100mm Insulation Boards

100mm is the most common insulation thickness in UK construction. It is the usual choice between rafters on a pitched roof, under a screed floor, in a timber frame or stud wall, and as the first layer in a loft. We hold more than 50 products at 100mm — PIR, phenolic, mineral wool, EPS, cavity slabs and flat roof boards — at our Rossendale warehouse for next-day UK delivery or trade counter collection. This page compares them so you can pick the right one for the job.

100mm insulation compared: R-value, lambda and where to use it

The R-value is the thermal resistance of the board at 100mm. Higher is better. Lambda (thermal conductivity) is the material property — lower is better — and R-value is simply thickness divided by lambda. All boards below are 100mm thick; the difference in R-value is entirely down to the material.

Product	Material	Lambda (W/mK)	R-value at 100mm (m²K/W)	Board / pack size	Typical use
Kingspan Kooltherm K107	Phenolic	0.018	5.55	2400 x 1200mm, 3 per pack	Pitched roofs where depth is tight
Kingspan Kooltherm K103	Phenolic	0.018	5.55	2400 x 1200mm, 3 per pack	Floors
Kingspan Kooltherm K112	Phenolic	0.018	5.55	2400 x 1200mm, 3 per pack	Timber and steel frame walls
Celotex GA4100	PIR	0.022	4.50	2400 x 1200mm, single boards	Roofs, floors, walls — general application
Recticel Eurothane GP	PIR	0.022	4.50	2400 x 1200mm, single boards	Roofs, floors, walls — general application
EcoTherm Eco-Versal	PIR	0.022	4.50	2400 x 1200mm, single boards	Roofs, floors, walls — general application
Sopratherm GA4000 (formerly Celotex)	PIR	0.022	4.50	2400 x 1200mm, single boards	Roofs, floors, walls — general application
Celotex CW4000	PIR	0.022	4.50	1200 x 450mm, packs	Partial fill cavity walls
EcoTherm Eco-Cavity	PIR	0.022	4.50	1200 x 450mm, packs	Partial fill cavity walls
Jablite EWI 70HPE Plus	Grey EPS	0.030	3.30	1200 x 600mm, 6 per pack	External wall insulation systems
Knauf DriTherm 32	Glass mineral wool	0.032	3.10	1200 x 455mm slabs, packs	Full fill cavity walls
Isover CWS 32	Glass mineral wool	0.032	3.10	1200 x 455mm slabs, packs	Full fill cavity walls
Rockwool RWA45	Stone mineral wool	0.035	2.85	1200 x 600mm slabs, 2.88m² per pack	Stud walls, floors, acoustic and fire
Knauf Rocksilk RS45	Stone mineral wool	0.035	2.85	1200 x 600mm slabs, packs	Stud walls, floors, acoustic and fire
EPS150	EPS	0.035	2.85	2400 x 1200mm	Floors under load
Knauf DriTherm 37	Glass mineral wool	0.037	2.70	1200 x 455mm slabs, packs	Full fill cavity walls
EPS100	EPS	0.036	2.75	2400 x 1200mm	Floors
Jablite Jabfloor EPS70	EPS	0.038	2.60	2400 x 1200mm	Domestic floors
Knauf Loft Roll 44	Glass mineral wool	0.044	2.25	13.89m² per roll, combi-cut	Loft, first layer between joists
Superglass Multi-Roll 44	Glass mineral wool	0.044	2.25	12.12m² per roll	Loft, first layer between joists
Isover Spacesaver	Glass mineral wool	0.044	2.25	Roll	Loft, first layer between joists

R-values are calculated from the manufacturer’s declared lambda at 100mm and rounded to the nearest 0.05. Board sizes, pack quantities and prices are on each product page.

Which 100mm insulation do I need?

Pitched roof — between rafters

100mm PIR between the rafters with 25mm to 50mm under is the standard specification for a warm pitched roof. Celotex GA4100, Recticel GP and EcoTherm Eco-Versal are interchangeable on performance, so price and availability usually decide it. If rafter depth is the constraint and you cannot fit an under-layer, Kingspan K107 at 0.018 W/mK gets you further in the same 100mm. Leave a 50mm ventilated gap above the insulation unless you are building a fully supported warm roof.

Floors — under screed or between joists

100mm PIR under a screed floor is the common new-build spec and typically gets you to a U-value of around 0.18 W/m²K depending on the perimeter-to-area ratio. Kingspan K103 is the phenolic alternative for a tighter target. For suspended timber floors, 100mm mineral wool such as Rockwool RWA45 between the joists is easier to fit around services, and EPS100 or EPS150 is the cheapest way to insulate a ground-bearing slab where loads are moderate.

Walls — timber frame and stud

Between the studs, 100mm PIR (or Kingspan K112 for the best performance) gives the highest R-value. Where fire performance or sound matters more — party walls, plant rooms, anything the fire strategy calls non-combustible — 100mm Rockwool RWA45 or Rocksilk RS45 is the answer: Euroclass A1, with good acoustic absorption.

Cavity walls

Full fill uses mineral wool slabs at 100mm: Knauf DriTherm 32, Isover CWS 32, Superglass Superwall 32 and URSA 32 all sit at 0.032 W/mK, with the 36/37 versions a cheaper step down. Partial fill uses 100mm PIR such as Celotex CW4000 or EcoTherm Eco-Cavity with a residual clear cavity. Which you need depends on the wall build-up and exposure zone — ask us if you are not sure.

Loft

100mm is the first layer between the joists. Building regulations want 270mm in total, so it is normally topped with 170mm cross-laid over the joists. Knauf Loft Roll 44, Superglass Multi-Roll 44 and Isover Spacesaver are all 0.044 W/mK and interchangeable. See our Knauf Loft Roll 44 page for the layering combinations.

External wall insulation

100mm grey EPS is the standard retrofit thickness for EWI on a solid brick wall, taking it to around 0.30 W/m²K. Jablite EWI 70HPE Plus at 0.030 W/mK is the board we stock, and we hold the full Ceresit render system to go with it — adhesive, mesh, beads, primer and top coat.

Flat roofs

For warm deck flat roofs, 100mm PIR flat roof boards — Kingspan Thermaroof TR26 and TR27, Recticel Powerdeck U and F, EcoTherm Inno-Torch — are supplied in packs with facings matched to the waterproofing system. Which one depends on whether you are bonding, torching or mechanically fixing the membrane, so check the product pages or ask.

What U-value does 100mm insulation give?

It depends on everything else in the build-up, not just the board. As a guide, in a typical pitched roof, 100mm PIR between rafters with 25mm under gets to around 0.18 W/m²K. Under a screed floor, 100mm PIR alone is around 0.18 to 0.20 W/m²K depending on floor size. In a timber frame wall, 100mm PIR between studs with a plasterboard lining is around 0.20 W/m²K. These are indicative — send us the construction and the target U-value and we will run the calculation, including condensation risk, free of charge.

How many boards do I need?

A standard 2400 x 1200mm board covers 2.88m². Divide the area to be insulated by 2.88, then add 10% for cuts and wastage. So a 40m² floor needs 14 boards plus one or two spare. Rolls are quoted by coverage: Knauf Loft Roll 44 at 100mm covers 13.89m² per roll. If you send us the dimensions, we will do the take-off and price it, and we do bulk and pallet rates on all of the above.

100mm PIR insulation — shop the range

All 2400 x 1200mm, 0.022 W/mK, R-value 4.50 at 100mm. Choose your thickness on the product page.

100mm Kingspan Kooltherm — shop the range

100mm mineral wool slabs

100mm loft and acoustic rolls

100mm EPS

100mm cavity wall insulation

100mm flat roof boards

Frequently asked questions

Is 100mm insulation enough?

For a single layer in a roof, floor or wall using PIR, 100mm is usually enough to meet current building regulations for a refurbishment and close to it for new build, depending on the rest of the construction. For a loft it is not — 100mm is the first layer and you need 270mm in total. For mineral wool or EPS you will generally need more thickness than PIR to hit the same U-value.

What is the R-value of 100mm PIR insulation?

4.50 m²K/W for any PIR board with a lambda value of 0.022 W/mK, which covers Celotex GA4100, Recticel Eurothane GP, EcoTherm Eco-Versal and Sopratherm GA4000. Kingspan Kooltherm at 0.018 W/mK is 5.55 m²K/W at the same thickness.

What is the difference between 100mm Celotex and 100mm Kingspan?

Celotex GA4100 is PIR at 0.022 W/mK. Kingspan Kooltherm K107, K103 and K112 are phenolic at 0.018 W/mK, so they perform around 20% better for the same thickness but cost more per board. Kingspan Thermapitch and Thermafloor are Kingspan’s PIR boards and are the direct equivalent of Celotex. Choose Kooltherm when you are short of depth; otherwise PIR is the more economical route to the same U-value with a slightly thicker board.

Is Celotex the same as Sopratherm?

Yes. Celotex is now owned by Soprema and the range is being rebranded as Sopratherm. Product codes, UK manufacturing and the declared 0.022 W/mK performance are unchanged. Celotex-branded stock is still fully valid, and we stock both.

Which 100mm insulation is best for soundproofing?

Mineral wool. 100mm Rockwool RWA45 or an acoustic partition roll such as Knauf APR, URSA APR or Isover APR between studs absorbs airborne sound far better than a rigid foam board. PIR and phenolic do very little for acoustics.

Which 100mm insulation is non-combustible?

Stone and glass mineral wool: Rockwool RWA45, RW3, Rocksilk RS45, Knauf DriTherm, Isover CWS and the loft and acoustic rolls are all Euroclass A1. PIR, phenolic and EPS are combustible and carry Euroclass E or F, so check the fire strategy before specifying them in escape routes, party walls or buildings over 11 metres.

Can I get 100mm insulation delivered?

Yes — next-day delivery across most of the UK on stock lines, with free delivery on orders over £500. Or collect from our trade counter at Alexandra Works, Crawshawbooth, Rossendale, open to the public Monday to Thursday 8–5 and Friday 8–4.

Other thicknesses

Looking for a different depth? See all PIR insulation boards, insulated plasterboard, or browse by brand: Celotex, Kingspan, Recticel, Rockwool, EcoTherm, Knauf.

Not sure which to order? Call 01706 584721, WhatsApp 07395 949258, or send the drawings to katy@pinksinsulation.co.uk and we will price the job and check the U-value.