New Build, Probably

[Paul_F]

OK, things have progressed a bit and we now have a design that we're happy with and a Quantity Surveyor estimate that's under our budget. A handful of tweaks still to do - mostly to the look of the exterior - and we're going to apply for planning.

The building itself is planned to be a timber frame with a brick outer skin - mostly to ensure that it's "conventional construction" for mortgage and insurance purposes. We're thinking that we will most probably go for Passivhaus certification, and given both the way the design has evolved and the relatively low-cost of roof integrated PV we're likely to find ourselves within spitting distance of the Passivhaus Premium standard, which is pretty remarkable. We wouldn't make any significant changes to the design to achieve this though, it's only of interest if it drops out naturally.

Building footprint is 8m x 12m, with the ridgeline (long axis) pointing NW-SE and houses on either side. It's essentially on 3 floors, although the third floor is planned to be a big empty store-room in the roof with limited headroom over most of it - the main stairs go all the way up to it though.

Intention is to heat with a GSHP (Kensa Shoebox) if the quote for it is sensible, and put a lot of PV on the roof - probably 16kW worth on both sides of the roof, going to a 12kW 3-phase inverter. Would also have a shower heat exchanger on the upstairs showers, since hot water is likely to be a large fraction of the total energy demand.

The way things are going we're looking at the final go/no go decision at the end of the year, which will be more than a little scary!

[Joeboy]

That's absolutely brilliant, looking forward to reading of the progress. We've extended our house twice and that's on top of an extension that the previous owner did and we finished when we moved in 24 years back. Its always in the back of my mind to to do the whole thing from scratch on anew plot and obsess PROPERLY on it

Roof integrated PV, that's the inset stuff that sits flush? What's the score if one goes wrong, easy enough to get out and back in all sealed up again?

[Paul_F]

Roof integrated PV, that's the inset stuff that sits flush? What's the score if one goes wrong, easy enough to get out and back in all sealed up again?

It's these things:

Basically it's a moulded plastic tray, which go together pretty much like interlocking roof tiles. PV panel goes on top and acts like a very large tile - no sealing necessary between panels. Standard lead flashing around the top and edges.
It's not absolutely perfect, but doesn't need to be - you've got roofing membrane under it and at least some air flow, so any small amounts of water to get in can dry out again.

[spread-tee]

I've never installed any of that stuff so not speaking from experience here, but fixing all those trays and then a load of flashings seems like just as much work as a conventional framed system. I suppose you will need quite a lot less tiles, but then you will need to cut lots of them around the tray assemblies. I personally wouldn't want to rely too much on the breather membrane either, I know it is good stuff but it still puts the battens at risk of rotting out and you can get pooling at the facia.

My gut feeling is to keep the roof as intact as possible and the PV as a stand off system, It might not look as tidy but to be honest we never even notice all the panels up on our roof anymore.

Sped

[Paul_F]

I've never installed any of that stuff so not speaking from experience here, but fixing all those trays and then a load of flashings seems like just as much work as a conventional framed system. I suppose you will need quite a lot less tiles, but then you will need to cut lots of them around the tray assemblies. I personally wouldn't want to rely too much on the breather membrane either, I know it is good stuff but it still puts the battens at risk of rotting out and you can get pooling at the facia.

My gut feeling is to keep the roof as intact as possible and the PV as a stand off system, It might not look as tidy but to be honest we never even notice all the panels up on our roof anymore.

Two key reasons it's interesting to me:

• Cost - the GSE trays themselves cost about £20/m² (slightly more expensive than a rack system), tiles are likely to be £50/m² and slates nearly £100/m². There's a bit more cutting, but it's more than paid for by the vastly reduced amount of tiles needed. The flashing detail is also very simple - basically a roll along each face, which is going to be easier to get right than individual flashings for each mounting point.
• Robustness - definite trade-off here, but my view is that the pluses (no risk of bird's nests, flashing easier to get right, much better in high winds, etc.) are no worse than the minuses which notably include the panels running hotter.

For us, I figure it's likely to be £300-500/kW cheaper, as robust and marginally better looking.

[nowty]

In 15 years time when my first FIT contract has expired, I shall replace my first 4kW system with totally black panels integrated into the roof. Nicer looking and impossible for pigeons to nest under them.

Even now after 10 years, I could swap the 4kW system for a 6kW replacement, so I am expecting, maybe an 8kW replacement in 2035.

[Paul_F]

Planning application has gone in - full details at https://publicaccess.aylesburyvaledc.go ... DLFCLJT000 if anybody is interested.

[Stinsy]

Looks good! Liking the pocket doors!

[Paul_F]

Looks good! Liking the pocket doors!

I think there's at least one set we'll probably get rid of, but they do help a lot with the internal flow and layout. There are quite a few other things the architect has done really well and which I'm really happy with:

• Having the stairs go all the way up to the loft makes a huge difference - gives us tonnes of storage and increases TFA by 20%, which in turn makes it really easy to hit Passivhaus. I'm pretty sure that the design as-is will hit Passivhaus Plus without any changes, and if we put a bit more PV on than planned (tempting but probably not very sensible) we're looking at Passivhaus Premium.
• The plumbing is really compact - quite apart from reducing the standing losses in the hot water system, it's also going to save quite a bit of money on the first fix.
• If for any reason in future one of us can't get up the stairs (old age, accident, etc.), everything we need is downstairs - shower, bedroom, etc. It's mostly aimed at visitors in the near-term and using it as a dedicated study, but it's nice to have the flexibility designed in.

[Stinsy]

I noticed the bedroom/shower room downstairs. Handy if you break you leg or become ill. I’d be tempted to lose the roof light in favour of more solar, and seriously consider solar on the NW facing roof.