During my research on green roofs, I found a lot of information about the potential insulating properties that a green roof can have. I was amazed at how hard it was to get a proper understanding of how and when a green roof can have an insulating effect, I set out to write down my findings in an accessible and understanding way. Here is what I found:

Green roofs can provide some insulating effect. The extent of the effect is dependent on the type of growing medium that is used, the thickness and moisture content of the sediment layer, and whether or not there is a dedicated insulation layer in the roof.

Although there are many reasons to install a green roof, there are lots of factors to consider before deciding whether the insulating benefits should be one of those reasons for your specific case, or if it is a reason to decide against installing a green roof. Let’s take a better look at these factors so you will be able to make your own assessment

Thicker green roofs provide more insulation

The first factor to consider is the thickness of the sediment layer. Thicker green roofs will provide more insulation that thinner green roofs. When trying to insulate something, you are trying to make it harder for heat energy to travel though your imposed boundary.

When the boundary you have installed, like for example a sediment layer of a green roof, is thicker, the heat energy has to travel through more obstacle before reaching the other side, which means it is a better insulator.

One factor that largely influences how thick the sediment layer of a green roof is going to be is the type of green roof. There three types of green roof, being extensive, semi-intensive and intensive. The thickness of the green roof is thinnest for the extensive type (6 to 20 cm), thicker for the semi-intensive type (12-25 cm) and thickest for the intensive type (usually in the range of 15-40 cm, but sometimes even up to 100cm).

On a home, the use of extensive green roofs is most common. So you will probably be looking at about 20 cm of sediment at most that will be insulating your home.

Lower density sediment provides more insulation

Another factor that influences the insulating properties of a green roof is the density of the sediment layer. The lower the density of the sediment, the better it is at insulating. This is because a lower density usually indicates a higher percentage of the sediment being air, which is a good thing for insulating.

The reason that a larger proportion of air is a good thing for insulation is because air is a really weak thermal conductor. In the sediment, air gets trapped in tiny pockets which provide insulation. The more air there is in the sediment, the more of these pockets are formed and the better the insulating properties of that particular type of sediment.

A dryer green roof insulates better than a wet green roof

Yet another factor that can have an effect on how much insulation a green roof can provide is its moisture content. When a green roof has a high moisture content (i.e. when it is wet), the insulating air pockets from the previous paragraph get filled with water instead of air.

Unlike air, water is actually a pretty good thermal conductor, which is not what we want when we are looking for insulation. The insulating effect that a green roof has to offer will take significant hits with increasing moisture content, as more and more of the insulating air pockets fill up with water and lose their insulating effect.

The presence of insulation layer might mitigate the insulating effect of a green roof

Even if a green roof is relatively thick and as a low density sediment, it will not necessarily improve your roof insulation drastically or even increase it notably at all. An important reason why no notable increase in insulation might occur is because there is already a dedicated layer of insulation present in the roof itself.

Although a green roof’s sediment layer can do some insulating, it shouldn’t come as a surprise that is does not get anywhere near the insulating properties of dedicated insulating materials like rock wool, Styrofoam or fibreglass. Therefore, adding a green roof on top of an already effectively insulated roof is very unlikely to add to the insulating properties of a roof to any practically useful degree.

On the other hand, if a roof does not have a dedicated and effective layer of insulation, any addition of insulating materials, like a green roof, can have a notable effect on the insulating properties of the roof. It will still not accomplish the same level of insulation as a dedicated insulation layer and is probably less cost-effective, but it is still a nice side effect if you are not only installing a green roof for insulation, but also for some of the other benefits a green roof has to offer.

Green roofs might cool down you home instead of keeping it warm

So even if the insulating effect is small, it can never hurt to have that little bit extra, right? Well, not quite… This is because one of the other reasons for installing a green roof is actually for lowering the temperature inside a building instead of keeping the heat in.

Green roofs do this through two processes, which are evapotranspiration and shading. Let me explain the two.

• Evapotranspiration is the process of turning water from its liquid form to its gaseous form, better known as water vapor. There are two ways that water can vaporize on a green roof. One of the ways is through evaporation from the water in the soil and the outside of plants.  The other way is through transpiration, which is a physiological process in plants in which water escapes through tiny holes in the leaves of the plants, called pores.
  
  Evapotranspiration is effective at cooling down buildings because turning water from its liquid state to water vapor costs a lot of energy. This energy is called the latent heat. To get an idea of how much energy: it takes more than 6 times as much energy to vaporize any amount of water that to take that same amount of water room temperature to the boiling point. All that energy is taken away from the roof by evapotranspiration, which has a drastic cooling effect

• Shading is the effect of the leaves of the green roof’s vegetation blocking and deflecting sunlight before it can hit the roof. Because the roof is not practically in the shade instead of having direct sunlight being blasted upon it, it does not get up to nearly the same temperature that a bare roof would.

Although the cooling effect is a big reason why people install green roofs in the first place, the cooling effect does not conveniently disappear in the heating season. An exception to this is when it gets so cold in the winter that the plants go dormant in the winter, stopping the shading and evapotranspiration effect.

In all other cases, the green roof will still be cooling your home when you are trying to keep it warm. So, if you sole reason for installing a green roof is to reduce heating cost by insulating your roof, it might be a good idea to reconsider and look into dedicated insulating materials.

Other ways green roofs can save energy

So if installing a green roof for insulation is not as effective as other insulation materials, is installing a green roof useless for saving energy? It definitely doesn’t have to! If you spend a lot of energy to keep your air conditioning running over summer, the cooling effect of a green roof can definitely help you cut down on that by taking some of the cooling work out of the hands of the airco. If winters are dry and so cold that the plants do dormant, shading and evapotranspiration are mitigated and the insulating properties of the green roof are as high as they can be.

To find out more about the potential of green roofs to cut down you energy bill, reading THIS ARTICLE will help you along your way. To get even more drastic in reducing energy usage, THIS ARTICLE will inform you on the benefits of combining a green roof with solar panels!

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