Koi pond insulation to prevent ice and freezing in winter

Which are the best materials to use for covering a Koi pond, and which ones aren’t? How much gap should I leave above the water surface? And how long should I cover my pond for?

Dear James,

Covering your pond overwinter can prove to be a very cost-effective way of protecting your koi from the extremes of winter. A simple DIY project can make sure that your koi overwinter safely and if used in conjunction with a pond heater (not the small 100W type used to keep your pond from freezing), a cover can actually pay for itself many times over in the heat that it prevents escaping into the atmosphere.

In order to safely recoup the cost of a cover (and to get it through the winter safely), you must build it to meet certain criteria. There are many home-built pond covers that have ended up being patched up and restored (even redesigned and rebuilt) before the end of the winter is over. Furthermore, because the cover will be in close proximity to your pond, the materials used must be pond and koi-safe. Even if you plan your cover not to come into contact with your pond water, there will inevitably be some interaction and mixing through run-off and condensation between the cover and your pond.

Design criteria for your pond cover

  1. Stability. Winter will be a testing time for your cover (after all, that’s why you want to protect your pond!) Strong wind and harsh elements will do their best to dislodge and perhaps even invert your cover (umbrella-in-a-gale-style). You will need to construct your pond cover and anchor it with strong winds in mind.
  2. Easily handles rain, snow and leaves. Your pond cover must be constructed to prevent snow and rain from settling or puddling on top of the insulating membrane. Rain and snow are notoriously heavy and if allowed to settle will burden your structure with an additional weighty load – a sure recipe for a collapse – disaster! Take your lead from the pitched roofs on our own houses – and even better still the arc of a polytunnel so that rain just runs straight off.
  3. Must reduce/deflect wind-chill. A pond cover must enclose your pond on all sides, preventing the wind from chilling your pond from any direction. (Imagine what it would be like sleeping in a tent over winter with one of the flaps open – very chilly!) A wind-proof material is ideal for this, but beware because if the wind gets inside your cover, it will act like a sail and blow straight off your pond.
  4. The cover must retain heat. This sounds quite obvious, but the choice of material will greatly affect your cover’s performance. Not only will your pond cover form a wind-chill barrier between the pond and the ambient icy air temperature, but it will also help to retain the heat stored within your pond water – especially if the pond is heated. It does this in 2 ways:
  • Insulation. The woolly jumper principle. By incorporating air pockets between warm and cold layers, the warmth is protected. The most popular material for this purpose is industrial bubble-wrap. Available off the roll in widths up to 2.5 metres (approx.), this material is ideal as a wind break but also a pond insulator. You may have seen a higher specification bubble-wrap material rolled up on the side of small, private swimming pools. This is usually blue (rather than clear) and is more tear-resistant.
  • Greenhouse effect. Clear PVC bubble wrap allows a degree of solar radiation to pass through into the pond void below the cover where it is retained as heat by the same bubble-wrap layer. So by using a clear cover, you can take full advantage of any sunlight by using your cover’s green house characteristics.
  1. Easy to install, disassemble and store. This is where you will have to make the compromise between the scale and structure of your cover, and its ease of installation and storage. Some of the most robust pond covers can take 2-3 men to lift on and off a pond, so robust was their timber construction. Others have used simple push-fit pipework to form a framework which is light, easily assembled and stored and can be anchored to prevent it from moving in the winter gales. Using plastic pipework is my preferred option as it also enables you to provide the highly effective arched polytunnel profile. It is very easy to disassemble in the spring as well. Furthermore it is inert and will not contaminate your pond in any adverse way. Wood on the other hand, although easy to work with, can be a little risky as it will often be pressure-treated with a preservative or require you to apply a preservative to prevent it from deteriorating in the rain and condensation. Another hazard associated with timber covers is the unavoidable use of metal fixings, screws or nails, which are likely to rust or leach heavy metals (if galvanised). If you do have to use wood in your construction (even as part of a pipework frame) do make sure that the preservative used is pond and fish safe.
  2. No weak spots. If adopting an arched profile, ensure that the transverse ribs are not spaced too far apart as this will allow you to stretch your membrane tightly over the structure and reduce the risk of sagging so that water has no chance to settle or put  additional stress on your membrane. The greatest risk to your cover’s integrity is a deterioration to the membrane. If it is not fixed correctly, or at too few points along the frame then it is more likely to tear; and once torn, it will deteriorate very quickly. For better tear-resistance, you could try a woven, rip-stop film that is now commonly available from DIY stores for covering garden furniture etc.

How much gap should I leave above the water surface?

Leaving a gap above your pond’s surface is the preferred option as this provides an insulating cushion of air directly above the water’s surface. It also allows the exchange of gases into and out of your pond. This is something that a layer of bubble-wrap laid directly onto your pond’s surface has in common with a layer of ice – and should be avoided. Having said that, I have seen bubble-wrap laid directly onto a very large pond ‘swimming-pool-style’ where a framework structure would have proved impractical.

How long should I cover my pond?

As your cover will be protecting your pond from the chilling effects of cold air, you should monitor the air temperature and place the cover on when nights are consistently below 8C. Likewise, you should not remove it before the air temperature rises above 8C in spring. You will find that the air temperature beneath the cover is warmer than the ambient and so you should take care not to remove the cover prematurely and subject your pond to a sharp chill. This of course would not be a problem if your pond is thermostatically heated.

BOXOUT: Top tips for covering a Koi pond?

  1. Stable and well-anchored against the strong winter winds
  2. Arched or pitched roof to prevent water from gathering
  3. Preferably a clear membrane to benefit from the greenhouse effect
  4. Easy to install, disassemble and store
  5. Use safe / inert materials – PVC pipework is ideal
  6. If using wood, ensure that you use a safe wood preservative
  7. Metal screws, nails and fixings could pose a hazard to your water quality
  8. When fixing the membrane, ensure that it is tight as it is likely to sag in colder weather

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