Methodology for calculating heat loss in premises and the procedure for its implementation (see SP 50.13330.2012 Thermal protection buildings, point 5).
The house loses heat through enclosing structures (walls, ceilings, windows, roof, foundation), ventilation and sewerage. The main heat losses occur through the enclosing structures - 60–90% of all heat losses.
In any case, heat loss must be taken into account for all enclosing structures that are present in the heated room.
In this case, it is not necessary to take into account heat losses that occur through internal structures if the difference in their temperature with the temperature in adjacent rooms does not exceed 3 degrees Celsius.
Heat loss through building envelopes
Heat losses in premises mainly depend on:
1 Temperature differences in the house and outside (the greater the difference, the higher the losses),
2 Thermal insulation properties of walls, windows, doors, coatings, floors (the so-called enclosing structures of the room).
Enclosing structures are generally not homogeneous in structure. And they usually consist of several layers. Example: shell wall = plaster + shell + exterior decoration. This design may also include closed air gaps (example: cavities inside bricks or blocks). The above materials have thermal characteristics that differ from each other. The main characteristic for a structural layer is its heat transfer resistance R.
Where q is the amount of heat that is lost square meter enclosing surface (usually measured in W/sq.m.)
ΔT is the difference between the temperature inside the calculated room and the outside air temperature (the coldest five-day temperature °C for the climatic region in which the calculated building is located).
Basically, the internal temperature in the rooms is taken. Living quarters 22 oC. Non-residential 18 oC. Water treatment areas 33 °C.
When it comes to a multilayer structure, the resistances of the layers of the structure add up.
δ - layer thickness, m;
λ is the calculated thermal conductivity coefficient of the material of the construction layer, taking into account the operating conditions of the enclosing structures, W / (m2 oC).
Well, we’ve sorted out the basic data required for the calculation.
So, to calculate heat losses through building envelopes, we need:
1. Heat transfer resistance of structures (if the structure is multilayer, then Σ R layers)
2. The difference between the temperature in settlement room and outside (the temperature of the coldest five-day period is °C.). ΔT
3. Fencing areas F (separately walls, windows, doors, ceiling, floor)
4. The orientation of the building in relation to the cardinal directions is also useful.
The formula for calculating heat loss by a fence looks like this:
Qlimit=(ΔT / Rolim)* Folim * n *(1+∑b)
Qlim - heat loss through enclosing structures, W
Rogr – heat transfer resistance, m2°C/W; (If there are several layers then ∑ Rogr layers)
Fogr – area of the enclosing structure, m;
n is the coefficient of contact between the enclosing structure and the outside air.
| Walling | Coefficient n |
| 1. External walls and coverings (including those ventilated by outside air), attic floors (with roofing made of piece materials) and over driveways; ceilings over cold (without enclosing walls) undergrounds in the Northern construction-climatic zone | |
| 2. Ceilings over cold basements communicating with outside air; attic floors (with a roof made of roll materials); ceilings above cold (with enclosing walls) undergrounds and cold floors in the Northern construction-climatic zone | 0,9 |
| 3. Ceilings over unheated basements with light openings in the walls | 0,75 |
| 4. Ceilings over unheated basements without light openings in the walls, located above ground level | 0,6 |
| 5. Ceilings over unheated technical undergrounds located below ground level | 0,4 |
The heat loss of each enclosing structure is calculated separately. The amount of heat loss through the enclosing structures of the entire room will be the sum of heat losses through each enclosing structure of the room
Calculation of heat loss through floors
Uninsulated floor on the ground
Typically, the heat loss of the floor in comparison with similar indicators of other building envelopes (external walls, window and door openings) is a priori assumed to be insignificant and is taken into account in the calculations of heating systems in a simplified form. The basis for such calculations is a simplified system of accounting and correction coefficients for heat transfer resistance of various building materials.
Considering that theoretical basis and the methodology for calculating heat loss from a ground floor was developed quite a long time ago (i.e., with a large design margin), we can safely talk about the practical applicability of these empirical approaches in modern conditions. Thermal conductivity and heat transfer coefficients of various building materials, insulation materials and floor coverings are well known, and no other physical characteristics are required to calculate heat loss through the floor. According to their thermal characteristics, floors are usually divided into insulated and non-insulated, and structurally - floors on the ground and on joists.
Calculation of heat loss through an uninsulated floor on the ground is based on the general formula for assessing heat loss through the building envelope:
Where Q– main and additional heat losses, W;
A– total area of the enclosing structure, m2;
tв , tн– indoor and outdoor air temperature, °C;
β - the share of additional heat losses in the total;
n– correction factor, the value of which is determined by the location of the enclosing structure;
Ro– heat transfer resistance, m2 °C/W.
Note that in the case of a homogeneous single-layer floor covering, the heat transfer resistance Ro is inversely proportional to the heat transfer coefficient of the non-insulated floor material on the ground.
When calculating heat loss through an uninsulated floor, a simplified approach is used, in which the value (1+ β) n = 1. Heat loss through the floor is usually carried out by zoning the heat transfer area. This is due to the natural heterogeneity of the temperature fields of the soil under the ceiling.
Heat loss from an uninsulated floor is determined separately for each two-meter zone, numbered starting from outer wall building. A total of four such strips 2 m wide are usually taken into account, considering the ground temperature in each zone to be constant. The fourth zone includes the entire surface of the uninsulated floor within the boundaries of the first three stripes. Heat transfer resistance is assumed: for the 1st zone R1=2.1; for the 2nd R2=4.3; respectively for the third and fourth R3=8.6, R4=14.2 m2*оС/W.
Fig.1. Zoning the floor surface on the ground and adjacent recessed walls when calculating heat loss
In the case of recessed rooms with a soil base floor: the area of the first zone adjacent to the wall surface is taken into account twice in the calculations. This is quite understandable, since the heat loss of the floor is summed up with the heat loss in the adjacent vertical enclosing structures of the building.
Calculation of heat loss through the floor is carried out for each zone separately, and the results obtained are summarized and used for the thermal engineering justification of the building design. Calculation for temperature zones of external walls of recessed rooms is carried out using formulas similar to those given above.
In calculations of heat loss through an insulated floor (and it is considered such if its design contains layers of material with a thermal conductivity of less than 1.2 W/(m °C)), the value of the heat transfer resistance of a non-insulated floor on the ground increases in each case by the heat transfer resistance of the insulating layer:
Rу.с = δу.с / λу.с,
Where δу.с– thickness of the insulating layer, m; λу.с– thermal conductivity of the insulating layer material, W/(m °C).
To calculate heat loss through the floor and ceiling, the following data will be required:
- house dimensions 6 x 6 meters.
- Floors - edged boards, tongue and groove 32 mm thick, covered with chipboard 0.01 m thick, insulated mineral wool insulation 0.05 m thick. Under the house there is an underground space for storing vegetables and canning. In winter, the temperature in the underground averages +8°C.
- Ceiling - the ceilings are made of wooden panels, the ceilings are insulated on the attic side with mineral wool insulation, layer thickness 0.15 meters, with a vapor-waterproofing layer. Attic space uninsulated.
Calculation of heat loss through the floor
R boards =B/K=0.032 m/0.15 W/mK =0.21 m²x°C/W, where B is the thickness of the material, K is the thermal conductivity coefficient.
R chipboard =B/K=0.01m/0.15W/mK=0.07m²x°C/W
R insulation =B/K=0.05 m/0.039 W/mK=1.28 m²x°C/W
Total value R of the floor =0.21+0.07+1.28=1.56 m²x°C/W
Considering that the underground temperature in winter is constantly around +8°C, the dT required for calculating heat loss is 22-8 = 14 degrees. Now we have all the data to calculate heat loss through the floor:
Q floor = SxdT/R=36 m²x14 degrees/1.56 m²x°C/W=323.07 Wh (0.32 kWh)
Calculation of heat loss through the ceiling
Ceiling area is the same as the floor S ceiling = 36 m2
When calculating the thermal resistance of the ceiling, we do not take into account wooden boards, because they do not have a tight connection with each other and do not act as a heat insulator. Therefore, the thermal resistance of the ceiling is:
R ceiling = R insulation = insulation thickness 0.15 m/thermal conductivity of insulation 0.039 W/mK=3.84 m²x°C/W
We calculate heat loss through the ceiling:
Ceiling Q =SхdT/R=36 m²х52 degrees/3.84 m²х°С/W=487.5 Wh (0.49 kWh)
The essence of thermal calculations of premises, to one degree or another located in the ground, comes down to determining the influence of atmospheric “cold” on their thermal regime, or more precisely, to what extent a certain soil insulates a given room from atmospheric temperature effects. Because The thermal insulation properties of soil depend on too large number factors, the so-called 4-zone technique was adopted. It is based on the simple assumption that the thicker the soil layer, the higher its thermal insulation properties (the influence of the atmosphere is reduced to a greater extent). The shortest distance (vertically or horizontally) to the atmosphere is divided into 4 zones, 3 of which have a width (if it is a floor on the ground) or a depth (if it is walls on the ground) of 2 meters, and the fourth has these characteristics equal to infinity. Each of the 4 zones is assigned its own permanent heat-insulating properties according to the principle - the further away the zone (the higher its serial number), the less the influence of the atmosphere. Omitting the formalized approach, we can draw a simple conclusion that the further a certain point in the room is from the atmosphere (with a multiplicity of 2 m), the more favorable conditions(from the point of view of the influence of the atmosphere) it will be located.
Thus, the counting of conditional zones begins along the wall from ground level, provided that there are walls on the ground. If there are no ground walls, then the first zone will be the floor strip closest to the outer wall. Next, zones 2 and 3 are numbered, each 2 meters wide. The remaining zone is zone 4.
It is important to consider that the zone can begin on the wall and end on the floor. In this case, you should be especially careful when making calculations.
If the floor is not insulated, then the heat transfer resistance values of the non-insulated floor by zone are equal to:
zone 1 - R n.p. =2.1 sq.m*S/W
zone 2 - R n.p. =4.3 sq.m*S/W
zone 3 - R n.p. =8.6 sq.m*S/W
zone 4 - R n.p. =14.2 sq.m*S/W
To calculate the heat transfer resistance for insulated floors, you can use the following formula:
— heat transfer resistance of each zone of the non-insulated floor, sq.m*S/W;
— insulation thickness, m;
— thermal conductivity coefficient of insulation, W/(m*C);
Typically, the heat loss of the floor in comparison with similar indicators of other building envelopes (external walls, window and door openings) is a priori assumed to be insignificant and is taken into account in the calculations of heating systems in a simplified form. The basis for such calculations is a simplified system of accounting and correction coefficients for the heat transfer resistance of various building materials.
If we take into account that the theoretical justification and methodology for calculating the heat loss of a ground floor was developed quite a long time ago (i.e., with a large design margin), we can safely talk about the practical applicability of these empirical approaches in modern conditions. The thermal conductivity and heat transfer coefficients of various building materials, insulation and floor coverings are well known, and other physical characteristics are not required to calculate heat loss through the floor. According to their thermal characteristics, floors are usually divided into insulated and non-insulated, and structurally - floors on the ground and on joists.
Calculation of heat loss through an uninsulated floor on the ground is based on the general formula for assessing heat loss through the building envelope:
Where Q– main and additional heat losses, W;
A– total area of the enclosing structure, m2;
tв , tн– indoor and outdoor air temperature, °C;
β - the share of additional heat losses in the total;
n– correction factor, the value of which is determined by the location of the enclosing structure;
Ro– heat transfer resistance, m2 °C/W.
Note that in the case of a homogeneous single-layer floor covering, the heat transfer resistance Ro is inversely proportional to the heat transfer coefficient of the non-insulated floor material on the ground.
When calculating heat loss through an uninsulated floor, a simplified approach is used, in which the value (1+ β) n = 1. Heat loss through the floor is usually carried out by zoning the heat transfer area. This is due to the natural heterogeneity of the temperature fields of the soil under the ceiling.
Heat loss from an uninsulated floor is determined separately for each two-meter zone, the numbering of which starts from the outer wall of the building. A total of four such strips 2 m wide are usually taken into account, considering the ground temperature in each zone to be constant. The fourth zone includes the entire surface of the uninsulated floor within the boundaries of the first three stripes. Heat transfer resistance is assumed: for the 1st zone R1=2.1; for the 2nd R2=4.3; respectively for the third and fourth R3=8.6, R4=14.2 m2*оС/W.
Fig.1. Zoning the floor surface on the ground and adjacent recessed walls when calculating heat loss
In the case of recessed rooms with a soil base floor: the area of the first zone adjacent to the wall surface is taken into account twice in the calculations. This is quite understandable, since the heat loss of the floor is summed up with the heat loss in the adjacent vertical enclosing structures of the building.
Calculation of heat loss through the floor is carried out for each zone separately, and the results obtained are summarized and used for the thermal engineering justification of the building design. Calculation for temperature zones of external walls of recessed rooms is carried out using formulas similar to those given above.
In calculations of heat loss through an insulated floor (and it is considered such if its design contains layers of material with a thermal conductivity of less than 1.2 W/(m °C)), the value of the heat transfer resistance of a non-insulated floor on the ground increases in each case by the heat transfer resistance of the insulating layer:
Rу.с = δу.с / λу.с,
Where δу.с– thickness of the insulating layer, m; λу.с– thermal conductivity of the insulating layer material, W/(m °C).
