Insulation calculator

How to calculate insulation for a UK property

Getting the right amount of insulation saves you money on materials and avoids wasted trips to the builders merchant. Whether you are insulating a loft, filling cavity walls, or fitting rigid boards to a solid wall, the process starts with knowing the area and working out what thickness will meet current regulations.

The calculator above handles the maths for you. Enter the length and width of the area, choose your insulation type and material, and it works out the thickness needed to hit Part L U-value targets, the number of rolls or boards, and a rough cost including labour.

Understanding U-values and Building Regulations Part L

Building Regulations Approved Document L sets the energy performance standards for buildings in England and Wales. Scotland and Northern Ireland have their own versions but the U-value targets are similar. For renovation and new build work, the key U-value limits are:

• Pitched roof (loft): 0.16 W/m²K
• Walls (cavity or solid): 0.30 W/m²K
• Ground floors: 0.25 W/m²K
• Flat roofs: 0.18 W/m²K

A lower U-value means better insulation. An uninsulated solid brick wall has a U-value around 2.1 W/m²K, so bringing it down to 0.30 requires a significant thickness of insulation material.

Choosing the right insulation material

Each material has different thermal conductivity, which affects how thick you need to go. Here is a quick comparison of the four most common options for domestic work:

Mineral wool (glass fibre or rock wool) is the cheapest and most widely available option. It comes in rolls for lofts or batts for walls. Thermal conductivity sits around 0.044 W/mK, so you need about 270mm for loft insulation to Part L standards. It is easy to cut and fit between joists or studs.

PIR boards (brands like Celotex and Kingspan) have much better thermal performance at 0.022 W/mK. That means you can achieve the same U-value with roughly half the thickness compared to mineral wool. They cost more per square metre but are ideal where space is tight, like in solid wall internal insulation or between shallow floor joists.

Expanded polystyrene (EPS) sits between the two at 0.034 W/mK. It is lighter than PIR and cheaper, making it popular for external wall insulation systems and under-floor applications.

Spray foam has excellent thermal performance (0.025 W/mK) and fills every gap, but it costs considerably more and some mortgage lenders have concerns about spray foam in loft spaces. Always check with your client before recommending it.

Loft insulation: the quick win

Loft insulation is the single most cost-effective energy upgrade for most UK homes. Heat rises, and an uninsulated loft can account for around 25% of heat loss. The Energy Saving Trust estimates that insulating a detached house loft from scratch saves roughly £590 per year on heating bills at current energy prices.

The standard approach is to lay 100mm mineral wool between the joists, then cross-lay a further 170mm on top. This gives the 270mm total needed to meet Part L. If the loft already has some insulation, measure what is there and top up to 270mm.

Quoting insulation jobs

When pricing insulation work, factor in material costs, your time, and any extras like loft boarding, vapour barriers, or access issues. Use this calculator to get your material quantities right, then add your day rate and any ancillary costs. For a professional quote, try the quote generator to create a detailed breakdown your customer can sign off on.

Once the job is complete, send a professional invoice with itemised materials and labour using InvoiceAdept. You can include the insulation type, thickness, area covered, and U-value achieved to give your customer a clear record of what was done.