Building Science UValues Surface Resistance The thermal resistance
Building Science U-Values
Surface Resistance • The thermal resistance of an open surface depends upon the conduction, convection and radiation at that surface. The air in contact with a surface forms a stationary layer which opposes the flow of heat. • Direction of Heat Flow - Upward/Downward • Climatic Effects - Sheltered/Exposed • Surface Properties - High/Low Emissivity
Standard Thermal Resistances Resistance Type Construction Element Heat Flow Surface Emissivity Standard Resistances (m 2 k/W) Inside Surfaces Walls Horizontal High Low 0. 12 0. 30 Roofs/Ceilings/Floors Upward High Low 0. 10 0. 22 Ceilings/Floors Downward High Low 0. 15 0. 56 Walls Horizontal High Low 0. 06 0. 07 Roofs Upward High Low 0. 05 Unventilated 5 mm Horizontal or Upward High Low 0. 11 0. 18 Unventilated 20 mm or greater Horizontal or Upward High Low 0. 18 0. 35 Outside Surfaces Airspaces (including boundary surfaces) Ventilated loft space with flat ceiling, unsealed tiled pitched roof 0. 11
Total Thermal Resistance • The total resistance is the sum of thermal resistances of all the components in a structural element • • Inside Surface Plaster Brick Outside Surface Rsi R 1 R 2 Rso
Total Thermal Resistance • RT = Rsi + R 1 + R 2 + Rso so • RT = ∑R where ∑R is the total of thermal resistances of all components occurring in a section through the element being calculated
U-Value Worked Example 1 Calculate the U-Value of a cavity wall with a 103 mm thick brick outer leaf, 50 mm of clear cavity, 40 mm of insulation board, then a 115 mm high performance aerated concrete block inner leaf with a 15 mm layer of lightweight plaster. Values of thermal conductivity in W/m K are: lightweight plaster 0. 18, aerated concrete blockwork 0. 11, insulation board 0. 025, brickwork 0. 77. Standard thermal resistances in m 2 K/W are: internal surface 0. 12, external surface 0. 06, cavity 0. 18
U-Value Worked Example 1 Diagram Outside surface 103 mm brick 50 mm cavity 40 mm insulation board 115 mm concrete block 15 mm plaster Inside surface
U-Value Calculation and Answer Layer Thickness (m) Thermal Conductivity (W/m K) Resistance (m 2 K/W) Internal Surface N/A Standard = 0. 120 Lightweight Plaster 0. 015 0. 18 0. 015/0. 18 = 0. 083 Aerated Concrete Block 0. 115/0. 11 = 1. 045 Polyurethane Insulation Board 0. 040 0. 025 0. 040/0. 025 = 1. 600 Clear Cavity 0. 050 N/A Standard = 0. 180 Exposed Brickwork 0. 103 0. 077 0. 103/0. 077 = 0. 134 External Surface N/A Standard = 0. 060 Total Resistance RT = 3. 222 U-Value Formula: U = 1 = 0. 31 W/m 2 K RT 3. 222
U-Value Typical Construction Elements Element Composition U Value (W/m 2 K) Solid Wall Brickwork 215 mm, Plaster 15 mm 2. 3 Cavity Wall Brickwork 103 mm, Clear Cavity 50 mm, Brickwork 103 mm 1. 6 Cavity Wall Brickwork 103 mm, Clear Cavity 50 mm, Lightweight Concrete Block 100 mm, Lightweight Plaster 13 mm 0. 58 Cavity Wall Brickwork 103 mm, Insulation In Cavity 50 mm Lightweight Concrete Block 100 mm, Lightweight Plaster 13 mm 0. 48 Cavity Wall Brickwork 103 mm, Clear Cavity 50 mm, Aerated Concrete Block 115 mm Insulation Board 55 mm less than 0. 30
U-Value Typical Construction Elements Element Composition U Value (W/m 2 K) Timber Frame Wall Brickwork 103 mm, Clear Cavity 50 mm, OSB or Sheathing Ply 9 mm, Timber Frame filled with Insulation 120 mm, Plasterboard 13 mm Less than 0. 30 Pitched Roof Tiles on Battens and Felt Ventilated Loft Airspace Mineral Wool across Joists 100 mm Mineral Wool between Joists 100 mm Plasterboard 13 mm 0. 15 Window Single Glazing, Metal Frames 5. 7 Window Single Glazing, Wood or PVC Frames 4. 8 Window Double Glazing, Wood or PVC Frames, Low Emissivity, 6 mm Gap 2. 7 Window Triple Glazing, Wood or PVC Frames, Low Emissivity, 12 mm Gap 1. 6 Door Solid Wooden 3. 0
U-Value Standards • The Technical Handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004 and are available in two volumes: Domestic Buildings and Non-Domestic Buildings Domestic Handbook 2013 • Section 6. 2. 1 - Maximum U-Values
U-Value Adjustment • It is sometimes necessary to calculate the effect that additional insulating materials has on a U -Value, or to calculate thickness of material that is required to produce a specified U-Value. • U-Values cannot be added together or subtracted from one another • Thermal Resistances, however can be added and subtracted. The resistances making up a particular U-Value can then be adjusted to produce the new U-Value
U-Value Adjustment Worked Example A certain uninsulated cavity wall has a U-Value of 0. 91 W/m 2 K. If insulation board is added to the construction, what minimum thickness of this board is needed to reduce the U-Value to 0. 35 W/m 2 K? Given that thermal conductivity of the insulation board is 0. 025 W/m K.
U-Value Adjustment Answer Step Calculation Target U Value U 2 = 0. 35 Calculate target total resistance (using R=1/U) R 2 = 1/0. 35 = 2. 857 Existing U Value U 1 = 0. 91 Calculate existing total resistance (using R=1/U) R 1 = 1/0. 91 = 1. 099 Extra resistance required is R 2 - R 1 Re = 2. 857 -1. 099 = 1. 758 k Value of proposed insulating material is 0. 025 R = d/k Use formula involving thickness of material d d=Rxk Calculate thickness of material d = 1. 758 x 0. 025 = 0. 044 metres = 44 mm So the minimum thickness of insulating board needed to give 0. 35 U-Value is 44 mm (A suitable manufactured size greater than 44 mm would be chosen)
Thermal Bridging • A thermal bridge is a portion of a structure with higher thermal conductivity that increases heat flow and lowers the overall thermal insulation of the structure. • Can be minimised by correct design and installation of thermal insulation
Thermal Bridging Examples • junctions of walls with roofs • junctions of walls with floors • mortar joints around high performance concrete wall blocks • timber framing between sections of wall insulation • timber joists and beams between sections of roof and floor insulation • steel lintels above windows and doors • window frames and sills • external door frames and sills
Temperature Gradient • The ratio of the temperature changes inside a structure is proportional to the ratio of thermal resistances ∆θ = R θT RT ∆θ R θT RT = = temperature difference across a particular layer resistance of that layer total temperature difference across the structure total resistance of the structure
Temperature Gradient Example
Next Week Recap of: • U-Values Introducing: • Fabric energy loss from Buildings • Ventilation heat loss from Buildings
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