Tuesday, 11 August 2015

Insulating your home makes you warmer - how much?

If you insulate your home you will find yourself warmer than before even if you don't change your heating controls at all. This can be because the heating system was previously incapable of supplying enough heat to reach the temperature you asked for, but even without that effect it happens because your home will take longer to cool down when the heating goes off than it did before. This is part of an 'automatic rebound effect' described by the book I reviewed in my last post 'The Rebound Effect in Home Heating' by Ray Galvin. It doesn't add much to your comfort but it is possible to take advantage to save some extra from your heating bills. First of all, how much difference does this really make?

I have explored this as an exercise while learning to use a building energy modelling package called Energyplus. I modelled a typical semi-detached home with solid walls, a solid floor and a well insulated loft. I gave it three heating zones - a living area zone comprising 2/3 of the downstairs, a bedroom zone comprising the corresponding area upstairs, and a side zone which contains the rest of both floors, with external doors, the staircase and some small rooms. The thermostat for the living areas is set to 21°C and the other areas are heated to 18°C. The side zone is next to the exposed side wall - the living area is next to the shared wall. I compared heating energy and temperatures in these zones with and without insulating the walls.

My model has an ideal heating system that is fully capable of providing the necessary heat but I found that insulating the home increases the average temperature in all three zones. The side zone is the most affected because it is next to a large area of exposed wall: during January the average temperature was 1.1°C warmer with insulation.

Table showing the increase in average January temperature due to adding solid wall insulation to the dwelling
January temperature increase
Living area
 - 2/3 of downstairs, heated to 21°C
Bedroom area
 - 2/3 of upstairs, heated to 18°C
Side zone
 - Two floors high against the exposed wall, including staircase, heated to 18°C

The temperature during heating periods was unaffected by the insulation; it is the temperature when the heating is off that has increased. The chart below shows the temperature variation through the day in the side zone, with a constant 5°C outside.

Temperature in the side zone of the house, on a day when it is a constant 5°C outside.

If you always have the heating on when you are home and awake, then this increase does not add to your comfort at all, though you will notice the difference if you have to get up in the night. In this case the side zone drops down to 12°C overnight before insulating, but never drops below 15 °C afterwards.

If you take a little trouble you can save on your energy bills by adjusting your heating timer. Since it takes a while longer for your home to cool down you can turn it off before you go to bed rather than after. Also, since there is less heating up to do in the morning you may be able to set it to turn on a little later.

In a previous post I discussed potential energy savings from not heating parts of the home that you don't use (see Energy Savings by not heating the spare room). I extended my model to investigate this too, by splitting the bedroom area into two parts and not heating the smaller part. Also there was no activity in the unheated area. The table below shows the difference in average January temperature between when the room was heated and when it was not. Adding insulation reduces the difference in temperature by about 30% in either case. It is also interesting that the solar gain on the south side makes a considerable difference.

Table showing the difference in temperature (average for January) between when the bedroom is heated and when it is not, with the bedroom on either the north or south side of the house.
North sideSouth side

These figures are indicative only: my analyses are based on a model, not a real house, and the exact values depend on the layout, insulation levels and draughtiness in the house as well as the heating regime and other activities going on in it. In case you are interested, here are some more details on my model:
  • I used climate data for Birmingham (UK).
  • The house was oriented with the front to the south. If you rotate it sideways so the exposed walls on the pair of homes are north and south you get quite a bit of difference between them.
  • The U-values of the uninsulated walls were 2.1. With insulation the U-value was 0.3.
  • I used heating times of 7-9am and 5-11pm on weekdays, 7am-11pm on weekends.
  • I assumed 2 adults and 1 child living the home. Overall gains from lighting and other activities were similar to the assumptions in RdSAP, the model behind your home's Energy Performance Certificate. I split them up between the zones and through the day in a plausible fashion, with less activity overnight and more in the evening, also more in the living areas. The balance between heat losses and gains is critical for the levelling off temperature when the heating is off.
  • For ventilation, I assumed 0.8 air changes per hour both with and without insulation. This is fairly cosy.
  • I put in some rather arbitrary assumptions about internal thermal mass. The internal mass is important for how quickly the house cools down, though it does not affect the temperature to which it would level off. Also I don't think this model has any air mixing between zones, which is approximately true if you keep the internal doors closed. The model assumes that the temperatures are uniform through each zone. 


  1. Good article. I call this 'passive rebound' (as opposed to 'active rebound', which is the fictional account that householders deliberately raise temperatures after a retrofit). Would be good to compare your model estimates against measured temperature increases/differences - a future article, perhaps.

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