Green roofs have been gaining attention from people looking to make their homes more sustainable and environmentally friendly. One of the main ways to make a home more environmentally friendly is to decrease energy usage. This made me wonder: can green roofs save energy? This is what I found
Green roofs can save energy. They can do so by providing a cooling effect during warm days, which reduces the energy used for indoor climate control like air conditioning. In some cases, they can also provide insulation, reducing the heating costs.
Yet, depending on the climate where you live, the energy-saving effect of green roofs can be mitigated. In some cases having a green roof on your home may actually cause your energy bill to go up instead of down.
The efficacy of green roofs to save energy is dependant on local climate
So how do you know whether installing a green roof on your own home or any other type of building will save you energy in the long run? You have got a couple of factors to consider there.
- If you live in a climate that is cooling dominant, i.e., most of the energy you spend on climate-controlling your house is through cooling, a green roof has a good chance of reducing your energy usage by helping to keep your house cool. Your electrical bill will probably turn out lower with a green roof installed through the combined evapotranspiration and shading.
- If you live in a climate that is heating dominant, whether or not a green roof will reduce heating cost depends largely on whether or not an insulation layer is already present, the thickness, density, and moisture content of the sediment layer, and whether or not the plants go dormant in the winter.
- Suppose you live in a climate in which both heating and cooling require large amounts of energy to be used. In that case, a green roof works best if it can cool your home or any other type of building, with a lot of evapotranspiration and shading in the summer. Simultaneously, the plants go dormant, and the sediment layer is relatively dry in the winter to reduce evapotranspiration as much as possible and still experience insulating effects from the sediment layer.
Green roofs safe cooling energy by evapotranspiration
The most important way in which green roofs can save energy is through the cooling effect of evapotranspiration. Evapotranspiration is the combined process of evaporation from water in the green roof, together with the process of transpiration, which is a physiological process in plants during which water is transpired through tiny pores in the plant’s leaves for numerous reasons, such as cooling and the transport of nutrients from the roots up into the plant.
Effectively, this process of turning water from its liquid state to water vapor is what cools the green roof down. This is because it takes a lot of heat energy to turn water into water vapor. The effect of evapotranspiration is very comparable to sweating; the water on your skin evaporates, cooling you off.
The cooling effect of evapotranspiration prevents the roof from getting up to the scorching temperatures that a conventional roof might get up to in the summer. As a result, the inside of the building stays much cooler, resulting in less energy having to be spent on cooling the house down through air conditioning.
Green roofs can also reduce cooling energy through shading
Next to evapotranspiration, there is another way in which green roofs can reduce the energy cost of cooling during warm and especially during sunny days, which is shading. Shading is the effect of the vegetation’s leaf areas catching the sunlight before it hits the roof itself.
Shading reduces cooling costs because the blocking of direct sunlight causes roof temperatures to remain much cooler than they would without the green roof. This is because the energy is partially reflected off the roof back into the sky without being turned to heat. Any radiation energy that does turn into heat can be removed from the roof by evapotranspiration.
How well a certain green roof works for shading is mostly dependant on the type and amount of vegetation on a green roof. Vegetation with relatively large horizontal leaves tends to be to work best for providing a shading effect.
How green roofs might reduce energy used for heating
Aside from the energy-reducing effect of evapotranspiration, green roofs can, under some circumstances, also reduce the energy spent on heating by providing an insulating effect to the roof. The most important way that a green roof provides insulation is through the sediment layer. The effectiveness of the green roof’s insulating properties depends on a couple of factors.
One of the factors is the thickness of the sediment layer in your green roof. This makes intuitive sense. Since there is more of the insulating material between the inside of your home or other building and the outside environment that you want to insulate it from, the heat from your home will have a harder time escaping through the sediment layer into the outside environment.
Another factor is the density of the sediment layer. The lower the sediment material’s density, the higher the amount of air that is trapped in there. Trapping air is a good way to provide heat insulation. In fact, the insulating materials that you probably know, like rock wool or Styrofoam, use this mechanism. This is because air has a low thermal conductivity, so heat does not travel through it easily.
The moisture content is also something to keep in mind. Water can fill up the cavities in the green roof that were previously filled with air. In contrast to air, water is a good thermal conductor, which is undesirable when insulation is what we are looking for. Therefore, a green roof’s insulating capabilities are at their highest when the sediment layer is dry.
Why a green roof might not save heating energy
Although a green roof could save you some energy used for heating, there is also a chance that it will not have that much of an impact or, in some cases, even increase the winter heating demand! Quite an obvious reason why a green roof might not save you heating energy is that you live in a place where you are not heating much in the first place because it simply does not really get cold year-round. However, there are also less obvious reasons why a green roof might fail to cut down the heating bill.
One of those reasons might be because you already have a dedicated layer of insulation in the roof itself. If the roof already has a layer of insulation, the green roof’s added insulating effect becomes relatively smaller, and most of the time is reduced to negligible levels.
On top of that, the evapotranspiration that helps you cool your house in the summer does not disappear in the winter in most cases. There is still water evapotranspiring from your roof, cooling your house down, which actually increases the heating cost. An exception to this is when you live in a climate that is cold enough for the plants to go dormant in the winters, which causes them to stop shading and transpiring. In that case, you will just be left with the evaporation, which will not draw heat from your home to as large of an extent as when the green roof is fully alive and kicking, so to say.
Cutting down the electric bill by incorporating solar panels with your green roof
Your green roof can save you some money on your electric bill by incorporating solar panels into your green roof. Even though I first thought that you had to choose between a green roof and solar panels, I recently learned that combining the two can be done.
Not only can it be done; it can actually provide some benefits. The green roof can increase the efficiency of the solar panels compared to a normal roof and the solar panels can provide the green roof with more biodiversity. I wrote a whole article about this, which you can read here.