Loft Insulation vs Double Glazing: Which Lifts an EPC?

Key takeaway: on the same certificate, in the same documented project, a loft insulation top-up delivered points at roughly £95 each while new double glazing cost about £2,700 per point. Glazing feels like the obvious energy upgrade; the scoring model disagrees. Fit windows for comfort, noise, condensation and saleability, buy your EPC points with insulation and controls first.

New windows are the improvement everyone can see, which is why so many owners reach for a glazing quote the moment a poor EPC lands. The scoring arithmetic runs the other way, and the gap is not close, it is more than an order of magnitude. Here are the documented numbers, why the model weights things this way, and the cases where glazing is still the right spend.

Head to head: the numbers

	Loft insulation top-up (to 270-300mm)	Double glazing (whole front elevation)
Typical installed cost	£300-£800	£3,000-£6,000+
Typical points gained	+5-15	+2-10 (documented case: +2)
Approximate cost per point	~£50-£100	~£1,000-£2,700
Share of the score it touches	roof heat loss, one of the biggest fabric levers	windows, typically only 10-15% of the score
Evidence to keep for the assessor	invoice plus photos showing installed depth	FENSA or CERTASS certificate
Funding position	qualifies for 0% VAT on energy-saving materials until 31 March 2027	standard replacement windows are not on the 0% VAT list (VAT Notice 708/6)

These are published typical ranges, property-specific, confirmed for your home by an assessment, never guaranteed. But the shape of the comparison holds across almost every certificate we see.

£95 vs £2,700 per point, the documented cost of a point from a loft top-up against a point from new front-elevation double glazing, on the same Victorian terrace, in the same improvement project.

The documented case behind the numbers

The comparison is not theoretical. In a published landlord case study (The Independent Landlord), Suzanne Smith documented her solid-wall Victorian mid-terrace going from EPC E (48) to C (77). The £800 loft top-up, from around 100mm to 300mm, gained approximately 8-9 points, per the surveyor guidance she cites: roughly £95 per point. The £5,400 of new front-elevation double glazing gained 2 points, per the EPC recommendation report itself: £2,700 per point.

Same house. Same project. One measure delivered points nearly thirty times cheaper than the other. The full measure-by-measure breakdown, alongside the government and English Housing Survey cost benchmarks, is in our guide to what it really costs to go from EPC E to C.

Why glazing scores so little

Windows typically carry only 10-15% of the score’s weight, because even poor glazing accounts for a modest slice of a home’s total modelled heat loss compared with walls and the roof. Improving a small slice, however dramatically, moves the total a little. That is the whole story, and no window marketing changes it.

There is a second, newer reason to be precise here. Since 15 June 2025, RdSAP 10 measures every window individually instead of assuming a typical window area for the property type. Accuracy cuts both ways: a home with modest glazed area may gain slightly less from new windows than the old assumptions implied, while heavily glazed extensions are finally scored as they really are. Either way, the assessor now records what is actually there, which also means a FENSA or CERTASS certificate for work already done is evidence worth carrying, as our guide to EPC re-assessment after improvements sets out.

Why loft insulation scores so much

Heat rises, roofs are large, and insulation is cheap per square metre, the model rewards exactly that combination. Topping up from a partial layer to 270-300mm costs £300-£800 and typically adds 5-15 points, which makes it the best sub-£1,000 fabric measure on almost any certificate. Propertymark’s analysis of points per measure puts the average loft top-up at +4.9 points and room-in-roof insulation at +9.91, the single most effective measure it studied.

There is also a forward-looking reason to favour fabric. The government’s partial response of 9 March 2026 confirmed that domestic EPCs will move to four headline metrics, energy cost, fabric performance, heating system and smart readiness, targeted from October 2026, subject to the 2026 regulations. Insulation feeds the fabric performance metric directly, so loft points earned today still count when the goalposts move. Glazing contributes there too, but at the same brutal cost-per-point disadvantage.

Where the rest of the fabric fits

Loft and glazing are the two ends of the fabric spectrum, and the measures between them slot in predictably:

• Cavity wall insulation, £400-£1,500 for typically +5-15 points, around £80-£150 per point, on cavity-walled stock (broadly post-1920s). After the loft, it is usually the next call, and the guarantee document (CIGA or equivalent) is the evidence RdSAP 10 wants.
• Suspended timber floor insulation, £400-£2,300 for typically +2-6 points. Where cellar access exists it can be startlingly cheap: the documented case above insulated the floor for around £150 in materials within a larger cellar-ceiling job.
• Room-in-roof insulation, the strongest single measure in Propertymark’s analysis at +9.91 points on average, relevant to converted lofts and dormered terraces.
• Solid wall insulation, £8,000-£15,000+ for typically +10-20 points. The big lever on pre-1919 stock, and deliberately last in the sequence: it carries consent and damp-risk considerations, and the documented Victorian terrace reached a high C without ever needing it.

The pattern is consistent: fabric measures beat glazing on points per pound at every price level, from an £800 loft roll to a five-figure wall job.

When double glazing is still the right call

None of this means never fit windows. It means fit them for the things they are genuinely good at:

• Comfort. Cold radiant surfaces and draughty frames make rooms feel colder than the thermostat says.
• Noise. On a main road, glazing does what no insulation measure can.
• Condensation and damp behaviour. Warmer internal glass surfaces cut streaming windows and the mould that follows.
• Security and saleability. Buyers and tenants notice windows; surveyors flag failed units.
• Heritage stock. Listed buildings usually cannot take uPVC double glazing, but secondary glazing is the consent-safe route, and it is scored.

If those are your reasons, a glazing quote is money well considered, our windows, glazing and draught-proofing hub covers the honest arithmetic. Just do not book it for the points, and note that draught-proofing delivers a slice of the same thermal benefit at £30-£250, typically +1-3 points, which is 1-2% of the cost.

The right order for the money

The sequence that keeps an improvement budget honest runs cheapest points first: LED lamps, a hot-water cylinder jacket, full heating controls, draught-proofing, the quick wins under £500, then the loft top-up and the rest of the insulation measures, then heating, then generation, with glazing justified on comfort grounds whenever it happens. Where solar sits on the same ladder, mid-table, well ahead of glazing, well behind the loft, is covered in do solar panels raise an EPC rating. That is the whole logic of cheapest EPC points first: buy points where they cost £50, not £2,700.

Timing sharpens the case. Insulation and draught-proofing qualify for the 0% VAT rate on energy-saving materials until 31 March 2027, when it reverts to 5%; standard replacement windows do not qualify at all. The tax system, unusually, agrees with the scoring model. Typical prices for every measure are on our cost guide.

Common questions

Will new windows take my D-rated home to a C?

Rarely on their own. The published range for whole-elevation double glazing is +2-10 points, and the documented case gained +2, so a mid-D home at 62 needing +7 points might get there only in the best case, while spending £3,000-£6,000. The same seven points are routinely available from a loft top-up plus controls for under £1,000. Run the gap arithmetic before the showroom visit, and see the FAQs for the points-per-measure figures we quote most.

My recommendation report lists new glazing, shouldn’t I follow it?

Read the column next to the measure, not just the measure. The recommendation report lists improvements with an indicative rating change for each, and glazing’s indicative change is usually small, in the documented case, exactly 2 points. The report is a menu with prices attached, not an ordered instruction: the sensible reading is to rank its lines by cost per point and work down from the cheapest. Glazing almost always lands at the bottom of that ranking, however prominently it appears on the page.

Is secondary glazing worth anything on an EPC?

Yes, secondary glazing is recorded and scored, and on listed buildings it is usually the only consent-safe glazing improvement available. It delivers part of the thermal benefit of full replacement at a fraction of the cost and without the planning risk. For heritage stock the sensible package is secondary glazing plus draught-proofing for comfort, with the EPC points bought through insulation, controls and lighting.

Spend where the points are

The loft-versus-glazing question answers itself once the numbers are on the table: roughly £50-£100 per point against roughly £1,000-£2,700. Spend on windows when comfort, noise or condensation demand it, and let insulation do the scoring work. For a sequenced, costed plan built from your actual score and gap, request a free quote, we rank every measure by cost per point before a pound is committed.