The hidden benefits of insulation

Improving the insulation in our homes is a well-understood good.  However, the benefits are even more than most people realise.

2020-09-06  [updated 2020-09-13]

by Richard Keech

Thermal comfort

Before we can understand the hidden benefits of insulation, we need to know a bit more about indoor thermal comfort.  To most people, being thermally comfortable at home is about having the right air temperature.  However, what makes us comfortable inside is actually a complicated function of:

  • the space we’re in:
    • air temperature;
    • mean radiant temperature;
    • humidity;
    • air velocity; and
  • the people in the space:
    • our metabolic rate; and
    • what we’re wearing.

It’s the role of mean radiant temperature that is under-appreciated, and which I want to mostly focus on.

Surface temperature

It’s been shown that the average temperature of all the surfaces around us (mean radiant temperature) plays a huge role in our thermal comfort because of the heat radiation emitted from the surfaces.  For a given level of thermal comfort, you can trade off air temperature and thermal comfort about one-to-one.  In other words, lowering air temperature by one degree, can be offset by raising the mean radiant temperature by about one degree (and vice versa).

When most people think of radiators, they think of relatively small, hot things.  But a large area at a lower temperature can have the same impact, so long as the temperature is above our preferred temperature set point.  So slightly warm walls, floor and ceilings act as subtle radiators.  Likewise, in winter, a poorly insulated ceiling, wall, window or floor will have a temperature below our preferred set point.  So they act to absorb thermal radiation and make you feel colder.  Think of them as negative radiators – they take rather than give radiant heat.

Insulation and radiant heat

This is where good insulation comes in.  When our homes are poorly insulated, the inside surface temperatures are further from the preferred inside temperature.  In a winter example, the more poorly insulated our home, even if heated, the more all those surfaces will be closer to the outside air temperature, i.e. the mean radiant temperature will be too low.  To compensate we need our heater set to a higher temperature.

Now imagine we insulate the same house (walls, ceiling, floor), and improve the glazing.  Suddenly all those surfaces are warmer, so the thermal radiation from them is more.  So, the air temperature doesn’t need to be as high for the same level of comfort.

Insulation vs hydronic heating

Hydronic heating systems get a lot of love because the radiant heat gives a lovely warmth.  However, a hydronic heating system in a home with a poor thermal envelope is a brute-force way of delivering decent mean radiant temperature despite the house’s poor fundamentals.   If we instead focus on getting an excellent thermal envelope, we can also get a great radiant heat benefit, but it’s completely passive.

In homes with great thermal envelopes, hydronic heating systems become a massive overkill.  A good example of this is in this article.  The author fitted hydronic in a building renovation with very high thermal performance and now finds that the hydronic system is completely unnecessary.

The other point is that hydronic systems are used for heating only, whereas the radiant benefit from insulation is in both summer and winter.

Insulation’s first and second benefits

When most people think about the obvious benefit of insulation, it’s about reducing the amount of thermal energy required to deliver a given air temperature.  However, the radiant benefit we’ve described here means there insulation benefit goes even further.  In winter, homes with great thermal envelopes will be comfortable with lower air temperature.  So this means extra savings on energy, and also heating systems that aren’t working as hard.  Likewise in summer, good homes can be comfortable at a higher air temperature.

Relaxed set point.  This characteristic operation of great thermal envelopes, where we can set our heating/cooling systems such that they don’t have to work as hard, is sometimes called having a relaxed set point.

Insulation’s third benefit

Resilience. Another consequence of this passive radiative benefit of good insulation is that homes cope better in extreme weather, regardless of your heating or cooling system.  In a heat-wave scenario it might mean staying comfortable where otherwise the cooling system might not be able to cope.  Another scenario might be a heatwave blackout scenario.  A house with a good thermal envelope will retain its comfort long after unimproved homes have become thermally unsafe to be in.

Insulation’s fourth benefit

As discussed, with a well-insulated thermal envelope, the inside wall surface temperatures are all much closer to the inside air temperature than would be the case (all other things being equal) in a home with poor insulation.  This smaller range of temperatures lessens the tendency of air to become stratified – hot up top and cold down below.   Stratified air is problematic, and less tendency to stratification means that conditioned air can mix more easily, leading to more comfort and less energy use.

Thermal stratification: notice the distinct horizontal line dividing cold air down low, and hot air up high.

Insulation’s bonus benefit

Over and above thermal benefits – well-insulated homes are better at stopping outside noise.  So quieter homes is a bonus extra.

Conclusion

When we insulate our homes properly, all those inside surfaces of the thermal envelope stay warmer act as subtle radiators in winter.  Likewise, in summer they stay cooler and give a similar benefit from avoided heat radiation.

The key take-away message is that getting insulation right is critical to homes that are cheap to run and comfortable to live in.

Lastly, having great thermal envelopes means that we can get the real benefits of hydronic heating at much lower cost.

Further reading

I go more into this subject in my article on insulation in Renew magazine, edition 127.

Jenny Edwards also talks about this here.