How to reduce your energy bills
The truth about thermal insulation
There are different approaches, but not all are scientificly proven. Also 'cost' is a significant factor.
For example; You could spend out on replacement windows. Having new 'triple glazing' with heat reflecting glass. The advertising firms present the advantages that it will 'save on your energy bills'.
Unless your windows need replacing for other reasons it isn't a cost efficient energy saver. Heat loss through glass is significant (5.6W/m²K). Using 2 sheets of glass with an air space between them reduces the passage heat to 2.8W/m²K.
Replace the air with Argon or Krypton gas and apply an extremely thin layer of metal (it is so thin you can see through it). The heat transfer can be reduced to about 1W/m²K.
Triple glazed sealed units with Argon or Krypton gas
A metalic coating on 1 of the sheets. Claimed U Value: 0.74 - 0.80W/m²K.
The edge of the glazed unit has 'spacers' holding the glass apart. To seal the gas in the unit mastic is used behind the spacers. The improved spacers are hollow rectangular extruded plastic tubes containing desiccant crystals.
The perimeter of the glass sealed unit enables thermal conduction to take place. Therefore, the window has 2 thermal values. The claimed 0.74W/m²K in the centre of the unit. Plus an area with a significantly less efficient thermal passage. The frame of the window will also have a less thermally efficient thermal passage.
Technically, the figure are based on 1m² of glass unit. The ratio of edge to window proportionally increases as the window area decreases.
The technical bit:
U Values are based on an area of 1m². The units of measurement are Watts (W) per metre square (m²) per Kelvin (K).
A Kelvin is the same temperature difference as Celsius. It is just a different starting point.
The number is how many Watts (energy) passing through 1m² surface area per difference in air temperature between the two surfaces.
A Watt is a measurement typically used for energy over a given time period. Energy is measured in Joules and time in seconds. [1J/sec = 1 Watt].
A comparison: A pair of French doors in a lounge. They are 1.60m wide and 2.0m tall (3.20m²). The glazed area is 1.92m².
A cold day outside is 5°C and the lounge is 22°C. A difference of 17°C.
If the glazing was:
- single glazed at 5.6W/m²K x 1.92m² = 11W x 17 = 187W
- double glazed at 2.8W/m²K x 1.92m² = 5 x 17 = 85W
- triple glazed* at 0.8W/m²K x 1.92m² = 2 x 17 = 34W
1,000W = 1kW. Energy is often quoted as kWh. That is 1,000W over 3,600seconds (1 hour).
A unit (kWh) of gas is currently capped at 5.93p. In contrast to electricity at 27.69p (March 2026)
As the figures above indicate, it will take a very long period of time to even 'break even' with having double glazing changed over to triple glazing with coated glass. To get a more realistic figure of energy saving, the outside temperature needs to be monitored as it is unlikely to remain constant. During the day, if a cold, but clear sky and a south to south west facing window there could be some solar gain.
Other factors include the time period the heating is on for. If you are out at work all day and only have the heating on in the evening and a short period before you leave your home the heating period may be 7hrs. (2 in the morning and 5 in the evening).
Comparing:
Double glazing 85W
with
triple glazing 34W (with coated glass).
A Watt is energy per second (1J/sec = 1W). There are 3,600 seconds in an hour.
Double glazing
85 x 3,600 = 306,000W per hour x 7 hours = 2,142,000W. Divide by 3,600 = 595 and by 1,000 = 0.59kWh
If gas is the energy provider 0.59kWh x £0.0593p = 4p. Compared to electricity 0.59kWh x £0.2769 = 16p
Triple glazed:
34 x 3,600 = 122,400W per hour x 7 hours = 856.800W. Divide by 3,600 = 238 and by 1,000 = 0.24kWh
If gas is the energy provider 0.24kWh x £0.0593p = 1p. Compared to electricity 0.24kWh x £0.2769 = 7p
Cost difference between double glazed and triple glazed per day when the weather is an average of 5°C outside and the home will only be occupied for 7 hours when the heating is on. Gas: 3p and electricity 9p. That is just for the French Door glazing. The frame will be letting more energy escape, however, if PVCu it will be very similar.
The figure look very small, but they are purely the energy being used and escaping through the glass of the door. There are much bigger heat issues - read on.
The calculations can become very involved and a Spreadsheet makes life a lot easier. If you want to carry out an approximate guide figure for comparison between double and triple glazing:
- Measure the width and then height of every window. To the nearest 10mm is fine as whatever people claim ALL figures are only APPROXIMATE as there are so many variances.
Then, using the same system as above you should be able to see a rough guide per day of the difference.
Heavyweight thermal curtains can be more effective than changing from double to triple glazing.
Pulling the curtains closed will significantly reduce heat loss from a room. Putting the bottom of the curtains on the window board so all the radiator is showing will enable A LOT MORE HEAT TO ENTER THE ROOM. It may not look too pretty but it makes a lot of difference to your heating bill and physical comfort.
Is retro cavity wall insulation a good investment?
It all depends upon location. Companies are supposed to carry out a preliminary investigation as to the suitability of cavity fill. If the location is exposed and subjected to high rainfall and strong winds then cavity fill may not be a good idea. Wind driven rain can pass through a single skin of brickwork. The cavity then lets the rain drop down and hopefully out of weep holes at the base of the wall. Filling the cavity takes away the gap so gravity cannot pull the rain down.
Blown Fibre
If the retro thermal insulation is blown fibre it will become saturated eventually. It cannot dry out so will soak through to the inner leaf. Now the inner leaf will contain more moisture so will be a lot more expensive to heat PLUS will physically feel colder.
Expanded Polystyrene Beads
Version 1. Expanded polystyrene beads (loose fill). Thermal quality is very good. The beads have small spaces between them so that gravity can still pull any rain that has passed through the outer skin down and out the weep holes. There are disadvantages: possible compaction over time. The result can be an empty gap at ceiling level of the wall. Consequence is a cold zone and possible condensation / mould growing.
The second, and quite amusing (sorry but watching it was funny). Window fitters had just taken the old window frames out and out poured all the polystyrene beads. The house had to be re-insulated after they had finished.
Version 2. Expanded polystyrene beads with adhesive. The bead are coated with an adhesive as they are fired into the cavity. The theory is, as the adhesive sets the beads cannot then compact. If any windows or doors are removed and replaced the insulation should remain in place. Disadvantages: If too much adhesive is used the beads clump together and leave pockets of empty cavity creating cold spots. The second disadvantage is if too much adhesive is pumped in it can produce a slab effect that prevents moisture (vapour) from leaving.
Injected Expanding Foam
Advertised as being the 'best' thermal insulation for use as retro cavity fill, loft insulation and in some 'Country Style' magazines advertised as a remedy for loose slaates and tiles. [The advertising target readers are more likely to own large old property in the countryside. To replace a very large roof covering when a few tiles or slates have failed appears to be a good option. The sprayed on urea formaldehyde foam sticks to virtually everything. It is an excellent thermal insulator. It is also a known carcinogen and banned in several countries on health grounds. Some schools had the cavity walls pumped with the foamed plastic. Then, they had to close the schools, in some cases for weeks, as the fumes escaping were making the occupants feel sick.
There are still firms offering to retro cavity fill domestic properties. Setting aside the 'known carcinogens' issue. It really only applies to people using the gas commercially or industrially. Formaldehyde gas is used to fumigate rooms that have toxic or harmful gases in there such as laboratories and fume cabinets. The gas will KILL everything it is in contact with.
Formaldahyde is also used as an adhesive agent bonding polyester to cotton. Your bed sheets, shirts and so on probably have formaldehyde in the fabric - that 'new smell'. That isn't the problem though. Sorry, but some people are particularly reactive to the gas even in small amounts found in fabrics.
THE PROBLEM - retro filling cavity walls with foamed plastic is like putting your home in a large polythene bag. Every home; Buckingham Palace to a bedsit has humid air in it naturally. Humidity is the gas state of H2O, yes water and condensation. If it is in a room which is warmer than outside, or another room, the vapour (humidity) will be pushed through the wall. (There is another blog explaining all about that).
The injected formaldehyde foam is gas proof. It has to be to contain the formaldehyde gas that has formed all the bubbles. The Canadian Government has carried out tests and concludes that the foam leaches out the gas over time and therefore the thermal qualities reduce.
What does that mean?
Having injected foam retro cavity wall insulation will create a barrier stopping humidity leaving a property. The walls will continue to absorb the humidity and will become damp. Damp walls require a lot of energy just to heat the water content. (Wet tee shirt V dry tee shirt syndrome). Your heating bill will RISE. Humidity will condense on all the cold surfaces and that is when mould will set up home. There are now firms offering to remove the foamed plastic from cavity walls. It is very costly though and not ideal.
Loft / under roof foam sprayed insulation
In theory it is the perfect answer. Farmers have been using it to thermally insulate barns to help store food during hot or cold weather. But that isn't buildings with roof tiles on battens and timber rafters etc. Spraying plastic foam under the roof tiles may help 'stick' broken and loose tiles in place. However, the air cannot let the roof timbers dry out. The rafters, purlins, tile battens etc. all need ventilation to let the humidity out. If it cannot escape the timbers will rot. A detached 1970s built chalet had the foam sprayed onto the underside of the roof members. The rooms in the roof had timber studwork walls. The small 'crawl' space roof eaves to room wall was sprayed with the foam using a lance. Physically the space was too small to enter and use quilt insulation.
The outcome: several years later black mould became an issue. The building surveyor monitored the problem, but it needed a builder carpenter to break the plasterboard for an 'intrusive survey'. Wow!!!!! Black mould had set up home behind the plasterboard and was feeding well on the timber studs. More plasterboard removed on the ceiling (the underside of the rafters). Foam had been used to fill the voids between the rafters and the plasterboard ceiling. The tile battens were saturated and wet rot had set up home. Further intrusion found that some of the rafters were also rotten. The end result: The whole upstairs plus the roof were taken apart. The rooted timbers replaced. Breather sarking with new tile battens and completely re-tiled using the original roof tiles.
The cost? I don't know. The builder hadn't finished when we spoke. He said that he had replaced several rafters and several of the timber studs of the room walls. Basically it was a new upstairs and roof covering. The previous owner of the chalet had had the foam sprayed 'thermal insulation' so there were no come back on the installation firm.
For more useful information about house maintenance issues including security, thermal insulation, and simple DIY jobs this book will help.
Buy it at the keenest price of:
£18,99 including Free postage
direct from the Publishers.
