Adding Thermal Mass in a Greenhouse or Polytunnel


Adding thermal mass to your greenhouse or polytunnel is a way to make sure that you make the most of the heat of the sun. By adding materials with high thermal mass, you are increasing the amount of heat energy that can be stored during the day. That heat will then be released as the air cools at night and can make a small but appreciable difference to the night time temperatures in your polytunnel. They might be enough, even, to keep off a hard frost in most parts, though will only be a partial improvement in colder areas, where other things such as bubble-wrap, horticultural fleece, mulches and/ or cloches will help to keep the cold off polytunnel plants.

Greenhouse pathway


Pathways in your polytunnel or greenhouse can be a good area of thermal mass. They can be made of a range of different materials such as brick, stone or concrete. Try to use reclaimed materials to be kind to the planet, reduce your carbon footprint and save money. Plants close to path edges will benefit from the heat dissipated from the dense structure during the night.

Raised beds greenhouse

Raised Bed Edges

Likewise, plants on the edges of your raised beds, if those beds have thermal mass on their edges, will also benefit from a little extra heat on cold nights. The effects may be marginal but all these small changes can add up and help make your polytunnel or greenhouse more productive. Rather than opting for a lighter weight wood planking or similar for your raised bed edges, consider something denser like stone, brick or clay.

Polytunnel hose

Water Stores

Water also absorbs and stores heat, releasing it slowly when the temperature falls in the evening and through the night. You may already catch and store rainwater. Consider moving stores of water inside a polytunnel, as these can make a difference to the night time temperatures. Some people even choose to have a well or reservoir within the greenhouse or polytunnel to act as a heat sink.

Adding thermal mass to your greenhouse or polytunnel could make a difference to how productive you can make the space, especially in the shoulder seasons. It is one, small way to prolong the growing season more than having a greenhouse or polytunnel does already.