Wall thermal design calculator: heat transfer resistance, insulation thickness, dew point

Choose a city, a building type and a structure, then enter the wall layers (material and thickness). The calculator finds the degree-days of the heating period, the required resistance to heat transfer by SP 50.13330, the actual resistance of the multilayer wall, the temperatures on the layer boundaries and the dew point, and checks moisture condensation inside the wall. For one layer (the insulation) it can find the thickness you need.

City (climate data)φ_ext, % (humidity of the coldest month; empty: by the city)BuildingStructuret_int, °Cφ_int, %Operating conditionOwn degree-days, °C·daym_p (norm reduction factor, 1)r (thermal uniformity, at most 1)α_ext, W/(m²·°C)Layer 1 (inner)Layer 2Layer 3Layer 4Layer 5Find the thickness of the layer

Climate: SP 131.13330.2012, table 3.1; coefficients a and b: SP 50.13330.2012, table 3; λ: appendix T. For another city choose «Own data» and enter t_ht, z_ht and t₅ or the degree-days directly. Operating condition A or B comes from table 2 of SP 50: a dwelling in the normal humidity zone with a normal indoor regime is B. For reference: check the data against the editions of the codes in force.

Calculate

Design thermal properties of materials

SP 50.13330.2012, appendix T, table T.1: λ for operating conditions A and B, W/(m·°C); μ is the vapour permeability, mg/(m·h·Pa); ρ₀ is the dry density (for masonry, the density of the masonry). For reference: check against the data sheet of the actual product.

Material	ρ₀, kg/m³	λ_A	λ_B	μ	Row of table T.1
Expanded polystyrene (boards)	15-17	0.041	0.047	0.05	5
Expanded polystyrene (boards)	25-30	0.038	0.044	0.05	8
Expanded polystyrene with graphite	15-20	0.035	0.04	0.05	11
Extruded polystyrene (XPS)	35-45	0.031	0.032	0.005	14
Polyurethane foam	40	0.031	0.04	0.05	17
Stone mineral wool (boards)	40-60	0.041	0.044	0.35	28
Stone mineral wool (boards)	80-125	0.042	0.045	0.32	27
Stone mineral wool (boards)	140-175	0.043	0.046	0.31	26
Glass wool (staple fibre boards)	45	0.041	0.045	0.51	33
Foam glass (boards)	121-140	0.051	0.051	0.005	237
Aerated and foam concrete on cement	400	0.14	0.15	0.23	172
Aerated and foam concrete on cement	600	0.22	0.26	0.17	171
Aerated and foam concrete on cement	800	0.33	0.37	0.14	170
Expanded clay concrete on clay sand	600	0.2	0.26	0.26	111
Expanded clay concrete on clay sand	1000	0.33	0.41	0.14	109
Masonry of hollow ceramic brick (1000 kg/m³ gross) on cement-sand mortar	1200	0.47	0.52	0.17	189
Masonry of hollow ceramic brick (1400 kg/m³ gross) on cement-sand mortar	1600	0.58	0.64	0.14	187
Masonry of solid clay brick on cement-sand mortar	1800	0.7	0.81	0.11	180
Masonry of silicate brick on cement-sand mortar	1800	0.76	0.87	0.11	183
Reinforced concrete	2500	1.92	2.04	0.03	199
Concrete on crushed stone or gravel	2400	1.74	1.86	0.03	200
Cement-sand mortar	1800	0.76	0.93	0.09	201
Compound mortar (sand, lime, cement)	1700	0.7	0.87	0.098	202
Lime-sand mortar	1600	0.7	0.81	0.12	203
Gypsum board (dry plaster)	800	0.19	0.21	0.075	55
Fibreboard and particleboard	800	0.19	0.23	0.12	40
Pine and spruce across the grain	500	0.14	0.18	0.06	192
Glued plywood	600	0.15	0.18	0.02	196

Climate data for the calculation (30 cities)

SP 131.13330.2012, table 3.1: t₅ is the temperature of the coldest five days, probability 0.92; t_ht and z_ht are the mean temperature and the length of the period with a mean daily temperature of at most 8 °C. For reference: check against SP 131.13330.2020.

City	t₅, °C	t_ht, °C	z_ht, days	Degree-days at 20 °C, °C·day	R_base, dwelling wall, m²·°C/W
Moscow	−25	−2.2	205	4551	2.993
Saint Petersburg	−24	−1.3	213	4537	2.988
Novosibirsk	−37	−8.1	221	6210	3.574
Yekaterinburg	−32	−5.4	221	5613	3.365
Kazan	−31	−4.8	208	5158	3.205
Nizhny Novgorod	−31	−4.1	215	5182	3.214
Chelyabinsk	−34	−6.5	218	5777	3.422
Samara	−30	−5.2	203	5116	3.19
Omsk	−37	−8.1	216	6070	3.524
Rostov-on-Don	−19	−0.1	166	3337	2.568
Ufa	−33	−6	209	5434	3.302
Krasnoyarsk	−37	−6.7	233	6221	3.577
Perm	−35	−5.5	225	5738	3.408
Voronezh	−24	−2.5	190	4275	2.896
Volgograd	−22	−2.3	176	3925	2.774
Krasnodar	−16	2.5	145	2538	2.288
Saratov	−25	−3.5	188	4418	2.946
Tyumen	−35	−6.9	223	5999	3.5
Izhevsk	−33	−5.6	219	5606	3.362
Barnaul	−36	−7.5	213	5858	3.45
Irkutsk	−33	−7.7	232	6426	3.649
Khabarovsk	−29	−9.5	204	6018	3.506
Vladivostok	−23	−4.3	198	4811	3.084
Yaroslavl	−31	−4	221	5304	3.256
Tomsk	−39	−7.9	233	6501	3.675
Kaliningrad	−19	1.2	188	3534	2.637
Sochi	−2	6.6	94	1260	1.841
Murmansk	−30	−3.4	275	6435	3.652
Arkhangelsk	−33	−4.5	250	6125	3.544
Yakutsk	−52	−20.9	252	10 310	5.007

How the wall thermal design is made

A thermal design checks that an external wall does not lose too much heat and that its inner surface does not get cold enough to collect moisture. The Russian code is SP 50.13330 «Thermal performance of buildings». The main quantity is the resistance to heat transfer R₀ (m²·°C/W): the larger it is, the warmer the wall. For a multilayer wall R₀ = 1/α_int + Σδᵢ/λᵢ + 1/α_ext with α_int = 8.7 W/(m²·°C) on the inner surface and α_ext = 23 W/(m²·°C) on the outer one.

Required resistance by degree-days

The norm depends on the climate through the degree-days of the heating period: D_d = (t_int − t_ht)·z_ht, where t_int is the indoor temperature, t_ht and z_ht are the mean temperature and the length of the heating period (the period with a mean daily temperature of at most 8 °C, and at most 10 °C for children and medical buildings). The base value comes from table 3 of SP 50: R_base = a·D_d + b. For walls of dwellings a = 0.00035 and b = 1.4, for public buildings a = 0.0003 and b = 1.2, for industrial buildings with a dry regime a = 0.0002 and b = 1.0. For example, for Moscow D_d = (20 + 2.2)·205 = 4551 °C·day and R_base = 0.00035·4551 + 1.4 = 2.99 m²·°C/W.

The sanitary requirement and the dew point

Besides energy saving, the sanitary requirement is checked: the difference between the air and the inner surface Δt = (t_int − t_ext)/(R₀·α_int) must not exceed the normative Δt_n (4.0 °C for walls of dwellings), and the temperature of the inner surface must be above the dew point of the indoor air. If the surface is colder than the dew point, condensate forms on the wall and mould appears. The page finds the dew point from the air temperature and relative humidity (ASHRAE formulas, as on the i-d chart).

Insulation thickness

If you pick the layer whose thickness is needed, the calculator subtracts the resistance of the other layers and of the surfaces from the required one and divides the remainder by the conductivity of the insulation. The result is rounded up to 10 mm. The factor r accounts for thermal non-uniformity (fasteners, reveals, joints): the reduced resistance is r·R₀.

Condensation inside the wall

A simplified check by the Glaser method: the vapour pressure falls from the inside to the outside in proportion to the vapour resistance of the layers δ/μ, and the temperature changes linearly within a layer. Where the vapour pressure exceeds the saturation pressure at the temperature of the layer, condensate is possible. The dangerous zone usually lies at the boundary of a warm dense layer and the insulation, on the side of the outer layers. Insulation outside and a vapour barrier inside lower the risk. The method ignores sorption, air flow and the yearly cycle, so it is approximate.

Limitations

The calculation assumes a one-dimensional flow through a flat wall. Windows, thermal bridges, junctions and the ground need separate calculations. The data are for reference: before using them in a design, check them against the editions of SP 50.13330 and SP 131.13330 in force.

FAQ

How do I choose operating condition A or B?

Use table 2 of SP 50.13330: first find the humidity regime of the room from the air temperature and humidity (table 1), then the humidity zone of the region (appendix B). For dwellings with a normal regime in the normal humidity zone (most of Russia) it is condition B, in the dry zone it is A. Condition B gives a slightly larger λ and so a more cautious result.

What are degree-days and where do t_ht and z_ht come from?

The degree-days = (t_int − t_ht)·z_ht. The mean temperature t_ht and the length z_ht of the heating period (with a mean daily temperature of at most 8 °C) are given in SP 131.13330, table 3.1. The page has 30 cities; for others choose «Own data». If the degree-days are already known, enter them in the separate field.

How thick should the wall insulation be?

Enough for the reduced resistance to heat transfer to be at least the required one: δ = λ·(R_req/r − 1/α_int − 1/α_ext − the sum of R of the other layers). It depends on the climate, the masonry and the insulation itself. Pick the insulation layer in the field «Find the thickness of the layer» and read the result: the better the masonry, the thinner the layer.

What is the thermal uniformity factor r?

It is the ratio of the reduced resistance to the conditional one: r = R₀ᵖʳ/R₀. For a solid wall without inclusions r = 1; insulation fasteners, reveals, lintels, joints and balcony slabs lower r (often to 0.7–0.9). The exact value comes from a temperature-field calculation by appendix E of SP 50. For reference: the value of r is entered by the user.

Why is it better to put the insulation outside?

Then the load-bearing wall stays in the warm zone and the temperatures at the layer boundaries stay above the dew point. Insulation inside leaves the cold wall behind a layer of low vapour permeability, and vapour condenses on its boundary. The temperature and dew-point graph on the page shows it: compare the order of the layers.