How Much of a Difference Does the Thermal Mass of a Wall Make for Plants and Trees in Winter?

self-watering garden planter against brick wall thermal mass

Warm season vegetables and fruit trees are often grown close to brick, stone and concrete walls facing the midday or afternoon sun. This is because these surfaces act as a thermal mass, heating up during the day, and releasing the heat at night as the temperature drops, keeping the air around the plants a bit warmer.

This has the effect of extending the growing season of warm season vegetables such as tomatoes, eggplants, chillies, cucumbers, pumpkins, etc by keeping the plants warmer and protecting them from frosts.

self watering garden planters with mesh support against wall
Self watering garden planters growing on mesh support against wall

There are many other objects and structures that can be used as a thermal mass, such as water tanks and boulders. The question is, how much do they really warm up, and is it a significant amount?

Testing How Much a Brick Wall Thermal Mass Warms Up in Winter

The only way to learn new things is start experimenting! To test how much of a difference a thermal mass makes to plants and trees, I devised an experiment which was conducted at the start of winter.

I measured the temperatures of two brick walls every hour, from midday to 8pm. One wall faced the midday sun, the other was opposite to it in the shade. The ambient air temperature was also measured as a comparison.

When testing commenced at midday, the temperature was a rather cool 13.5⁰C (56.3⁰F).

Using an infrared laser thermometer, I measured the temperature of the sun-facing brick wall at approximately head height level, and was shocked that the temperature was 38.7⁰C (101.66⁰F), nearly three times warmer than the ambient air temperature!

infrared thermometer measuring wall temperature
Infrared thermometer measuring sun-facing wall temperature, measurement taken at 12:25 PM

How warm was the low brick wall in the shade, facing the opposite direction? Predictably, it was very close to the air temperature, at just 13.7⁰C (56.66⁰F). So, a wall in the shade will be at the same temperature as the surrounding air, while a wall facing the sun will be a lot warmer.

infrared thermometer measuring wall temperature
Infrared thermometer measuring shade-facing wall temperature, measurement taken at 12:26 PM
digital thermometer humidity meter measuring air temperature
Digital thermometer & humidity meter measuring air temperature, measurement taken at 12:27 PM

Having determined that sun-facing wall heat up really well, the next question was how well does the thermal mass retain heat.

To answer this question, I took a measurement every hour, for 8 hours, allowing enough hours for the sun to set and for it to turn completely dark outside. The results are tabulated below.

TimeAir temp (⁰C)sun-facing wall temp (⁰C)shaded wall temp (⁰C)
12:00pm13.5 38.713.7
1:00pm14.4 40.514.2
2:00pm14.1 30.815.3
3:00pm14.0 30.214.7
4:00pm11.2 26.314.5
5:00pm7.8 21.014.0
6:00pm6.5 16.913.0
7:00pm7.0 16.212.4
8:00pm5.7 13.712.4
Table of hourly temperature measurements of a sun-facing wall, a shaded wall, and outside air.

Graphing the results of the experiment, we can clearly see how the thermal mass heats up well, peaking at 40.5⁰C (104.9⁰F) at 1:00pm, then slowly cooling down over several hours.

By 8:00pm, the sun-facing wall was still warmer than the shaded wall by 1.3 degrees, and both walls were considerably warmer than the outside air temperature, which had plummeted to 5.7⁰C (42.26⁰F) by nightfall.

This shows that even the shaded wall equalised with the highest air temperature reached during the day, and managed to maintain a fairly constant temperature throughout the day over, the duration of the experiment.

Graph showing heat accumulation and retention of brick wall thermal mass in sun and shade
Graph showing heat accumulation and retention of brick wall thermal mass in sun and shade

Even though this experiment was not exhaustive (we didn’t take measurements into early morning), we can conclude that a brick wall will attain a temperature considerably warmer than the surrounding air during winter, and maintain a temperature of a few degrees warmer, for a few hours after sunset.

How Much Heat Do Different Materials Retain?

Specific heat is defined by the amount of heat needed to raise the temperature of 1 gram of a substance 1 degree Celsius (°C).

The higher the specific heat capacity of a material, the more energy it requires to heat it up an additional 1 degree Celsius, and therefore, the more energy/heat it can hold.

Which substance has highest specific heat capacity? It’s actually water!

It takes more energy to increase the temperature of water compared to any other substance, therefore water can hold the most heat. A large repository of water, such as a water tank, pond, lake or dam can store a lot of useful heat!

How do different materials compare? I’ve included a table below to show the differences in specific heat capacity.

Specific heat capacity table of common materials, showing the amount of energy (measured in joules) required to raise the temperature of 1kg of the material by 1 degree Celsius (°C).

MaterialSpecific Heat Capacity J/kg.°C
Water 4187
Oak Wood 2380
Soil 1810
Asphalt 915
Aluminium 887
Concrete 879
Clay 878
Brick 841
Glass 792
Sand 780
Cast Iron 554
Iron 462
Table showing energy required to raise temperature of 1kg of various common materials by 1 degree Celsius (°C)

The table shows us that 1kg of water takes 4,187 joules of energy to raise its temperature by 1 degree Celsius (°C) , which is almost five times higher than brick, which requires only 841 joules.

If water can hold almost fives times more heat than bricks, it would have been interesting to see the results of the experiment if a water tanks was used instead of a brick wall.

How can we use this information? By knowing how various materials respond to heat, we can use them to our benefit when designing a more sustainable garden, which can sustain cold-sensitive plants through the cooler seasons, and become productive earlier in the warmer seasons.

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