Wednesday, 19 October 2011

Insulating the greenhouse

All the signs are now indicating the rapid approach of winter. A cold drenching day yesterday left the ground running with water and best left alone. However there is plenty to be done in the polytunnels and greenhouse.

This morning a thin crust of ice on the poultry water and a forecast of much colder weather tonight means that there should be no delay to putting up the insulation in the greenhouse.

Each year I put up a bubble wrap layer next to the glass and hang more over a framework of training wire to reduce the height of the internal space. It means that the heater has less volume to heat and reduces heat loss through the glass. It does make the greenhouse feel more enclosed and leads to reduced light levels, but as most of the plants are in a semi-dormant state that is not a big problem.

I do not run a warm greenhouse through the winter, it is more of a frost free place. Tubular heaters suspended beneath the benches provide a gentle continuous heat and a fan heater cuts in if the temperature drops too low. The heaters are usually switched off during the day, unless the frost or snow remains and there is no sunlight. A back up paraffin heater has thankfully not been needed for many years now, they require more attendance and are difficult to adjust to ensure the right heat level. I think in a sustained power cut I would probably resort to moving plants into the potting shed or house rather than use the paraffin heater.

Most of the heating is required during the early spring when nursery and vegetable plants are actively being grown. Then the insulation can interfere with ventilation and light transmission and it is important to be vigilant with seedlings to prevent disease and weak growth. Delaying the sowing of seeds to avoid the worst of the weather conditions is a necessary tactic; this year I had to delay sowing for three weeks or more due to the heavy snow and very low temperatures. Also avoid sowing seeds of tender plants, these should always be left for a later sowing. The hardy seedlings will withstand some drops in temperature but the tender seedlings will succumb or suffer a check in growth which will lead to a weaker plant.

It is always a relief to remove the insulation, though I tend to leave the sections that are on the lower half of the greenhouse and just remove the top half first. Suddenly there is more light and the greenhouse feels larger. This year April was a sunny warm month and the insulation was interfering with the ventilation to such a degree that the build up in heat beneath it was causing significant stress to the plants. The heat cannot escape through the roof lights with the insulation in place and the increase in daylight means that a decision needs to be made: reduce the heat lost at night or gain more daylight and heat during the day, with better ventilation too. Adding some means of absorbing that daytime heat inside the greenhouse will mitigate some of the loss during the night as the absorbed heat is released during the cooler hours. I have a large black tank of water and some concrete paving slabs, all of which act as a thermal store.

The polytunnels have no insulation or heating. If you want to  use them for propagating early in the season consider adding a heated bench with some fleece for keeping the heat in at night, or erect a cold frame or small tunnel inside the large tunnel as a dedicated propagating area or area to keep more tender plants.