A second film over the first is the least expensive insulation a film house has, and the easiest to use badly. The question is rarely whether two layers hold more heat than one, because they do, but whether the heat is worth the equipment, the power and the light they take in return.

The covering concerned here is standard clear greenhouse film, listed at a covering material thickness of 0.15 mm or 0.12 mm, run as a single layer or as two and held with aluminum clips. It sits on a frame of hot dip galvanized steel pipe under an arch roof, with spans of 6.4, 8, 9.6 or 10 metres, a total structure height of 4.3 to 6.5 metres and a design wind load of 0.35 kN/m2, and it is chosen on this model to prevent heat loss and cold air intrusion.

Which thickness grade to buy, and how a cover is tensioned at the channel, are separate subjects with their own pages. What follows is the layer count and the air cushion behind it.

A single layer is one sheet pulled over the frame and held at the channel. It keeps the wind off the crop, holds a little warmth into the evening and loses it again through the night, because a single polyethylene sheet is thin and passes heat readily.

A double layer is two sheets with a gap held between them. The gap is not empty air sitting still by accident: a small blower keeps a slight pressure inside it so the outer sheet stays taut over the hoops and the inner sheet does not fall onto the crop. That cushion of air is the insulation, and the inner sheet is the surface the crop actually lives under.

What changes is the rate at which heat leaves, not the width of the house. The frame, span and height are the same whether one sheet or two are fitted, so a house can be ordered as single layer and run as double later, provided the fixing points for an inner sheet are there. The cover grade is a separate decision, and the trade offs between film thicknesses are set out in the note on choosing between the cover grades.

A double cover is not two films thrown over the same frame; it is a covering system, and the parts are worth listing before the decision is made. There are two runs of film rather than one, and the inner sheet is usually fixed to its own line of rails or purlins so that it hangs clear of the crop.

On the model described here the cover is held with aluminum clips and the ventilation is manual or electric, so a blower and its air line are simply the equipment that makes the layer count real. Where a house is ordered as a single layer, the same frame can usually be fitted with the inner line later, but it is better to decide before the frame leaves the works.

The gain from a second layer is the still air between the sheets. Moving air carries heat away quickly, which is why a windy night pulls warmth out of a single cover so sharply, while air trapped in a thin layer and held there does not circulate and instead slows the passage of heat through the whole assembly. The crop is then protected not by the film alone but by film, air and film together.

Two things follow, and both are felt after sunset rather than at noon. The house loses heat more slowly through the night, so the temperature inside falls less steeply between the last hour of light and dawn, and a heating system has less ground to recover when it runs. The inner surface also stays closer to the air temperature of the crop zone, which matters on the days when condensation forms, a subject treated on the page about light transmission and condensation.

It is also the sense behind the good insulation and low energy consumption listed for this model: the extra layer is judged against a heating bill over a season, not against one thermometer reading.

Nothing on a greenhouse is free, and the second sheet is paid for in three currencies: light, power and attention. Light first. Two films with air between them pass a small part of the daylight less than one, because there are now four surfaces and a diffuse cushion in the path of the sun. On a bright summer crop that is nothing; in a dull winter growing period, under a house that already fights for every hour of light, the few percent lost are real.

Power next. The blower is the price of the air gap and it runs for as long as the gap must be held, so it belongs in the running cost of the double layer rather than the purchase order for film. And attention last, which is where a double cover quietly fails: a puncture in the inner sheet, a slipped tube or a stalled blower deflates the gap, and a deflated inner sheet hangs onto the crop instead of floating above it. That failure does not show through the outer film from the yard, so it is found by looking up inside the house.

The layer count is a site decision before it is a crop decision. Sites with cold nights and long heating seasons, where the difference between a single and a double cover is measured in fuel over many months, carry the extra cost easily. Sites with mild winters, inexpensive heating or a crop that tolerates a cool night gain much less, and there a single cover kept in good order is hard to beat.

Between those two ends the argument is usually about what is growing. A house given to seed nursery stock, young plants or high value vegetables that stall below a certain temperature rewards every degree held overnight, and a double layer is a modest way to hold them. The same house growing a hardy field crop, or one only run for the frost free season, is better served by a single sheet and a well fixed one. The crop list for this model is vegetables, fruit, flowers and seed nursery stock, and it is quoted to the customer's own drawing rather than a fixed footprint, so the layer count is settled with the house.

Fitting order matters more on a double cover, and the inner sheet goes up first. It is run onto its own rails or purlins, tensioned evenly and left clear of the crop line before the outer sheet is pulled over the hoops and held at the channel. If the two sheets are fitted from opposite ends, the air path between them is pinched.

Once the house is covered, the routine is short. Walk the inside line at eye level each season and look for a sagging inner sheet, the first sign of lost pressure; check that the air line has no split and the blower intake is clear; and keep the channel and clips on the outer sheet as on a single cover, since the weather sheet is fixed no differently. How the outer sheet is tensioned at the channel is set out in the note on fixing a cover at the channel and clips.

The covering on this model is standard clear greenhouse film at a covering material thickness of 0.15 mm or 0.12 mm, run as a single or a double layer and held with aluminum clips on the frame channel, and its purpose is to prevent heat loss and cold air intrusion. The frame is hot dip galvanized steel pipe under an arch roof, with spans of 6.4, 8, 9.6 or 10 metres, a total structure height of 4.3 to 6.5 metres, ventilation that is manual or electric, and a design wind load of 0.35 kN/m2. The galvanized coating is specified at 70 to 85 micrometres of zinc to BS EN ISO 1461 and assessed under BS EN ISO 14713-1, so the working life of the frame follows the corrosivity class of the site: class C2 in clean country air, C3 in urban air, C4 industrial or a C5 coastal setting. Cooling and heating, irrigation, ventilation, fertilization, shading and benches can be added on request, each quoted as its own line. Where the covering sits inside the total price of a house is worked through in the note on what the covering adds to a house budget.

The company is a manufacturer rather than a trading house, and its greenhouses are produced in its own works, which is how quality control and competitive pricing are held on one order, and it has built frames in its own factory for above 35 years. A house leaves in three groups, metal structures, covering system and install accessories, and a double layer is ordered as the covering group with the inner film, the clips and the air equipment matched to the frame in place. Samples of the products, including film components, are free, with sample freight borne by the customer. After an order an engineer supplies layout drawings and the installation manual and can travel to the customer's country for on site supervision.

Reporting on how commercial growers run covered houses is published by Hortidaily, and public information on protected cultivation and covering materials is published by the United States Department of Agriculture. The wider range this covering belongs to is shown under our polyethylene film greenhouses. To settle one layer against two for a site, call +86 191 5068 3942 or write to sales@cngreenhouses.com with the winter night temperature, the heating you expect to run and the crop.

Q: What is an air-inflated double layer on a greenhouse?
Two films with a narrow gap held between them. A small blower keeps a slight pressure in the gap so the outer sheet stays taut and the inner sheet floats instead of resting on the crop, and the still air trapped between the sheets is what slows heat loss. The outer sheet is held at the channel with aluminum clips, the inner one on its own rails or purlins.

Q: How much heat does a second layer keep in?
It slows the rate at which heat leaves through the roof, so the gain shows as a gentler fall in overnight temperature rather than a fixed number of degrees, and it is judged against the hours of heating the house would otherwise run.

Q: Is a double layer worth it on a 6.4 to 10 metre span house?
On these spans it is practical, because the inner sheet follows the same arch as the outer one. It earns its place where nights are cold and the crop stalls below a certain temperature, such as seed nursery stock or high value vegetables, and is harder to justify on a house used only through the frost free season.

Q: What does the second layer cost me in light and upkeep?
Two films and the air between them pass a small part of the daylight less than one sheet, which matters most in a dull winter period. The blower adds a small continuous power draw, and a punctured inner sheet, a split air line or a stalled blower deflates the gap, which is checked from inside the house.

Q: What covering and frame figures apply to this house?
Cover: standard clear greenhouse film at 0.15 mm or 0.12 mm, single or double layer, held with aluminum clips. Frame: hot dip galvanized steel pipe under an arch roof, spans of 6.4, 8, 9.6 or 10 metres, total height 4.3 to 6.5 metres, ventilation manual or electric, design wind load 0.35 kN/m2, the zinc coating to BS EN ISO 1461 and corrosivity class to BS EN ISO 14713-1. Crops are vegetables, fruit, flowers and seed nursery stock, quoted to the customer's own drawing.

Q: Are samples free, and who pays the freight?
Samples of the greenhouse products, including film cover components, are provided free of charge, with the freight on a sample delivery borne by the customer. For a covering order the films, clips and air equipment are quoted as the covering group and matched to the frame channel already in place.