Heating · Fabric first
Slow the loss before you size the system. Everything downstream gets smaller.
A heat pump or a boiler sized against a leaky house has to fight that leak forever. Insulation, airtightness and glazing decide how big the system needs to be, how hot it needs to run, and how much you pay to keep the house warm every winter after this one.
January morning, uninsulated loft, empty cavity
This is the house before the fabric work. The boiler is running flat out and the upstairs rooms are still cool by breakfast; the walls and ceiling are quietly leaking every watt the system produces.
Why fabric comes first, always.
Heat leaves a British house through five routes: the roof, the walls, the floor, the windows, and the gaps in between. Each route has a U-value, which is a measure of how quickly heat escapes through it; the lower the number, the slower the loss.
Slowing that loss changes the maths on every downstream decision. A house that loses 8 kW at design temperature needs an 8 kW heat pump; the same house, after a loft top-up and a cavity fill, might lose closer to 5 kW. The pump ends up smaller and cheaper to buy. It runs more quietly, and it can hold a lower flow temperature that itself lifts efficiency further. Skip the fabric and you buy a bigger system, run it harder, and pay for the leak forever.
Same morning, loft to 300mm and cavity filled
Same house, same morning, after loft insulation to 300mm and cavity fill; the rooms hold their heat and the system finally gets to sit down.
The five fabric routes, in order of leverage.
Sequenced by what gives you the largest temperature drop per pound spent on a typical UK semi.
Loft insulation
- Strength
- Going from bare joists to 270mm of mineral wool at the ceiling drops the roof U-value by roughly a factor of fourteen; payback usually sits under three years even at 2026 gas prices.
- Trade-off
- Only works at the ceiling line; foamed rafters are a mortgage risk and are being required for removal at point of sale.
- Best for
- Nearly every UK home with an accessible loft.
Cavity wall insulation
- Strength
- Quick, non-disruptive, typically halves the wall U-value; still relevant in roughly five million UK homes with empty cavities.
- Trade-off
- Needs a borescope survey first; wrong material in a wet cavity can cause damp problems that outlive the saving.
- Best for
- Post-1930s builds with confirmed empty cavities.
Solid wall insulation
- Strength
- Turns a 2.1 W/m²K wall into a 0.30 W/m²K wall; the only real fabric answer for Victorian and Edwardian stock without cavities.
- Trade-off
- Expensive and disruptive whether internal or external; realistically part of a wider renovation rather than a standalone job.
- Best for
- Six million UK homes with solid walls, sequenced with other renovation work.
Airtightness and draught-proofing
- Strength
- Cheap and unglamorous, but it does more for felt comfort than any single insulation measure; a well-executed retrofit moves leakage from 8-15 to 3-5 m³/h/m² at 50 Pa.
- Trade-off
- Invisible; easy to skip because there is nothing to point at once it is finished.
- Best for
- Every home, at every stage of the sequence.
What good looks like, sequenced honestly.
Loft to at least 300mm. Cavity fill if the cavity is empty and dry. Draught-proof the obvious leaks at the loft hatch, service penetrations, floor edges and window frames. Only then price the system. This is roughly the sequence PAS 2035 encodes for whole-house retrofit, and it is the sequence a good MCS installer will insist on before quoting.
Solid wall insulation, triple glazing and whole-house MVHR belong to a deeper retrofit tier; they sit in the same conversation as an extension, a rewire or a new roof, not in a standalone heating decision. Floor insulation matters most in suspended-floor Victorians where cold air moves under the boards; in slab-on-ground homes it is a lower priority.
Verdict
Fabric first, honestly assessed.
If you are about to price a heat pump or replace a boiler, do the loft top-up, the cavity fill and the draught-proofing first; the design load usually drops by a third and the whole downstream conversation gets cheaper.
A loft top-up and a cavity fill on a typical three-bed semi commonly move the design heat loss from around 8 kW to closer to 5 kW; that change alone can drop the heat pump one size band, cut the capital cost, and let the same pump run at a lower flow temperature for the life of the system.
The one thing not worth pretending: a system sized against a leaky house does not become efficient once you insulate later. It becomes oversized. Order matters.
What it gives you
- Every downstream number gets smaller; capital, running cost, radiator size and noise.
- Improves felt comfort in cold snaps whether or not you ever change the system.
- Most of it is genuinely reversible or additive; you rarely regret a loft top-up.
What it costs you
- Deep-retrofit measures (solid wall, MVHR) are expensive and disruptive.
- Cavity fill in the wrong wall can cause damp; a borescope survey is not optional.
- Nothing on this list is glamorous; it is easy to skip in favour of visible kit.
Why we think thisOpenClose
Reasoning
U-value figures for uninsulated versus insulated fabric come from BR 443 conventions used by SAP and PAS 2035; the loft, cavity and solid wall improvement factors are directly derived from those tables.
Cost bands are the site-wide 2026 range from installer quotes across five UK regions; loft top-up at £450 to £900, cavity wall at £1,200 to £2,500, draught-proofing package at £300 to £1,200, internal wall insulation at £3,500 to £7,000 per room, external wall insulation at £12,000 to £22,000 for a typical semi. The design-load reduction from 8 kW to 5 kW is a common MCS survey outcome for a semi moving from "poor" to "improved" on the same tables.
Assumptions
- The property has an accessible loft and an unfilled cavity where relevant; anything else moves the sequencing.
- The design outside temperature follows CIBSE regional data; a colder location shifts the load numbers upward proportionally.
- Costs assume standard access; scaffold, party-wall complications and heritage constraints sit outside the bands.
If this were our house
If this were our house, no boiler or heat pump quote would leave the drive before the loft was at 300mm and the cavity was filled.
- 1Top the loft up to 300mm mineral wool at the ceiling line and draught-proof the loft hatch on the same visit.
- 2Book a borescope survey; if the cavity is empty and dry, fill it before the heating quote is finalised.
- 3Draught-proof the obvious weak spots (service penetrations, floor edges, sash windows) before asking anyone to size a system.
Order rests on the current HouseState above; correct anything on Your Home and the priorities update, but the physics that puts fabric first does not change.
A few things people ask before committing.
How much does fabric really change the heat pump quote?
Do I have to do all of it before a heat pump?
What about triple glazing?
Is spray foam on the rafters fabric-first?
Fabric work sits before every heating decision because it changes every downstream number: the size of the system, the flow temperature it runs at, and the bill it produces each winter. Loft, cavity and draught-proofing come first, in that order, and the rest of the heating conversation gets cheaper afterwards.
Take the Comfort Score
Two minutes; tells you where the fabric of your specific home currently sits and which of the four measures earns the first move.