Hi all. I'm Peter, and my wife Marysia and I hoping to start a new-build in Gloucestershire over the next few months. We are in our sixties and looking to make our final (unforced) move, so design needs to be wrinkly-friendly, comfortable and cheap to run. The site is elevated, south-facing and shade-free, so ideal for maximising solar energy one way or the other. The outline planning consent at this stage seems to allow for a number of build styles, and there is already a relatively radical self-build going up on the next plot.

So far, so good, but in a way we have too many options, and the possibilities seem endless, both in house design and in 'green' options. Our particular concerns are over heating the house, and as I'm sure you all aware, for every possible solution there seems to be several opinions and even more disagreements, with few hard facts, so reaching firm conclusions will be a leap of faith.

We are planning a 2-storey build on an 11 x 8 metre footprint, with the long side pointing south on an unobstructed site in Gloucestershire. I've (hopefullly) attached a Sketch-up mock-up, not perfectly crafted, but a fair likeness. Can't see how to insert into text.

External features include the 50º front roof with PV or PV/solar thermal mix. Lots of south-facing glass, and little on the other sides. I'm slogging through a SAP spreadsheet at the moment to get an idea of rating. Likely to do timber-frame or SIPs with a 0.15 wall rating.

Would like a thermal mass inside- perhaps a 250mm concrete floor with wet UFH set in the slab about a third down from top. The roof and brise-soleil are angled to allow winter sun to reach back of rooms, but only a metre or so in summer. Main aim is to make things as Passiv'ish as we can afford and minimise secondary heating. Hopefully solar energy will heat a thermal water store with off-peak electric top-up as and when, and be directed under a good chunk of the ground floor when needed. There is a void above the central third of the ground floor and an open landing/gallery to help circulation of air, plus MVHR. Maybe an opening roof light for summer cooling.

I was a 'tech' teacher and have a working knowledge of control systems and have done lots of re-furbs, re-wiring and plumbing, so should be able to fit lots of the heating kit myself.

I'd be happy to expand on any of the above, and look forward to any replies- for better or worse! Thanks, Peter F

Slight clarification: this sentence should read "8m2 of solar thermal is only sufficient to contribute 25% of your HW requirement, and nothing to your heating in December/January"

You may also want to consider your solar PV FIT payments versus solar thermal RHI depending and whether you are more interested in saving the planet or gaining an income from your energy company (and other consumers). My understanding is that the solar thermal RHI, because it is based on the number of residents may go against you and thus solar PV might provide a better income and for longer (7yrs v. 20 yrs)?

Personally I would look at getting a full building simulation done for the house if you are worried about overheating. Because both SAP and PHPP are parametric models and thus aren't great at capturing the impacts of either long runs of either warm sunny weather or cold dull weather, they are never going to provide you with as good an understanding as a building simulation model.

Even at Passivhaus levels you will need heating if you plan on maintaining your home at 20C or above. The Passivhaus criteria is 15 kWh/m2/yr for heating and most Passivhaus's end up being close to that, so on a 176m2 house you should plan on 2,600 kWh/yr of heating, unless you are prepared to put up with the temperature dropping.

Given I assume there are only going to be 2 of you in a relatively large home, your internal winter gains excluding solar are likely to be overestimated by SAP and PHPP as they assume occupancy levels based on floor area. PHPP has the advantage that you can override some of the occupancy but has a relatively crude 2W/m2 gain model compared with SAP which is a little more nuanced, but you can't easily change the occupancy in SAP.

If you want to do the building simulations yourself you could use the free US EnergyPlus model for which I think there maybe some Sketchup front ends? I think (free?) IES also used to have a Sketchup interface.

If you want some ideas for external shading you could look at the Barratts development at Hanham Hall in Bristol, which has a very similar sectional profile to your proposed design (largely mono-pitch north facing roof, with south facing lip) - the show homes use independent balconies for shading, the more recent builds use sliding external shutters. They are also constructed out of SIPs with a passive slab type floor, they were originally specc'ed for Code 6, but because they messed up their renewables didn't even meet Code 4.

Hi ActivePassive - looks like you'll be useful to have around.

There's the free Canadian Hot3000 simulator, front-ending the esp-R engine. AFAIK the free IES Sketchup plugin is very basic.

Posted By: tonyI love solar panels at a nice steep angle even steeper would be OK

Depends what you are trying to achieve from them.
Yes you may get a winter peak that is higher than the normal 30 or 35°C, but you will get less energy in total.
If you use your winter production to preheat some water then no a problem, but if you are hoping to rely on it to heat your water, then it may be.

I would concur with Ed's comments, if it were my house I would only use PV. Personally I would consider flipping the roof design to a largely south sloping/facing mono-pitch which would maximise space for PV. The downside is that the pitch being shallow wouldn't provide quite so much electricity in the winter, you would need to choose your panels carefully for those which work well in background/ambient light as opposed to direct sunlight.

As Ed suggests if you are weighing up the efficient use of finite roof space between solar PV and solar thermal, you are comparing the 35% to 40% conversion efficiency of solar thermal system with the 15% to 20% of solar PV, but in the solar thermal case you loose most of its gain because you have to dump the heat in summer. Although it is a close call I think PV is the better option. Also, if you go down the large solar thermal route you need to think carefully about how you are going to dump the heat in the summer. Electricity is a whole lot more useful than relatively low grade thermal energy.

In the extreme, if you plastered a south facing shallow 88m2 mono-pitched roof with PV, you might get about 12 kWp on it, yielding perhaps 10,500 kWh/year; cost is likely to be about £10,000 with a £1750 annual FIT/export income, so a 6 year payback. When I have looked at similar designs in the past, you should, with a heat pump be able to get away with almost no mains electricity demand for heating and HW even in the winter. It really depends on whether you can finance the PV upfront and whether you like the aesthetics of PV. Also be aware you will need a 3 phase supply if you go much about 6 kWp, which might add to your costs, but would prove beneficial if you end up with an electric car in future! A lot of these decisions come down to personal taste.

In terms of construction look at the methods where EPS is used externally (Viking House or SteamyTea posts) and then rendered.

Overheating - I suffer that with less glass and a colder climate (Scotland) so it is a real issue. Fitting roof lights to open may well help. Tilt and turn doors (so they can be tilted. Mediterranean countries of course use external shutters - that may be an option or a canopy.

There have been a number of threads on using PV and diverting it to a thermal store - I'd check those out.

Highly recommend the Akvaterm tansk - brilliant.

Your enemy is time if you want to start soon. These things do take time to iron out. I would take a breath and take the time to plan it right.

It's also a function of how far from due south the panels look - if you have to drift east or west, then also reducing the tilt to maximize diffuse irradiance can help offset the reduction in performance.

As a general rule - as the latitude starts to exceed about 50 degrees, then a relaxing of the tilt angle can improve the overall panel performance - I've seen simulations that suggest even relaxing by up to 25% can actually gain about a 5% improvement in total annual performance and improved winter performance (excepting snow which more easily covers a shallow panel)

Regards

Barney

Posted By: borpinmethods where EPS is used externally (Viking House or SteamyTea posts) and then rendered.

and fostertom's, in all modesty!

Ahhh ST - now if I lived where you lived, then this snow stuff would be a mystery to me as well - but that nice warm bath that surrounds you gets a bit cooler by the time it's got past Welsh Wales and up the river to Gloucester - up onto the escarpment it can get a bit nippy I'm told

Regards

Barney

Hi and welcome, looks like a great project you're about to start! I like the non traditional styling and different wall coverings on each storey for extra interest.

I'm not so sure about the unequally pitched roof as it's not ideal for the solar PV and I can't see that it adds anything visually. I guess there is a conflict between wanting more glazing on the south elevation which would lead to a mono pitch north facing roof and more south facing roof which naturally leads to a south facing mono pitch. Another option I've seen in a new house near here in Bath is a having an inverted roof... I'm not sure of the technical name but instead of a ridge at the centre there is a valley. They have solar PV on the south facing half of the roof and the south facing elevation is mostly glazed.

Ed

Overheating only becomes an issue in the UK (and hotter climates) if no allowance is made for ventilation. In Malta we used internal shafts or chimney to get air flowing through a building to clear out stale hot air and replace with cool clean air. I have followed this in the UK buy having opening roof windows at the tops of stairwells or an atrium with good effect. Stick some roof windows in at the top of your stair well and in the north facing roof and you will not be overheated!

Really, PHPP has all these answers very precisely, and results do correspond with its predictions (unlike SAP); thermal modelling is in theory even better.

Subscribe to Passive House Plus magazine - very good indeed - or read most of it online.

Or join AECB for £76 I think it is, and get that quarterly mag free (latest just out), as well as our own Keith's Green Building mag also quarterly free.

PH+ article emphasises the importance of getting very local weather data files. Meteonorm software can generate weather files for wherever you specify, on similar basis to the brilliant PVGIS website which Ed linked to above.

You can make a data file of your actual horizon, as viewed from your solar-receptor surface, upload it to PVGIS and see just how much low-angle winter sun you'll actually get. Sounds like plenty, on your site.

Correction- house aims at SSW, just west of south.