Earth Notes: On Building a New Efficient Home in (the South of) EnglandUpdated 2020-01-09 17:29 GMT.
It happens to us all. A thoughtful (rich) friend asked in the pub a few days ago how he should build himself a new home. He's not a million miles from London. He mentioned "Passive House". He is keen to do things "right". Whatever that might be.
I have not self-built. (I have neither money nor time!) So I should not be confused with an expert. Consult some of the sources listed on this page and elsewhere on this site. Hire a grown-up early, for a good steer.
Finding a Site
Finding a site for new build or refurb can be a major effort and take a long time. Balance cost, access to services, something far enough away from (or close enough to) civilisation to taste, and that ties in with how you earn a living.
In this case, my friend already has his plot. So I'll gloss over that usually painful run-up to the self-build process!
Refurb vs New Build: Air-tightness, VAT
A super-efficient house is much nicer to live in than a bog-standard build. The cost difference, eg on the mortgage for a refurb, is not excessive.
There's an old joke about the easiest way to make a small fortune. Start with a large one, and lose most of it. Likewise there's two common ways to get an energy-efficient and comfortable house. Gut and refurbish an existing one. Or knock it down and rebuild from scratch.
In both cases airtightness is a key ingredient of efficiency. But how you go about achieving it is rather different in each case...
Arguably, the only way to make an existing build as airtight as a good new-build is to wrap a new skin around it. Think External Wall Insulation (EWI). AKA a 'tea cosy'. Else your best bet may be to make each room tight on its own as best you can. Tapes and squirty foam around your Internal Wall Insulation (IWI) panels, aka 'dry lining'. For this to work in practice you need a builder who 'gets' that unplanned ventilation, ie air leakage, is responsible for huge amounts of heat loss. So detailing is key. No mash-hammers at dawn to 'adjust' walls for pipes and cables. Even if out of sight!
Watch out for elements such as cut rigid foam board in wooden structures that move, eg between roof rafters. They will be exceedingly difficult to make and keep tight too.
Such a refurb may well benefit from MHRV (Mechanical Heat-Recovery Ventilation). But it may work with a less elaborate room-by-room scheme. Think single-room MHRV aka SRMHRV. Especially if retrofitting air ducts everywhere doesn't feel like fun...
New build to a Passive standard is designed from the get-go to be 'built tight, ventilated right'. So it will leak much less. And is less likely to come apart at the (taped) seams later. But it usually implies whole-house central ducted mechanical ventilation to match.
You likely do need a specialist builder with specific experience, in the UK at least, properly supervised, to get such a build right. Bodging this as happens in stick build construction just doesn't work.
There are other strange factors to take into account. New build does not attract VAT. Refurb does. Definitely a perverse incentive to waste the embodied energy in existing built stock...
Most of us haven't done this before. We don't know what all the pitfalls are. So it's wise to take advice from someone seasoned. Yes, that responsible adult alluded to earlier:
On some projects they could lead the whole effort and be a lead/principal contractor. On others they would advise.
You don't want to spend all your time fighting uphill against "But that's how we've always done it!" Nor against trades making foot-wide holes in your airtight membrane to get a pipe or cable through quickly!
We have repeatedly used Mansell And Woodland who are pleasant to deal with, reasonably priced, and prepared to try new things.
Take a look at Superhomes' My Green Builder list.
Some Self-Build Sites
Start with these UK-based self-build forums for loads of useful advice. People who have been there and done that (including the mishaps):
Woodstoves: Just Say No Kids
Short version: you'll cook if you ever light it!
Longer version: you'll have at least three problems with a woodstove, however pretty it is:
- If it is not a sealed-to-room stove (with a dedicated air inlet from outside, as well as a flue), then you're undermining one of the main heat-retaining features of your shiny new house: airtightness.
- You've now put at least two pipes through your lovely insulated walls. Guess which way heat will be moving when it's cold and you don't have a fire lit?
- If your shiny new house is working well then even a minimal ~3kW unit will cook you. Unless you open all the windows and strip down to underwear. OK for parties where eggnog has already been served. Otherwise not ideal!
Besides all that, it is difficult to operate a woodstove without hurting local air quality, especially particulates.
Passive, Minergie, Building Regs
Existing building regulations are an inadequate minimum. Especially as the UK scrapped its zero carbon homes plan in 2015. Don't only build to meet regs. You would be building in discomfort and excessive heating bills for the life of the building if you did!
A Passivhaus or 'Passive House' is one where "... the heating requirement ... is reduced to the point where a traditional heating system is no longer considered essential."
Note that Passive House is not the only game in town for new build, and has its own wrinkles that can be gamed. There are alternatives such as Minergie.
Zero vs Net-Zero
So why does PH (or similar matter)? Why not just slap more PV on the roof? The issue, dealt with in "The problem with Net Zero: the grid is not a bank" is the dinner-party topic of interseasonal energy storage. In the UK there is about 1/5th the energy per day arriving as sunlight in mid-winter as in high summer. You can't really 'bank' it in the grid since inherently the grid has no storage at all (and is balanced over fractions of a second between supply and demand), and what explicit storage there is (pumped hydro, a sliver of battery, etc) covers hours not months. So overproducing and exporting madly in summer doesn't actually cover your winter imports as the number of us doing so gets significant. "Net zero" (as I have at home) is not ultimately a scalable strategy.
So, finesse all that and basically avoid the need to import as far as possible at all in winter. The bulk of energy import is commonly for heat. Insulate well enough that you only very rarely need heating at all. Be a real "zero" not "net-zero". Now maybe slap on some PV anyway. Become a net generator and a grid hero!
(At some point soon battery prices and appliance demands may drop enough to make becoming near zero on electricity—eg light—sensible. But don't hold your breath. That's still a lot of storage!)
Solar PV vs Thermal, Storage, Sunamp
I'm hugely in favour of distributed renewables, out to a wider definition that includes heat pumps. We should potentially be turning every square metre of otherwise unused unshaded space such as roofs and even windows into solar PV energy collection. Simply so that we can cover a greater share of demand on overcast days. We can just ignore the extra energy easily and safely when we don't need it ("overbuilding" or "overprovisioning").
Solar PV is not the only game in town. Solar thermal for hot water can work well in some circumstances. (Though may often be undercut by PV's cost and operational simplicity.)
Generally forget wind power unless you are somewhere very rural or windy. Stornoway or the top of an exposed hill are probably good locations.
If you have access to a watercourse then you may have the possibility of hydro power. Excellent if you can do it!
Because renewables are usually intermittent, some sort of storage is beneficial. From second to second or season to season. Even if you have a grid connection, some storage may help if there is a power outage. You may also save the carbon footprint from the few percent losses incurred in sending power back and forth over the local distribution network. At some point soon the grid may be 'full' for parts of the day (eg at solar noon from PV). So storage may help you hang on the the energy for when you or the grid can use it later.
Storage can be in many forms. Heat (eg hot water tanks) and electrical (in batteries) are common. Stored heat is less versatile than stored electricity. But if what you want is heat, eg for a bath, it's good.
Hot water tanks have been with us for a long time, but though water's heat capacity is good, we can do better. It is also much harder to retain heat in a hot water tank than appearances suggest. Typical standards only deal with retaining heat for relatively few hours. In a world powered by intermittents, a much better rule of thumb is a few days' storage.
Sunamp heat batteries, for example, store heat in phase-change materials far more densely than water can manage. They are insulated with vacuum panels so heat leaks out far more slowly than from a water tank.
Warehome: Grand Designs 2017-10-26 featured Passivhaus, including my favourite Sunamp heat storage, and coming soon is a link to Sunamp's blog ("Grand Designs: Energy from just 6 lightbulbs heats this Self build London home")...
The house has one Sunamp Heat Battery in the kitchen to provide instant hot water. A second in the basement with an expansion unit. There is a third Sunamp unit and small pressurised circuit to feed two towel rails in the bathroom and en-suite. The batteries run on Economy 7 tariff. They charge overnight at lower cost.
See The House at Mill Orchard. How to build an efficient livable home from scratch. From getting planning permission to moving in. This house has a Sunamp too. It replaced a far larger water tank. The tank leaked enough heat to make the adjacent room uncomfortably warm. The new Sunamp has no such issues.
Also see this 2017 build in the Passive House Database.