Just Enough Science To Understand Your House In A Heatwave (Pt.2)

This is part of a three part article that could also be called “Just Enough Science To Understand Your House In A Heatwave

To put this article in context I suggest you read part one first. You can find it back on my articles Page.

How heat enters (and exits) your home

Think back to your high school science lessons for a few minutes. Heat enters and exits your home in different ways – conduction, radiation and convection. It’s a dense topic and easy to get lost in the weeds – but I’ll do my best to keep it short and sweet.

Conduction

Heat entering your building through “direct contact”

Heat will transfer from a warmer object to a cooler object by way of conduction.  So, when its hot outside and cool inside, the heat will transfer through your building envelope (walls, floor, roof). The stuff your house is made of – brick walls, clay roof tiles, timber floorboards, glass windows etc will resist the heat coming in. The ability for a material to resist the heat coming in and out is expressed as it’s r-value. (higher the number, the better it is). As an example, a single pane of 6mm glass has a pretty low r-value (R 0.16) whereas an insulated weatherboard is much better at resisting heat (r-value of 2.5) A double brick home sits somewhere in the middle (R 1.5).  When it comes to heat conduction thermal mass plays a big part in a material’s ability to retain heat (we’ll get into this soon).

Be aware while searching online about this – an R-value is common here in Australia, but a material’s U value, is more common in other parts of the world.

Convection

Heat will enter and exit your home through air currents. As we know warm air rises – so we lose heat out of the roof of our house. If your house is on timber posts, you can also gain heat as it rises from the floor below. 

Much more common is heat entering through the wall cavity. A double brick wall is made up of two layers of brick with a small gap in between them (about 3 to 5 cm). We call this gap a cavity. As the outer layer of bricks warm up, they will heat the air in this wall cavity and as the cavity heats, it will heat the inner layer of brick. So, when you feel the heat coming off the wall inside your bedroom at night, that is a combination of conduction heating your bricks and convection heating the gap of air in between. 

A double brick wall is made up of two layers of brick with a small gap in between them (about 3 to 5 cm). We call this gap a cavity. As the outer layer of bricks warm up, they will heat the air in this wall cavity and as the cavity heats, it will heat the inner layer of brick. So, when you feel the heat coming off the wall inside your bedroom at night, that is a combination of conduction heating your bricks and convection heating the gap of air in between.

Radiation

Heat transferring through the light rays

AKA like that warm, sunny spot on the floor that your cat sits on in winter

Heat comes into your building through the sun’s direct rays warming the interior of your home.  This is why the orientation of your home (often called solar orientation) and shading of nearby trees (and awnings) are so important in the design of your home.  Large openings to the east, north and west can allow a lot (read too much!) direct sun into your building. We won’t go into it much here – but trust me for now, solar orientation is a big deal in building design (well, it should be anyway)

*If you’re a sticklers for detail, heat also enters via infra-red radiation too – like a warm fire. 

Thermal Mass

How much heat a material will absorb

(remember, thermal just means relating to heat)

Materials that absorb a lot of heat (concrete, rammed earth, brick) have a high thermal mass. When you feel the warmth of a brick wall in a cool evening, this is because it has stored a lot of heat during the day and is still releasing it. The same goes for a concrete path that has been baking in the sun all day. Other materials (timber, steel, fabric) have low thermal mass – they absorb very little heat.

Using thermal mass to our advantage can be great in winter. A textbook passive solar design move is to have a north facing living room that gets lots of direct sunlight onto a concrete floor during winter. The sun warms the concrete floor, which releases heat into the room in the afternoon.

However, the same affect is at play in your double brick home during summer – and it can be uncomfortably hot. The hot afternoon sun beats down on your west facing brick wall and your bricks are sucking in all of that heat – which they release all through the night, due to thermal lag.

Thermal Lag

How fast a material absorbs and releases heat

AKA warm bricks in a cool evening

Some materials absorb and release heat quickly. Lightweight construction materials like timber and steel draw in heat quickly during the day, and release it quickly after it starts to cool outside.

It takes longer for other materials to do the same, concrete, rammed earth, brick. We say these materials have a high thermal lag. This is because on a really hot day it takes a while for the brick to really heat up (about 7 hours if you’re counting). This is good right – it means it takes a long time for the heat of the day to enter your home! True, but it becomes a problem when releasing that heat. In the evening as it cools outside, the brick wall (which due to thermal mass has absorbed a ton of heat) is very slow to release that heat. So ,as you’re going to sleep at night, the bricks are still releasing heat – yeah it’s not very comfortable. 

But what’s worse is that the morning after a really hot day your house hasn’t had the chance to ‘reset’ by getting rid of all the heat it gained the day before. When this happens over the course of a few days, like in a heatwave, the heat inside really starts to build up.

We can do our best to avoid this by flushing out all the built up heat overnight – through night purging - we’ll get to that in the next article.