Insulation & Energy

Loft Insulation Cost and U-Value Calculator

Tell us the size of your loft and what is already up there, and we will work out what you need to buy, what it should cost in Northern Ireland, and how much the ceiling U-value actually improves. If you already have enough insulation, we will tell you that instead.

Rates last reviewed 1 August 2026 · how we work these figures out

How big is the loft?

Measure the loft floor, not the footprint of the house. Pace it out if you have to — it does not need to be exact.

Length

Width

Loft floor area 0 m²

How much of it are you insulating?

Water tanks, boarded storage decks and awkward corners often get left out.

The whole loft Wall plate to wall plate, everything except the hatch. Most of it About three quarters. A tank or a storage area is being left. ≈75% Part of it About half. One side, or one section of a split loft. ≈50%

What is up there already?

Push a ruler down through the quilt to the plasterboard. Joists in an older Northern Ireland house are usually 100mm deep, which is a handy reference.

Existing insulation depth

We only price the difference between this and the depth you want, so it is worth measuring properly.

None Bare joists, or a token skim you would not call insulation. ≈2.3 W/m²K About 50mm A thin old layer, well below the joist tops. ≈0.766 W/m²K About 100mm Level with the joists. Very common in NI houses from the 1970s to the 1990s. ≈0.428 W/m²K About 150mm Standing proud of the joists by roughly two inches. ≈0.297 W/m²K About 200mm A cross-laid second layer over the original. ≈0.227 W/m²K 250mm or more Already close to current guidance. ≈0.184 W/m²K Not sure We will assume 100mm and say so in the result.

What total depth do you want?

This is the finished depth, existing plus new — not the amount you are adding. The U-values shown are for standard mineral wool.

Finished total depth

Technical Booklet F1 asks for 0.16 W/m²K when you insulate a roof in Northern Ireland.

100mm Level with typical joists. Well short of current guidance. ≈0.428 W/m²K 150mm The old 1980s standard. Better than nothing, not much more. ≈0.297 W/m²K 200mm A common compromise where headroom or storage matters. ≈0.227 W/m²K 250mm Close to target, and enough with a high performance quilt. ≈0.184 W/m²K 270mm The long-standing recommendation. Just misses the F1 target in mineral wool. ≈0.171 W/m²K 300mm Recommended The first standard depth that meets the Northern Ireland roof target in ordinary mineral wool. ≈0.155 W/m²K

What are you laying, and who is laying it?

Boarding matters more than people expect: quilt only works at full depth, so boards have to go on legs above it.

Insulation material

Mineral wool roll The standard loft roll. Cheapest per m² and easy to lay yourself. λ 0.044 Glass wool (high performance) Thinner for the same performance. Useful where headroom is tight. λ 0.032 Sheep wool Natural, handles moisture well, pleasant to handle. Considerably dearer. λ 0.039 Blown cellulose Recycled paper fibre blown in. Fills awkward gaps, but needs a machine. λ 0.038

Who is doing the work?

Professional installer Supplied and fitted. Installation of energy saving materials is currently VAT zero-rated. Doing it myself Materials only. Allow a full day, and buy a proper mask before you start.

Boarding

Boards laid straight onto deep quilt crush it. Raised boarding on legs keeps the insulation at full depth.

No boarding Open loft, insulation only. The loft is already boarded The boards have to come up and go back on raised legs. I am boarding it over A new raised storage deck on legs, as part of this job.

Where are you, and how is the house heated?

The postcode sets the regional rate. The fuel sets what a kilowatt hour of saved heat is actually worth.

Postcode

The first part is enough. We only use it to pick a regional rate.

Main heating fuel

Northern Ireland is unusual in the UK: most homes are on heating oil, so that is the default here.

Heating oil Still the most common heating fuel in Northern Ireland. Mains gas Available across Greater Belfast and the main towns. Electric heaters Direct electric. The most expensive heat, so the biggest saving per unit. Heat pump Uses far less energy per unit of heat, so the money saved is smaller. Solid fuel Coal, wood or a multi-fuel stove.

What this includes

Quilt or blown insulation to the depth you choose, with a wastage allowance
Roll or bag count, based on stated coverage per roll
U-value before and after, on the same RdSAP defaults an EPC assessor uses
An indicative annual heating saving, as a band, and a simple payback
Raised loft boarding, where you have asked for it

What it does not include

Clearing the loft out first, and taking away anything already up there
Repairs to the roof covering, the felt, or any rotten timber found on the way
Insulating the loft hatch, the cold water tank and the pipework
Re-routing or derating electrical cables buried in the new insulation
Insulation at rafter level for a room in the roof — that is a different job entirely

Ready for real prices?

An estimate tells you whether a quote is fair. A quote tells you what it costs. Get three, free, from builders working in your area.

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Questions

Loft Insulation costs: the questions people actually ask

How deep should loft insulation be in Northern Ireland?

Current guidance points to 270mm to 300mm of mineral wool quilt, and it is worth understanding why the two numbers differ. 270mm was the long-standing recommendation and it gets a typical ceiling to roughly 0.17 W/m²K. Technical Booklet F1 of the Building Regulations (Northern Ireland) asks for 0.16 W/m²K when you insulate a roof, and with ordinary mineral wool at a lambda of 0.044 you need about 290mm to reach it once you allow for the joists bridging the insulation. That is why we mark 300mm as the recommended depth: it is the first standard roll depth that actually meets the Northern Ireland target. A higher performance glass wool gets there at 250mm.

Is it worth topping up from 100mm to 300mm?

It is worth doing, but be realistic about the size of the prize. Going from nothing to 300mm takes a ceiling from around 2.3 W/m²K to around 0.16, which is a very large change. Going from 100mm to 300mm takes it from around 0.43 to around 0.16 — still a worthwhile improvement, but the saving is a fraction of what the first 100mm delivered, because insulation has strongly diminishing returns. The top-up is cheap, though: you only buy the 200mm you are missing, the labour is the same either way, and payback is usually still inside a handful of years. Run both depths through the calculator and compare the numbers before you decide.

Should I lay loft insulation myself or pay an installer?

Quilt in an ordinary rectangular loft with a decent hatch is a genuine DIY job. It is not skilled work — it is unrolling, cutting and being careful where you put your feet. Allow a full day for a typical three-bedroom loft, and budget for a proper FFP3 mask, goggles, gloves, long sleeves and crawl boards, because mineral wool fibres are unpleasant and a foot through the ceiling is an expensive afternoon. Pay an installer if the loft is awkward, if the hatch is small, if there is a lot to clear first, or if you want blown insulation, which needs a machine. Note too that professional installation of energy saving materials in a home is currently zero-rated for VAT while buying the rolls yourself from a merchant is not, which narrows the gap more than people expect.

Are there grants for loft insulation in Northern Ireland?

There usually are, but the schemes change often enough that any figure printed on a website goes out of date quickly, so we deliberately do not quote amounts here. Support in Northern Ireland has historically come through the Affordable Warmth Scheme, administered by local councils for owner-occupiers on lower incomes, and through the energy suppliers' obligation schemes. Eligibility normally depends on household income, benefits received, the age and condition of the property and how much insulation is already there. Start at NI Direct and your local council's home energy pages, and speak to them before you commit to paying for the work, because grant-funded work generally has to be arranged through an approved installer rather than reimbursed afterwards.

Can loft insulation cause condensation or damp in the roof?

It can, if the ventilation is got wrong, and this is the one part of the job that is worth taking seriously. Insulating the ceiling makes the loft above it colder, which means any warm moist air leaking up from the house is more likely to condense on the underside of the felt. Two things prevent that. First, keep the ventilation path at the eaves clear — do not stuff quilt tight into the wall plate; use eaves vent trays if you are working close to the edge. Second, reduce the moisture getting up there in the first place: seal around the loft hatch, seal service penetrations, and make sure bathroom and kitchen extract fans duct outside rather than into the loft. If you already have black mould on the sarking or the felt, sort the ventilation before you add insulation, not after.

Does loft insulation improve my EPC rating?

Yes, and it is one of the cheapest ways to move an EPC. RdSAP records loft insulation as a depth in millimetres, and the assessor works from what they can see and measure, so the improvement is picked up directly rather than being argued about. Going from an uninsulated or thinly insulated ceiling to 270mm or 300mm typically moves a three-bedroom house several SAP points, and it usually appears on the EPC's own recommendations list beforehand. Two practical points: the assessor measures the depth they find on the day, so top up before the survey, not after; and if you board over the loft, make sure the insulation under the boards is still at full depth and visible somewhere, because an assessor who cannot see it has to record the default.