Insulation R-value Calculator

What This Calculator Does

This insulation R-value calculator adds up the layers of a wall, roof or floor and tells you how well the whole thing keeps heat in.

You get three numbers. Rc is the score of the layers together, the number Dutch and Belgian builders use. The U-value says how much heat leaks through, so lower is better. The US R-value is the same Rc in the American units you see on insulation packs in the US.

It also shows how much each layer adds, so you can see straight away which layer does the real work. For the basics behind these numbers, read how insulation traps air.

How To Use This Calculator

1. In What are you insulating?, pick wall, roof or floor. This changes the U-value a little, because heat leaves a surface faster going up than going down.
2. Under Layers, inside to outside, pick a material for each layer and type its thickness in millimetres.
3. Have a product datasheet? Pick Known R-value and type the R it lists, in m²K/W. A US R-value? Divide it by 5.678 first.
4. A closed, still air gap between two layers? Pick Still air gap and type its width.
5. Building with timber studs? Tick Studs on the layer the studs run through, and type the Timber share: how much of the wall's face is wood. The UK convention (BRE BR 443) uses 15% for a timber-frame wall, or 12.5% with careful detailing.
6. In US climate zone, pick your zone if you want the US code check. Zone 5 covers places like Chicago and Denver.
7. Use Add layer for more rows and the bin icon to remove one.
8. Read the result card. It compares your Rc with the Dutch new-build minimum and your U-value with the US code for your zone. That comparison is a guide, not a code check: see below.

The Formula, In Plain Words

Every material has a number called lambda (λ). It says how easily heat flows through one metre of it. Small lambda means a good insulator.

The R-value of one layer is its thickness in metres divided by its lambda. So 140 mm of stone wool at λ 0.035 gives 0.14 ÷ 0.035 = 4.0.

Add all the layer R-values together and you have Rc.

The air right next to each surface also slows heat down a little. The European standard EN ISO 6946 gives fixed values for this: 0.13 inside and 0.04 outside for a wall, 0.10 and 0.04 for a roof, 0.17 and 0.04 for a floor. Add those to Rc and you get the total, Rt.

The U-value is 1 divided by Rt. The US R-value is Rc times 5.678.

Studs make it a little harder, because heat can go two ways: through the insulation or through the wood. EN ISO 6946 works this out twice. Once as if the two paths sit side by side, which gives an upper limit. Once as if the stud layer were one mixed material, which gives a lower limit. The answer is the average of the two.

A Worked Example

The calculator starts with a timber-frame wall, inside to outside:

• 12.5 mm gypsum board: 0.0125 ÷ 0.21 = 0.06
• 140 mm stone wool: 0.14 ÷ 0.035 = 4.00
• 12 mm OSB: 0.012 ÷ 0.13 = 0.09
• 60 mm rigid wood fibre board: 0.06 ÷ 0.045 = 1.33

Rc is 0.06 + 4.00 + 0.09 + 1.33 = 5.49 m²K/W. That beats the Dutch minimum for walls of 4.7.

With the surfaces, Rt is 0.13 + 5.49 + 0.04 = 5.66. The U-value is 1 ÷ 5.66 = 0.177 W/m²K. In US units the wall is about R-31.

The stone wool does 73% of the work and the wood fibre 24%. The gypsum and OSB together add only 3%. They hold the wall together, they don't keep it warm.

Now add the studs: 15% timber through the stone wool. The wood path alone has Rt 2.73, because 140 mm of wood is only R 1.08. The upper limit comes out at 4.87 and the lower limit at 4.50. The average, 4.69, gives Rc 4.52 and a U-value of 0.213 W/m²K. The studs cost 0.97, about 18%, and the wall now falls just short of the Dutch 4.7.

Now try a 450 mm straw bale wall with the careful straw value. One bale gives Rc 5.63, about R-32 in US units, with no other insulation at all.

What To Watch Out For

Without a timber share, the calculator treats each layer as solid and even, so a timber-frame wall looks better than it is. Official Dutch and US checks include the studs, so set the timber share before you compare. Metal fixings, corners and gaps in the insulation are not counted.

So "Above it" on the result card is a rough guide, not proof that your wall meets the rules. In the Netherlands the official Rc is worked out with NTA 8800, which also counts fixings and air gaps in the insulation. Let a builder or energy advisor check the real design before you apply for a permit.

The lambda values are typical figures from published tables, such as the UK BRE guide BR 443 and the Upper Austria energy agency's insulation guide. Products vary, and a datasheet always beats a typical value.

Straw insulates better when heat flows across the stalks (λ 0.052) than along them (λ 0.080). How the stalks lie depends on how the bales are stacked, so pick the careful value if you're not sure.

A layer behind a ventilated gap, like timber cladding on battens, doesn't count. Outside air flows behind it, so it keeps the rain off but not the cold out.

Damp insulation works much worse than dry insulation. Read about moisture traps before you close up a wall.

Sources

• BRE BR 443, Conventions for U-value calculations (2019)
• IPLO, Energiezuinigheid nieuwbouw (Bbl article 4.152)
• Energiesparverband Upper Austria, Ökologische Dämmstoffe
• DIBt, approval Z-23.11-1595 for straw bale insulation
• Passipedia, Straw as insulation material
• NIST SP 811, Appendix B.9 conversion factors