SCHOOL OF MECHANICAL AEROSPACE AND
CIVIL ENGINEERING
BUILDING AND ENVIRONMENT
Tutorial 1 Answers: The Big Plan
The objectives of the tutorial are
- to give you practice in using and manipulating the U value calculation
procedure
- to give you practice in calculating energy consumption and carbon emissions
- identify missing information and deal with the situation
To be able to fully answer this question, you need to
- Calculate the total wall and ceiling surface areas of the bungalow, allowing for the
windows
- Determine the thermal resistance of the multifoil layer
- Calculate the total heat losses from the bungalow with and without the multifoil
layer and compare the results
- Upgrade the existing windows, recalculate the results and compare to the original
Firstly, calculate the total wall surface area.
Total area of external envelope = (8+8+12+12)*2.8 = 112m2
The surface area of wall = Total area of external envelope – area of doors and windows
= 112 – 24
= 88m2
The area of the ceiling is 12*8 = 96m2
No mention is made of the floor in the brief. However, we know that heat loss through
the floor is important, so we must include the floor in the calculation.
The area of the floor is also 12*8 = 96 m2
Now calculate the U values of walls ceiling and floor for the basic building.
Walls
Thermal Thermal
Resistivity Thickness
Layer (m.K/W) (m) Resistance (m2.K/W)
Internal Surface 0.12
Plasterboard 6.3 0.015 0.0945
Concrete block 1.8 0.2 0.36
Foam insulation 45 0.025 1.125
Ventilated cavity 0.18
Brick 1.5 0.11 0.165
External surface 0.06
Total resistance 2.105 (m2.K/W)
U Value 0.475 (W.m2.K)
Ceiling
Thermal
Resistivity Thickness Thermal Resistance
Layer (m.K/W) (m) (m2.K/W)
Internal Surface 0.12
Plasterboard 6.3 0.015 0.0945
Mineral wool insulation 29 0.15 4.35
External surface 0.06
Total resistance 4.625 m2.K/W
U Value 0.216 W.m2.K
For the floor, I am assuming a U value of 0.5W/m2K. This would be a
reasonable value for a house of this type. You may care to test some
designs of your own.
Total thermal transmittance = transmittance through walls
+ transmittance through ceiling
+ transmittance through floor
+ transmittance through doors and windows.
= 88*0.475 + 96*.216 + 96*0.5 + 24*1.5
= 41.8 + 20.74 + 48 + 36
= 146.54W/K
The tutorial sheet does not mention air infiltration – let’s not go there on this occasion,
we are only concerned with fabric heat losses.
Now, add in the multifoil layer to walls and ceiling. The multifoil is the equivalent of
0.15m mineral wool insulation. From the ceiling calculation above, it can be seen that the
thermal resistance of 150mm of mineral wool is 4.35 m2.K/W. All we have to do is add
4.35 to the total thermal resistances for the basic walls and ceiling, giving 6.46 m2K/W
and 8.98 m2K/W respectively and U values of 0.158 W/m2K and 0.111 W/m2K.
The overall fabric transmittance is now 88*0.158 + 96*.111 + 96*0.5 + 24*1.5
= 13.90 + 10.66 + 48 + 36
= 108.56 W/K
The daily primary energy saving may now be calculated. We are not told the length of
time for which the central heating is switched on. In the Edwards Bungalow, this time is
usually 8 hours. During the bad weather last year, this was closer to 16 hours. For this
calculation, we will stick with 8 hours. Don’t forget that one Watt is one Joule per
second. There are 3600 seconds in an hour.
Total delivered energy saving is equal to
consumption of basic house – consumption with multifoil
= (146.54 – 108.56)*(20-0)*3600*8
= 21.88MJ per day
= 6.07kWhr per day
Of course, savings would be bigger if the central heating was left on longer.
The question specifies a boiler efficiency of 0.6.
From this, the daily primary energy saving is
6.07/ 0.6 = 10.12kWhr per day
From the notes, the carbon factor for natural gas is 0.198 kg per kWhr.
This means that the total carbon emission saving per day would be 6.07 * 0.198
= 1.20 kg per day.
If the windows and doors are upgraded to 0.5 W/m2K, then the primary energy saving
over the basic building would be
24*(1.5-0.5)*(20-0)*3600*8 / 0.6
= 23.03 MJ per day.
= 6.40kWhr per day.
From this, the total carbon emission saving per day would be
6.40*0.198 = 1.27kg per day.
The saving is not as big, but the two options are very close. However, there are issues of
disruption to consider.
POSTSCRIPT
The Big Plan never happened. Mrs Edwards was unhappy with the prospect of the house
being taken apart from the inside.
I ended up spending my money on replacement windows built to current regulatory
standards. It looks nice. Energy consumption has decreased by around 12%. In addition
to the decrease in overall U value, the new windows have well fitting, properly sealed
opening elements and this has reduced air infiltration.
The old boiler died on 13 March 2013. A new high efficiency combination boiler will be
installed on 8 April 2013. Another £2000 to wave goodbye to…….
My faith in multifoil insulation has been somewhat undermined.
http://www.buildersmerchantsnews.co.uk/news/archivestory.php/aid/7039/
Multifoil_trade_body_acts_on_insulation_standards.html
basically tells us that there is still a major disagreement as to how to reliably determine
the U values of multifoil insulation products – and this is close to a decade on since these
products first appeared.
