Earth Notes: On Building a New Efficient Home in (the South of) England
Updated 2020-12-04 08:36 GMT.
Where to start, what to remember, and useful resources. Save your wallet, warm toes, and the planet! #greenBuilding
Building a house that is comfortable and truly eco-friendly, rather than just feel-good eco-bling, is a complex jigsaw with many pieces that have to be "just right". Some important elements are outlined here.
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!
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...
Sensible Architects
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.
Sensible Builders
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.
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!
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.
Exemplar Houses
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.