Passive Solar Design in a Polytunnel

A polytunnel, no matter what it is used for, can be a fantastic way to make full use of the heat and light that the sun can provide. It does not matter whether you want to grow food in your polytunnel, or use if for a wide range of recreational uses – there are a number of hints that can help you incorporate excellent passive solar design into your structure. In this article, we will examine the concept of passive solar design before looking at how to incorporate it in your polytunnel in a little more depth.

What is Passive Solar Design?

Passive solar design is all about making direct use of the energy that our sun can provide. In the design of a structure this involves looking at positioning and orientation. It also involves looking at increasing capacity to catch and store solar energy, and reducing the amount of solar energy that escapes (over winter or at night). Layout is very important.

Passive solar design is usually a concept that is applied to sustainable housing and business premises. But the same principles and techniques also apply to a polytunnel.

A polytunnel already makes significant strides towards good passive solar design, simply by virtue of allowing the sunlight inside through its covering, and retaining some of that heat it provides.

What is more, a polytunnel makes it easier to grow food or other plants all year round. Since plants obviously use sunlight to grow, and convert sunlight (along with carbon dioxide and water) into carbohydrates through photosynthesis, this is one direct way to use and store solar energy.

Placing a Polytunnel for Good Passive Solar Design

The first thing to consider when thinking about good passive solar design is the location and positioning of the polytunnel. The perfect location will, of course, depend on where in the world, or where in the country the structure is to be built. While in some parts of the world, shade structures and additional shading from external factors like trees could be beneficial for summer growing. Here is the UK, however, you should aim to place a polytunnel in as sunny a spot as possible if it is to be used for growing.

Understanding the Movement of the Sun

The sun, unlike so many elements in gardening, behaves in an entirely predictable way. It will rise and set in the same point in the sky at a certain time of year, depending where you live, and the angle at which it hits the intended polytunnel location will alter in the same, dependable way throughout each year.

You can find out where the sun will be at any given time on any given day in any given location simply by observing the natural cycles throughout the year. However, since we cannot always spare the time to simply watch and wait for an entire year before placing a polytunnel, we can consult online sun position charts.

Understanding the movement of the sun and how this will effect the heat and light on your intended polytunnel location will help you to determine whether or not it is a good place to put a polytunnel. Think not only about the direction from which heat and light will flow throughout the year but also about how high the sun will appear in the sky at certain times of day.

Determining The Best Orientation For a Polytunnel

Once you have determined the sun position throughout each day and throughout the year, this can also help you to decide which way round to position your polytunnel. Though often other considerations will come into play and a polytunnel will often be placed along a site boundary, for example, a polytunnel should ideally be positioned on either an east-west or a north-south axis.

When positioned with a long side facing south, one side of the polytunnel will be more shaded than the other. When positioned with the long sides facing east and west, the same amount of sunlight will reach both sides, however the northern end of the tunnel will be marginally less warm and sunny. Either way can work. Just make sure you consider which option might be better for the way you eventually lay out your polytunnel, and for the plants you wish to grow. (We’ll look at some layout options a little later in this article.

Incorporating Thermal Mass in a Polytunnel

Some materials are able to store heat from the sun and slowly release it later when the air temperature falls. Incorporating thermal mass into your layout can help in general to keep a polytunnel at a more even temperature throughout the year. This is an important feature of passive solar design and will reduce other forms of energy that must be expended on heating (or cooling) the structure.

Incorporating Thermal Mass in Bed Edging

The first way that you can consider adding thermal mass is through bed edging. It can help to create a cosy micro-climate along bed edges that can be helpful to tender plants and young seedlings. Materials with good thermal mass include stone, brick, clay or adobe, for example.

Adding Thermal Mass in Staging or other Inside Structures

You might also add thermal mass to a polytunnel through using such materials in the construction of polytunnel staging or other structures such as seating areas, for example, that you might decide to create within the space.

Storing Water in the Polytunnel

Water also has good heat-storage capacity. So storing a rainwater tank or creating another water reservoir in your polytunnel can be another way to introduce more thermal mass and to catch and store the energy that our sun provides.

Creating an Earth-Sheltered (Or Partially Earth Sheltered) Polytunnel

Another option that you might be able to consider for your polytunnel is making it an earth-sheltered (or partially earth-sheltered) structure. By adding banked earth, earth bags or even a stone wall along the north side of polytunnel, you can help to retain energy and keep the polytunnel a more consistent temperature throughout the year.

Increasing Insulation in a Polytunnel

Good passive solar design may also include taking measures to improve the polytunnel’s ability to retain heat during the winter months. It is important to remember that good insulation does not mean removing all ventilation, as good ventilation and air flow are still also vitally important.

You might increase insulation by adding internal row covers or mini polytunnels, or simply by adding a layer of bubble wrap or off-cuts of polytunnel cover to create an extra barrier to prevent as much of the sun’s heat as possible from escaping when it most required.

Internal Layout for Passive Solar Polytunnels

As mentioned above, before you place your passive solar polytunnel it is important to decide where to position it, and how it should be oriented with reference to the cardinal directions.

In a North-South Oriented Polytunnel:

In a polytunnel with the long sides to the east and west, both sides of the polytunnel will get equal amounts of light, though the northern end of the structure will be marginally cooler and more shaded. In such a polytunnel, it is best to place taller structures, such as any trellising for vines or climbing plants, trees or large fruiting bushes etc. towards the north end of the tunnel so as to prevent them from casting too much shade on other plants you are growing.

However, you could also consider taking advantage of the shadier and slightly cooler end of the polytunnel to grow plants such as lettuces and spinach that can be prone to bolting in the hottest months. For example, if you have a trellis near the northern end of the tunnel,you could create a vertical salad garden on its northern side.

In the longer sides to the east and west, it may be helpful to think about grading plants roughly from shortest to tallest as you move northwards through the structure.

In an East-West Oriented Polytunnel:

In a polytunnel with longer sides to the north and south, the south side will be sunnier and warmer than growing areas to the north. This can be useful where you want to maximise diversity and create a range of different habitats to suit different plants.

To the southern side, for example, you might plant summer crops such as tomatoes & peppers or sweetcorn,squash and beans, while to the north you might create a shadier space for brassicas and certain other leafy greens.

However you decide to orientate your passive solar polytunnel, it is important to think carefully about the patterns and movement of light through the space throughout the course of a day and throughout the course of a year.

Have you thought about passive solar design in your polytunnel? Have you any tips or suggestions to share? Let us know in the comments below.