1. Heat loss, U-values and all that stuff

2. First a bit of Science First Law of Thermodynamics You can’t get more heat out of a closed system than you put in (100% efficiency is as good as it gets) Second Law Heat will only pass from a hotter body to a colder one (nothing you do can stop it) Third Law Heat can’t be transferred between things of equal temperature (pretty obvious really) Burnt hand

3. What is Heat Loss? The movement of heat from a warmer space to a colder space Heat is lost through all surfaces where there’s a temperature difference General formula for Heat loss is Sum [Uvalue x Area x temp. difference] or Q=ΣU.A.ΔT In an average house, heat will be lost through all external walls, windows, doors, ground floors and roof. If there are internal unheated areas or areas heated to a lower temperature, there may also be heat loss through internal walls, internal floors and internal ceilings.

4. What is a U-value? Each part of a building will have different thermal properties. The property that governs how easily heat is lost is the U-value (or heat transmission) The lower the U-value the less the heat loss. It’s expressed as the amount of heat energy lost through a square metre of the structure for each degree Kelvin (Celsius) U-value is expressed as Wm 2 K

5. What is a k-value? Whereas U-value relates to the heat loss from the space on one side of a structure to the space on the other side, K-value relates to the thermal property of a single material. It shows how easy it is for heat to pass through a one metre thickness of the material (conductive) K-values are usually used by manufacturers to indicate the thermal value of an particular insulating product and is expressed as W/mK. If a structure contains several different materials, you can’t add the K-values together to get the U- value of the whole structure you have to add the resistances and invert the answer. K-value

6. What is an R-value? A building structure usually contains several different materials and each may have different properties. The properties of a single material are expressed in terms of its resistance to heat loss (R), this is the opposite (inverse) of U-value. Resistance = thickness of material / K-value (R = L / K) Resistance is expressed as m 2 K/W The bigger the R-value the better the insulation. Surfaces between materials may also have a resistance to heat transfer but since they have no substance, they don’t have a K-value. U-value is the inverse of the sum of all the R-values Or U = 1 / Σ (R1+R2+R3 etc) R-value

7. Some typical materials- thermal properties Building Material KR=L/K Brick (110mm)0.840.14 Sandstone (100)1.500.07 Concrete block1.630.06 Thermal blocks0.190.53 Glass (4mm)1.050.00 Timber (20mm)0.140.14 Roof tiles (20mm)1.100.02 Plasterboard (12)0.160.08 Render (20)0.500.04 Insulation (100mm) KR=L/K Mineral wool0.0372.70 Cellulose fibre0.0352.86 Glass fibre0.0402.50 Polystyrene0.0283.57 Urethane foam0.0234.35 Phenolic foam0.0185.56 Spacetherm0.0137.69 Sempatap (10mm) 0.0631.58 Multi-Foil 5.25

8. R-values for surface resistances Ro=0.06 Ri=0.10 Rc=0.18 Rc=0.15 Ro=0.04 Rc=0.18 R=0.14 Ri=0.12

9. Height- 5m Floor size- 5m x 8m Calculating the heat loss House perimeter-(5+5+8) x 5 = 90m 2 Window area-90 x 15% = 13m 2 Wall area-(5+5+8) x 5 = 77m 2 Floor area-5 x 8 = 40m 2 Roof area-5 x 8 = 40m 2 Heat is also lost through ventilation 2. Measure the house volume Room volume5 x 8 x 5 = 200m 3 1. Measure the area of all external surfaces

10. The amount of heat lost is proportional to the temperature difference between inside and outside 3. Assume design temperatures Say 19 0 C internal with -1 0 C external So temperature difference is 20K

11. A Typical house heat loss calculation Area Δ T Uvalue heat loss 2006 1960s 1900s Wall- 77m2 20K 0.30 = 0.462kW (24%) [31%] [36%] Floor- 40m2 20K 0.22 = 0.176kW (9%) [11%] [13%] Roof- 40m2 20K 0.16 = 0.128kW (7%) [25%] [11%] Windows- 13m2 20K 1.80 = 0.468kW (24%) [21%] [22%] Volume- 200m3 20K 0.175*= 0.700kW (36%) [12%] [18%] Total heat loss = 1.934kW 6.45kW 8.91k * Assumes 0.5 air change/hr Room heating cost* £145 (49%) £484 (76%) £668 (82%) Hot water cost * £150 (51%) £150 (24%) £150 (18%) £295 £634 £818 *Assuming heating 1500hrs/year *Assuming gas at 5p/kWh

12. Temperature gradient through a solid wall Solid wall 19 external insulation 19 dew point @ 30% RH 4 10 2 15 18

13. Temperature gradient through a solid wall Solid wall 19 internal insulation 19 dew point @ 30% RH 4 10 2 15 18

15. Temperature gradient through a cavity wall 19 4 dew point @ 30% RH 8 3 16 18 Uninsulated cavity wallCavity wall insulation

17. Air Leakage Air leakage Standards @50pa pressure differential Best Practice = 3m 3 /hr/m 2 Passiv Haus = 0.6m 3 /hr/m 2 Building Regulations 2010 = 10m 3 /hr/m 2 Example- 170m 2 total surface area @ 10m 3 = 1700m 3 or 8.5 a.c./hr @ 3m 3 = 510m 3 or 2.5 a.c./hr @ 0.6m 3 = 102m 3 or 0.5 a.c./hr