What Thickness of Insulation Do You Need for Walls?

There is no single thickness of insulation that is right for every wall. The answer depends on the type of wall (solid, cavity or timber frame), which side the insulation goes on, which material you use and the U-value you need to hit. This guide gives well-established starting points for each wall type, points to the published tables behind them, and shows what we stock so you can price it up.

Quick answer

• New cavity wall on an extension (partial fill): around 100mm of PIR cavity board is a typical starting point for a 0.18 W/m²K target, depending on the blockwork.
• Existing solid wall, insulated from the inside: the target for a renovated wall is 0.30 W/m²K. Insulated plasterboard (we stock 37.5mm to 92.5mm) is the usual choice, but the thickness needed depends on the wall, so get a calculation first.
• Timber frame: the stud depth sets the thickness between the studs (for example 90mm or 140mm mineral wool), with PIR over the studs if you need to improve it.

These figures are indicative only. Always check your actual build-up with a U-value calculation, such as our PIR U-value calculator, and with Building Control before you order.

Why wall insulation thickness depends on the U-value

Building Regulations in England do not specify a thickness. They set a maximum U-value for a wall, in W/m²K, and the thickness you need is whatever gets your wall under it. The most common targets for England are in Approved Document L, Volume 1: Dwellings: 0.18 for a new wall on an extension, and 0.30 for an existing wall that you are renovating or upgrading with internal or external insulation (0.55 for a cavity fill). Check the current edition for your project. Our PIR thickness guide sets out the tables, and our building regulations guide gives the wider picture.

Cavity walls

For a new cavity wall with a partial-fill board and a clear cavity, Kingspan's published tables for Thermawall TW50 show roughly 0.18–0.20 W/m²K at 80mm and 0.15–0.17 at 100mm, depending on the block type. That is why 100mm is a common starting point for 0.18. We stock the board in 40mm to 100mm: for example Kingspan Thermawall TW50 100mm at £74.73 per pack (£27.68 per m²), and Recticel Eurowall in the same thickness range. The cavity boards are 1200 × 450mm.

If you are filling an existing cavity, or building one with full fill, a mineral wool batt is the alternative: Knauf DriTherm 37 Cavity Slab 100mm is £31.82 per pack at £4.86 per m². Check that the wall construction and exposure are suitable for the product, as a full-fill cavity needs a design that allows for the exposure of the site. Our cavity wall insulation guide explains the options.

Solid walls insulated from the inside

Insulating the inside face is the usual method for a solid brick or stone wall, and it uses insulated plasterboard or PIR board between and over battens. Every millimetre of insulation reduces the size of the room, so the thickness is a trade-off. Approved Document L allows a lesser standard where meeting the figure would reduce a room's floor area by more than 5%, which you should agree with Building Control.

Our insulated plasterboard comes in a range of thicknesses. Kingspan K118 runs from 37.5mm to 82.5mm, for example 72.5mm K118 (R 3.15 m²K/W), and Celotex PL4000 from 37.5mm to 77.5mm, for example 77.5mm PL4000 (R 3.00 m²K/W). To compare them, see our K118 vs PL4000 guide, and for fitting, see how to fit insulated plasterboard to a solid wall.

Solid walls insulated from the outside

External wall insulation leaves the rooms their full size but needs a system with a render or cladding finish, which is normally a specialist job. We stock boards used in external systems, such as Jablite EWI graphite EPS (100mm £56.10 per pack of 4.32m²) and Kingspan Kooltherm K5 external wall board (50mm £143.94 for 7.2m²). The thickness is set by the system design, so follow the system supplier's specification.

Timber frame walls

In a timber frame wall the insulation goes between the studs, and its depth is the depth of the stud, commonly 90mm or 140mm. URSA FRAMETEC 35 140mm is £43.10 per roll covering 4.93m² (£8.74 per m²). PIR can be added over the studs to reduce thermal bridging and improve the U-value. Our thermal bridging guide explains why this matters.

Stud walls and acoustic walls

If the wall is an internal partition, insulation is for sound rather than heat. Mineral wool in the stud cavity is the usual choice and the thickness is set by the stud, see our guide to soundproofing a stud wall.

Thickness by wall type at a glance

Wall	Insulation position	Typical product	Indicative starting point
New cavity wall (extension)	In the cavity, partial fill	PIR cavity board	around 100mm for 0.18 W/m²K
Existing cavity wall	Filled cavity	Mineral wool slab or batt	set by the cavity width
Solid wall, internal	On the room face	Insulated plasterboard or PIR on battens	37.5–92.5mm range, calculation needed
Solid wall, external	On the outside face	EWI board in a render system	set by the system design
Timber frame	Between and over studs	Mineral wool roll plus PIR	stud depth, plus PIR if needed

Frequently asked questions

What thickness of insulation do I need for external walls?

It depends on the wall type and the target U-value. For a new cavity wall on an extension, around 100mm of PIR cavity board is a typical starting point for 0.18 W/m²K.

How thick should internal wall insulation be?

For a solid wall, insulated plasterboard is the usual choice (we stock 37.5mm to 92.5mm), but the exact thickness depends on the wall and the target. Get a calculation first.

Is 50mm of insulation enough for a wall?

Sometimes. A 50mm PIR board (£21.03 per sheet) can be enough for a stud wall or where the existing wall already performs well, but it is unlikely to reach 0.18 on its own for a new wall.

Can I use 100mm PIR in a wall?

Yes, for example in a cavity or between studs. 100mm Eurothane GP is £36.83 per sheet. See our 100mm PIR U-value guide.

Do I need a vapour control layer or ventilation?

Internal wall insulation changes how the wall dries and where moisture can condense, so a damp and ventilation assessment is important before you start. Ask a qualified surveyor or Building Control if you are unsure.

Prices are shown ex VAT and vary, so check the product page before you order. We deliver to mainland England only. Insulation and plasterboard orders have a £400 ex VAT minimum and free delivery (see delivery information). Outside England? Call us or contact us for a quote. Call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm), email sales@insulationwarehousedirect.co.uk, or order online.