overheating risk ie. reduce glazing area, recess windows further into reveals, larger eaves overhang, bris soleil etc.

 

If, like me, you decide you don't want to reduce the glazing area because you like it, or can't increase overhangs or add bris soleil due to planning restrictions then you could consider low emissivity glass and/or add external shading and/or active cooling.

 

But, you need a thermal model to determine what's the most cost effective, and what gives you the house that best suits your lifestyle.

 

Edited to add:

Expanding on ProDave's "and importantly the type of insulation", it's not all about U Value, when it comes to resisting incoming heat, choosing an insulation with a "decent" decrement delay will aid in reducing any heating requirement overnight when the sun goes down.

This may be what you infer by your use of the term "thermal mass".

Edited September 30, 2016 by IanR

[Mikey_1980]

Congratulations, where abouts in Hants are you, we are in between Newbury and Winchester and MBC built our house. 

 

I would have thought, depending how MBC do that their flat roofs that the insultaion thickness would have a significant delay in allowing the heat through, our bedroom has a vaulted ceiling and with the 400mm of insulation it doesn't overheat in the summer thourgh the roof.

[TerryE]

The main villain in tenrms of solar gain is a large area of sun-facing  window. 

[jack]

Welcome!

 

We have an MBC house with a completely flat roof south of Farnborough. We have 400mm of cellulose insulation and there's no way you get enough heat through that to cause overheating.

 

Do think carefully about roof structure. In the end we went for a cold roof with cellulose. If doing it again I'd go for a warm roof, possibly with rigid insulation. I'd also consider going for fibreglass rather than the high tech German EPDM/modified bitumen system we went with (although that's largely driven by dissatisfaction with the way it was installed)

[Bitpipe]

We have a traditional pitched slate roof house  (Berkshire, M4 corridor) with an east west aspect, built this/last year by MBC. We have ample glazing on the east and west with minimal north and south.

 

All out east glazing and south roof windows have electric external blinds / shutters and these have been extremely effective at minimising solar gain during the day. The only times the upstairs and room in roof were uncomfortably hot were on very warm days this summer when trades left all doors and windows open, letting in warm air. 

 

When we suggested they kept them closed the following day the house remained comfortably cool. 

 

Only regret on my behalf was not speccing a blind for the south corner kitchen window (it's east counterpart has one)  - does not get a lot of direct sun as the neighbours gable shades it and privacy is not an issue but it would have looked nice to have both looking the same!

 

+1 on GRP roofs, we have a few flat sections and our roofer would not use anything else, it is pretty bullet proof. 

[SteamyTea]

Definitely go for a GRP roof.  These don't have to be totally done on site, any half decent GRP company could make you some 8 by 4 ply sheets with some GRP laminated to them.  Then they would only have to seal the joints once installed.  This reduces most of the risks caused by weather i.e. too hot, sunny, dark, cold, wet.

 

As for a green roof, they don't change the thermal properties any more than a mud roof from the experiments I did a few years back.  I was never happy with the explanation that the plants transpire and help regulate temperature.

If you want to reduce thermal transmission, start by painting it white, or cover it in something that will stay shiny.

 

As for decrement delay, the calculations are fairly simple:

https://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia

There is no 'magic' involved, just basic thermal engineering.  There is an awful lot of bollocks spoken about it though.  As a general rule, the more mass of any material you have, the closer to the mean daily energy levels you can store, once you know the energy levels, you can calculate the temperature range.  It is a non linear function though, which is where it gets tricky.

 

So you only need to put in enough insulation to keep the place warm at the coldest time, and then deal with any overheating by other methods i.e. a reflective outer surface or forced cooling.

And just remember that half the time the world is dark, and a good 2/3rds of the daylight time it is cloudy.  of the remain 1/6th of the day, there is probably not enough temperature difference to worry about, leaving you with a few hours a year when your house may go over 26°C.  A portable aircon unit may be your best option for those few times.

Edited October 1, 2016 by SteamyTea

[divorcingjack]

We are building an MBC house with a flat roof too - it's cellulose filled and i don't have too man concerns about overheating through it. The massive south facing window on the other hand.... 

 

Hope you don't mind me hijacking your thread a bit, but @Bitpipe, could you offer some additional details about your external shutters? We are having some engineering difficulties with a cantilevered shading canopy and I'm just wondering if external shutters might be a better option. 

 

Thanks, 

dj

[IanR]

I'm going with Hunter Douglas external venetian blinds that will be tucked behind the timber cladding. For a 2.1m drop you need about 220mm above the window to retract them and they need a head box 120mm deep (for an 80mm slat), they have guides on the sides, either an extruded channel or wire.

 

When we first quoted HD installed as well as supplied, but when I've gone back to order they no longer install so I've been passed to one of their "partners" to requote.

[oldkettle]

On 01/10/2016 at 07:46, jack said:

Do think carefully about roof structure. In the end we went for a cold roof with cellulose. If doing it again I'd go for a warm roof, possibly with rigid insulation. 

 

Jack, I bit off-topic, but why would you prefer a different roof structure? Or is it for flat roof only?

[jack]

Only if I were doing a flat roof. I'd be happier with the same build up in a pitched roof.

 

The other possibility (for a cellulose cold roof) would have been to carefully detail an internal vapour-tight layer rather than relying on a breathable structure with ventilated head space. 

 

I'm assuming, perhaps a little histrionically, that something will go wrong in 10 or 20 years and we'll need to take serious steps to rectify. Hopefully it won't come to that.

 

If I had my time again I probably would have found a way to avoid the flat roof entirely. 

 

Hope that helps but let me know if you need more info.

 

 

[jack]

8 hours ago, IanR said:

I'm going with Hunter Douglas external venetian blinds that will be tucked behind the timber cladding. For a 2.1m drop you need about 220mm above the window to retract them and they need a head box 120mm deep (for an 80mm slat), they have guides on the sides, either an extruded channel or wire.

 

When we first quoted HD installed as well as supplied, but when I've gone back to order they no longer install so I've been passed to one of their "partners" to requote.

 

I believe these are the blinds that we have (although under a different brand name).

 

One thing I will say is that they definitely don't block out 100% of the light. We didn't allow for internal window coverings and really felt it with summer's early sunrises.

 

You also need to think about how you're going to control them. Ours are integrated with our home automation system. 

 

If anyone wants more info, let me know.

[Trw144]

8 minutes ago, jack said:

 

I believe these are the blinds that we have (although under a different brand name).

 

One thing I will say is that they definitely don't block out 100% of the light. We didn't allow for internal window coverings and really felt it with summer's early sunrises.

 

You also need to think about how you're going to control them. Ours are integrated with our home automation system. 

 

If anyone wants more info, let me know.

 

Are you auto shading based on room temperature? I ve pretty much programmed my loxone system now - was pretty impressed with how intuitive it was - managed it in a couple of nights after work. Only left with the 0-10v signal for the boiler temperature control, and a couple of rooms and switches to assign

[IanR]

12 minutes ago, jack said:

One thing I will say is that they definitely don't block out 100% of the light. We didn't allow for internal window coverings and really felt it with summer's early sunrises.

 

You also need to think about how you're going to control them. Ours are integrated with our home automation system. 

 

If anyone wants more info, let me know.

 

Where does the light "leak" through? I knew those with wire guide can let the light through the holes for the wires, but I was hoping the tracked version, with edges tucked behind the cladding, would give pretty good coverage.

I'm assuming yours is connected up to your Loxone. That's the way I'm going.

 

 

Edited October 19, 2016 by IanR

[IanR]

2 minutes ago, Trw144 said:

Are you auto shading based on room temperature? 

 

I'm hoping to use sun position to define slat angle, and temp to decide if they are needed at all.

[jack]

14 hours ago, Trw144 said:

Are you auto shading based on room temperature?

 

No.  All rooms to the south and west have some form of overhang or shading via trees, so it isn't really necessary.  Also, with many of the rooms we've decided we like the effect of the blinds being down but 70-80% open.  Nearly all the blinds are programmed to come down at sunset.  Many of the downstairs ones are programmed to open or partly rotate first thing in the morning.  The rest (eg, bedrooms) we operate manually as needed.

 

14 hours ago, Trw144 said:

I've pretty much programmed my loxone system now - was pretty impressed with how intuitive it was - managed it in a couple of nights after work. Only left with the 0-10v signal for the boiler temperature control, and a couple of rooms and switches to assign

 

Amazing, isn't it?  As a company they have a few flaws (their GBP pricing model drives me mental), and the cost of their proprietary products is at times silly.  But if you have any sort of technical or programming background their block-based programming interface is awesome.  Have you looked at the Loxone google groups mailing list? 

 

14 hours ago, IanR said:

 

Where does the light "leak" through? I knew those with wire guide can let the light through the holes for the wires, but I was hoping the tracked version, with edges tucked behind the cladding, would give pretty good coverage.

 

Two places: the wire guide holes and the edges.  I don't know whether there was an element of safety built into the measurements, or an allowance needs to be built in to allow installation, but on most windows there's at least 20mm lateral allowance.  In some cases it's 25+mm.  That means you have a total of around half an inch gap both sides, on average.  I personally can't see why such a big gap is needed, but at the time they were installed there were so many other issues that this particular one didn't seem that important.  

 

[Bitpipe]

On 19/10/2016 at 13:11, divorcingjack said:

We are building an MBC house with a flat roof too - it's cellulose filled and i don't have too man concerns about overheating through it. The massive south facing window on the other hand.... 

 

Hope you don't mind me hijacking your thread a bit, but @Bitpipe, could you offer some additional details about your external shutters? We are having some engineering difficulties with a cantilevered shading canopy and I'm just wondering if external shutters might be a better option. 

 

Thanks, 

dj

 

Sure, happy to help.

 

We purchased Gaulhofer windows from a UK reseller.

 

The external shutters were specified to five east facing windows (our street view) as an add on for specific windows and the window drawings and dims included them.

 

I got the technical guy at the reseller to work with with MBC to ensure that a pocket was built above each relevant window to accommodate the blind box - obv. sacrificed a little insulation here. The blind box front sat flush with the MBC frame and this made the exterior of the frame 100mm back from the frame which was perfect for us.

 

Windows arrived with the blinds fitted so felt like they were an integral part of the unit. The units themselves are Roma blinds (roma.de).

 

The electrical connection (a 220v 4 core cord) ran into the house within the frame. No issues in making everything airtight from the inside and as the blinds were flush, the render batten & board was able to run flush over the top.

 

Looks very neat, when the blinds are recessed, they are invisible, however we keep them down most of the time and just tilt them as required. Very effective in keeping sun out and can almost act as a blackout.

 

Electrician wired them into the room light panel and we have a mix of rockers and latching switches - they do take about a minute to fully extend or retract so holding down a switch for that long is a bit of a pain.

 

Only downside is I omitted to spec a blind for the south kitchen window (east has one) as I felt it was shaded enough and looks out onto a hedge and neighbours gable. However the lower sun angle allows it to shine in, not a massive solar gain issue, more of a nuisance.

 

Practically impossible to retro fit a matching external bind now so will have to fit something to the interior.

 

[divorcingjack]

Thanks @Bitpipe, very useful. Our windows are Internorm, but the architect advised us that the canopies form an important part of the look of the facade, so they are staying, engineering difficulties and all! I might look at externals for some of the other windows though.