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The 'Climate Frame' Outdoor Terrarium with Natural UV-B and WiR-A

Andy C. Highfield
Tortoise Trust, Almeria, Spain


UV-B Transmitting sheet fixed to angled frames

Lumisol UV-B transmitting sheet on angled panels (Photo: Big Joe)

The design described here is the product of many years research, testing and development. The concept behind it was to produce a highly versatile and easy-to-adapt outdoor terrarium suitable for reptiles that could substantially reduce the reliance placed on artificial heat sources such as basking lamps in colder climates. Basking lamps have many drawbacks, including a failure to provide the ideal type of heat (water-filtered IR-A) or pattern of heat distribution (ultra-wide field) required by basking reptiles. They also consume large amounts of energy and are costly to operate and maintain. The 'design spec' for this unit included maximum use of natural UV-B and the ability to provide a thermally complex microclimate optimised for reptiles.

Traditional vivaria and terraria tend to offer a very limited range of temperatures, and in a very simplified, limited format: a 'hot spot' and a 'cool end'. This bears little resemblance to the complex thermal environments observed in natural habitats. It is important to stress that UV-B and Infrared are a dynamic pair, and that in order to be able to adequately utilise UV-B, reptiles need a high quality and physiologically well-matched source of Infrared in addition. Keepers have tended to concentrate on UV-B provision, while overlooking the deficiencies of Infrared provision.

This design was evolved from early trials with 'polytunnels' (sometimes known as 'hoop houses' in the US) and smaller outdoor 'cloche' systems. These were generally effective in raising body temperatures, but provision of UV-B was problematic due to the technological limitations of materials available at that time. New advances in plastics technology have now overcome this primary deficiency. In addition, these basic designs have been developed much further using data from high resolution thermal imaging systems to provide a far more 'naturalistic' range of temperature choices, and also to maximise solar energy inputs. There are a number of ways in which these units can be expanded, for example by adding additional solar-thermal collectors and thermal stores. In cold climates use of these methods can greatly expand the 'season' where outdoor maintenance remains possible.


The basic construction is quite simple, and very flexible.  The base unit can consist of four walls to low-medium height, and the UV-B transmitting plastic panels are fixed via frames to a strong sub-structure. In one sense, the concept is similar to a traditional "cucumber frame". Wood and concrete blocks can both be used to create the walls.... decking board is particularly useful and weathers well. This is one of our (very) early prototypes in the early phases of construction. The base and supporting framework for the plastic sheet are clearly visible.

An early
                  prototype during construction

When complete, that particular prototype served as a high humidity tropical house for a number of species. A combination of (non-UVB transmitting) twin-wall polycarbonate sheet and UVB transmitting panels may be used in any desired combination to produce a balance of UVB and heat conservation. This particular building was 300 square M., one of the larger units of this type we have used. It included an additional oil-fired hot-water central heating system permitting year round use, even in the winter months.

                  beams for sheet roof

High humidity tropical habitat inside unit

These enclosure are normally best used as part of a larger 'free air' outdoor enclosure with easy access to the 'greenhouse' part. Animals should not be confined to the enclosed part only. It is critical to note that these enclosures are intended for use in cold climates and that over-heating is a serious danger if they are used incorrectly. Their purpose is to reduce reliance upon artificial light and heat and they would normally be used only at times when indoor maintenance would be employed, for example, in early Spring and Autumn/Fall. It is worth repeating that this type of enclosure can generate extremely high interior temperatures very quickly in full sunlight. It is therefore vital that there is adequate ventilation, some form of effective temperature control, and adequate internal cool retreat zones (such as a burrow). Intelligent use of 'reflectors' and 'collectors', such as black or white/silver material, also offers different ways to control internal temperatures.

A Climateframe terrarium inside a larger pen

The 'Climateframe' supports a roof of 'Lumisol' material mounted on wooden or metal frames. A partial shade zone is made from graded mesh material with some holes to replicate the kind of shade found under plant cover. This can be added to, or reduced, by simply adding or removing frame overlays containing suitable material. A cool burrow zone is also provided by means of a 'box' end with white or other reflective material on its 'roof' to reflect heat. This 'burrow zone' includes soil and stones to allow the animals to create their own cooler microclimates.

The exact amount of 'seasonal extension' that can be gained using these principles very much depends upon geographical location and the size of the terrarium. In general, larger units are more efficient than smaller ones. There really is no limit to the maximum size. These designs can be as large as required. Some users have reported very substantial increases in the amount of outdoor vs. indoor time achieved, from as little as less than 6 -8 weeks a year to 3.5 months. In warmer climates, up to 9 months a year can be achieved – all with near zero running costs and natural UV-B and Infrared at more than acceptable levels. To date, test units have been used with tortoises, turtles (with an interior pond), green iguanas, bearded dragons and snakes.

One of the key features is that the design is inherently flexible and may be adapted easily to suit different seasons or weather conditions. In this example, the base unit, intended for terrestrial tortoises,  is covered first by removable panels with anti-predator wide mesh:

Anti-predator mesh over tortoise habitat

Next, modular removable panels are constructed to support the UV-B transmitting film. These can be used collectively, or singly, as required:

Modular framework in construction

Here, just one frame is in position, providing extra warmth at one end of the habitat only. In summer, frames with partial or 'dapple' shade provision can also be added or removed as necessary (all these photos (c) Big Joe):

Partial frame in use

The design consists of five principle functions:

1) Natural UV-B provision by means of new-technology thin-film plastics.

2) Natural water-filtered IR-A from sunlight.

3) The ability to offer any kind of humidity option, from arid to humid tropical, easily, in almost any environment, to suit a wide range of species. Internal ponds can also be installed.

4) A complex thermal environment with full sun, full shade, partial shade and burrowing microclimates.

5) Expandability and minimal energy inputs.

In addition, it was essential that the design was straightforward to build, with readily available materials, and that the cost to build was as reasonable as possible. Only a design that is simple, cost-effective and versatile would prove attractive to keepers.

Natural UV-B provision

Until recently, options for making use of natural UV-B in enclosed terraria were very limited. There were a few (very expensive) types of UV-B transmitting glass and plastics available, but they were difficult to source and not necessarily easy to install and use. The main drawback, however, was cost. These materials were very expensive indeed. And being rigid, equally expensive to ship and handle. Recently, however, some new thin-film flexible UV-B transmitting plastics became available, and we spent 18 months testing them to establish their suitability for use by reptile keepers. On the face of it, they offered many advantages. They were very reasonably priced compared to rigid sheet materials, easy to handle and install without specialist tools, and appeared to offer very acceptable rates of UV-B transmission, Infrared transmission and heat retention. Two brands were trialled. Lumisol and Sunmaster. Both delivered excellent results and furthermore, withstood two seasons outdoors without visible degradation. Manufacturers of both claim a useful life of 5 years or more in general use. Given their low initial cost and ease of replacement, this is more than acceptable.

I am indebted to Frances Baines who independently tested these materials for UV-B transmission with the following results.

UV-B Transmission Properties UV-B Transmission

This may be compared to other available materials:

Comparison of Glazing Materials - UV-B

It is also informative to look at a simple indoor-outdoor comparison of UV-B levels where UV-B under natural sunlight are compared to simultaneous readings beneath a sheet of 'Lumisol' film. Readings were taken of both the UV Index and of microwatts per square centimeter:

UV-Index Lumisol

Microwatts Sq Cm

This may be compared with a standard type (non-UVB transmitting) horticultural plastic film, 'Tuftane':

UV-B levels Tuftane

Tuftane UV-B

The difference is dramatic. The net result is that both Lumisol and Sunmaster provide very adequate UV-B transmission at the critical wavelengths for vitamin D-3 synthesis and as such, are of considerable importance to reptile keepers.

Both materials are easy to install either on 'hoops' as used in agricultural poly-tunnels as greenhouse replacements, or upon rigid frames, as used in our own design. It can be used alone, or in combination with other materials where structural limitations dictate, i.e., as UV-B transmitting panels in an otherwise standard structure. While the most expensive materials (UVB transmitting cast Perspex sheet) do offer technically superior performance in terms of UV-B transmission, the practical difference is relatively slight, and the levels achieved under Lumisol and Sunmaster are very acceptable indeed at the most critical wavelengths. We were able to rear juvenile Testudo graeca graeca using these materials exclusively, without any additional artificial UV-B or D3 supplementation, to bone densities equivalent to wild examples. We are therefore quite satisfied that they are safe and effective in use. In northern climates the clear varieties, which offer maximum transmission across the full spectrum should be used for most species, though further south, or for some rainforest species, the diffuse varieties may be preferred.

A further advantage of this material when used to create an outdoor terrarium is that natural frequencies of daylight are also present, together with normal and natural UV and IR diffusion and 'scattering'. This contrasts sharply with the conditions present under artificial light and heat sources. Such 'scattered' and diffuse sources of heat and light have important implications for reptile behaviour and physiology, some of which we are only beginning to understand.

Water-Filtered IR-A (WiRA)

All of these specialist films allow animals to take advantage of natural Infrared and greatly minimise (or even eliminate entirely, depending upon location) the need for artificial basking sources. This is of enormous importance to overall health and development. For a more detailed discussion, and numerous examples, of this topic see our earlier report into the health effects of basking lamps on reptiles.

The use of these new plastics permits not only natural UV-B usage, but critically, allows this in combination with natural wavelengths and patterns of Infrared use.

The results speak for themselves. There is no discernible difference between patterns of natural (outdoor) infra-red in reptiles and the patterns observed using thermal imaging cameras of infrared use under these films. By comparison to the patterns observed under artificial basking lamps the results can only be described as dramatic. There is 'deep' and very even heating, even to the extremities and the unnatural surface heating seen with typical artificial sources is entirely absent. It should also be noted that the 'dehydrating' effect of typical basing lamps is also entirely eliminated.

This is immediately obvious when comparing results from a thermal imaging camera.

Inside 'Climate Frame'

Heating effect inside Climateframe Inside Climateframe
                - Plastron

Note the particularly even heat, right to the plastron and extremities. Subjectively, the entire animal feels like a 'hot rock', which is exactly how they feel after basking in the wild. The 'top to bottom' even heating pattern is very evident in this image:

Thermal image of tortoise basking inside unit

Under artificial basking lamp

Uneven heating effect under basking lamp

Humidity control

Different species require very different levels of ambient humidity. In captivity, many keepers have tried to 'overcompensate' for the severe drying effects of artificial lamps by providing completely unnatural elevated levels of humidity using 'moist hides'. This exposes the animal to sustained levels of humidity that it may never experience in nature, and this is not without consequences. One particular effect is over-softening of the keratin and an increase in susceptibility to bacterial and fungal pathogens. This author has always maintained that it is far better to provide a truly suitable environment in the first place than to attempt to 'fix it' using artificial and potentially risky methods such as this. As to what constitutes a 'suitable environment' for any given species, the point of reference for this should be reliable data from the environment in which the species inhabits in nature. It is of course extremely important that this understanding is based upon genuine, reliable data and not upon ill-informed guesswork.

If a truly suitable environment is provided, there is no need to provide completely artificial and extreme measures to compensate for inherent defects caused by poor design elements elsewhere in the habitat.

Humidity is primarily controlled by means of airflow restriction and by the amount of water added to the enclosure. High rates of airflow with little additional water will soon result in an arid environment, while lower rates of airflow with larger amounts of added water will rapidly cause a rise in ambient humidity. This is absolutely no different from the methods employed in agricultural greenhouses. To obtain a correct or desired level of ambient humidity at any given temperature it is simply necessary to balance the rates of airflow and water inputs. By this means, it is relatively simple to create a wide range of humidity levels, either on a constant or upon a cyclic basis.

Passive side vents

Open-mesh passive side-vents (Photo: Big Joe)

Various means may be employed to add water (their suitability depending upon the species being maintained) but electronic 'self-watering' irrigation systems, misters and sprayers can all provide useful options. These can range from simple to more complex, with timers and 'electronic leaf' type sensors being used to carefully create and control almost any type of microclimate.

These illustrations show just some of the heat-flow and environmental humidity possibilities.

Heatflow and Humidity Control

A= Substrate for burrowing inside insulated compartment, B = Air within insulated compartment, C= insulating material overlaid with white reflective layer, D= substrate within the unit (this should be in good thermal contact with the subsoil), E= Fan assisted or passive air vents, F= Water tank or source for G = Sprinkler or mister system with sensors or timer control.

The 'Climateframe' can also be put into 'reverse' mode where the cool, or burrowing zone, becomes warmer than the outer covered zone. This is achieved by removing the reflective 'roof' and replacing it with a dark, heat absorbing material. The heat flow then changes to:

Changing the cool zone to a warm zone

This kind of adaption is only likely to be required in exceptionally cold climates and only with certain species. In this case 'C' = the heat reflecting and insulating layer is replaced with a heat absorbing layer.

 Some experimentation in your own climate and particular situation may be required, but our results indicate that it is fairly easy to produce just about any required level, from those demanded by desert dwellers to those suited to rainforest species. Indeed, one person involved in our trials successfully used such a unit for tropical amphibians, and another used a vertical, larger variant of this design for Green iguanas.

An extended version with external solar panels

In this extended example, extra heat was collected via additional solar panels and stored in a large, buried, insulated thermal store. At night the collected heat (in hot water) was re-circulated to radiators in the warm zone. An additional cool and burrowing zone is also included, and in this example, a pond is also present to raise ambient humidity for tropical species.

Our own trials concentrated upon use for arid habitat dwellers. It should be noted that with larger, more agile species some form of physical protection for the UV-B film is invariably required. This can be achieved using metal twin-weld mesh on a sub-frame. This will prevent contact, tearing and escapes. UV-B loss is minimal with medium-wide meshes. Very fine mesh will of course block substantially more UV-B. According to figures provided by Frances Baines ( typical chicken wire blocked 7% of UV-B, while 1/4” twin-weld mesh blocked 35-37% of UV-B. It is advisable therefore, to use as wide a mesh as possible, consistent with preventing damage or escapes. The more the total surface area of the wire used, the more UV-B will be blocked. Of course, some species may require no such protective measures in the first place.

It is also worth noting that misters and evaporation effects can be used in hotter climates or in peak summer seasons in moderate climates as a means to reduce interior temperatures. This can be done in such a way that heat will be rapidly lost via evaporation without a large effect upon ambient humidity, as the water vapour created is immediately vented outside – though this method is best suited to housing located in areas that have naturally low ambient air humidity. In humid climates it will not work and will merely increase the humidity within the unit. We found that it was highly effective at our test site in Almeria, Southern Spain, and should work well in similar climatic zones, however, such as Arizona in the US.

Different panels (vented or closed) can be added
                and removed

Different, interchangeable panels can be added and removed as required to suit different seasons. In the example shown these open mesh panels can be changed to fully closed panels if higher temperatures are required or if humidity needs to be adjusted (Photo: Big Joe)

In moister environments, increasing the rate of airflow through the unit is generally the most effective way of reducing interior temperatures when required, either passively or by means of fans. These too can be controlled by means of electronic thermostats and it is quite easy to construct a fan system based on low voltage fans (e.g, computer type fans) that will run directly from photovoltaic panels.

Side view

Forced air circulation based upon computer fans

Front view

Front view of forced or passive air circulation

Temperature gain and stability

Units employing the UVB transmitting film were tested at various locations from Southern Spain to Scandinavia. In all cases significant gains over ambient were recorded. Air temperatures within and outside the units were compared, and in addition, body temperatures of tortoises inside the unit were monitored. As the units were tested under a very wide range of conditions, at various latitudes, and the sizes of the units on test varied considerably, it is impossible to provide too many specifics, but examples of ambient air temperature comparisons include:


Ambient: 9C -

Inside unit: 20C

Gain: 11C


Ambient: 8C

Inside unit: 18C

Gain: 10C


Ambient: 13C

Inside unit: 25C

Gain: 12C

Canterbury (Kent)

Ambient: 17C

Inside unit: 31C

Gain: 14C

A general overview of measurements taken suggests that an air temperature gain between 10C to 15C would be typical in early Spring in moderate or somewhat cloudy conditions. Temperatures do of course vary hour by hour, and there are other variables such as time of year, cloud cover and orientation of the units to the sun. This is a typical report compiled by one of our beta-testers in the UK:

“Thursday 15th May 2014
All times GMT
Weather clear sky full sun
09.00 all three frames in place back panels off
Outside temp 15.5
Inside 16.2 shade
Inside 16.7 in sun
10.00 all three frames in place back panels off
Outside temp 19.6
Inside 18.1 shade shade
Inside 24.1 in sun
Carapace 40.0
Plastron 35.0
11.30 all three frames in place with back panels on
Outside temp 22.7
Inside temp 22.0 in shade
Inside temp 30.0 in sun
Torts now free roaming in garden
12.20 all three frames in place with back panels on
With all frames and panels on temps have shot up.
Outside temp 20.9
Inside temp 28.0 in shade
Inside temp 37.4 in sun”

These results are entirely typical of other reports we received and collated over a 14 month period.

Another report from Sweden cited these figures:

“Sunday 18th May 11:00, Västerås, Sweden
Weather sun
Outside temp 17°
Inside temp. 27°

Tort temps (T g ibera 50-75 g)
Tort one in enclosure
Carapace. 32°
Plastron 29°

Tort two outside inclosure
Carapace 29°
Plastron. 24° “

By June, even greater differentials are recorded:

“Saturday 7th June 2014 13.40 GMT
Frames have been on since last night
Weather sun / cloudy light breeze
All frames in place including rear panels
End sections open
Outside temp 24.8
Inside temp. 39.8
Ground temp shade area 29.2
Ground temp full sun highest 49.1 temp taken off welsh slate lowest 36.0 temp taken off soil/sand mix
Inside hide temp 20.7 ( this reading was taken right at the deepest part of the hide ) this hide has full sun
Inside hide temp 16.5 ( this reading was taken right at the deepest part of the hide ) this hide has full shade.

Tort one in enclosure / tort half in/ out of hide ( hide in full sun )
Carapace. 36.7
Plastron 33.7

Tort two outside inclosure roaming in full shade.
Carapace 29.8
Plastron. 29.6”

Some comparisons obtained using data-logger temperature recorders:

External Air Temperatures

External Air Temperatures

Internal Air Temperatures

Internal Air Temperatutes

The key point is that in all cases very substantial increases of both ambient air temperature inside the units, together with significant increases in both carapace and plastron temperature are experienced over conditions and body temperatures outside of the units, and that this is achieved with no additional energy inputs. No basking lamps were used, and no artificial background heating employed.

It is important to note that by using various basking strategies, for example, angling of the body, limb extension, and partial isolation from a cooler substrate, reptiles can substantially raise their overall body temperature above that of the ambient air temperature as long as some infra-red is present. So, it is quite typical to see body temperatures 12-15C above the ambient air temperature. In terms of these units, this can translate to:

External Air Temperature = 12C

Internal Air Temperature = 22C

Reptile's Body Temperature = 33C

The basic design of these units can be extended. It is entirely possible to add additional solar collectors and accumulators to save heat during the day, to provide additional heat at night – although with many species low temperatures at night may not only be normal, but beneficial. The need for this (or for supplementary artificial overnight heat) is entirely dependent upon the species concerned. It may not be required at all for temperate species in some locations, while tropical species will almost certainly require it. In any event, such need will be considerably reduced over typical methods of maintenance as a significant proportion of total heat requirements will be met from solar resources.

It is extremely important to understand that smaller units, by themselves, will not “hold” heat to anything like the same degree as larger units. In this respect, the larger and the greater the volume and mass of the unit, the more heat will be retained and for longer periods and the more minute-to-minute variations will be “evened out” (though as pointed out, adding more thermal storage externally, by means of circulating hot solar water systems is certainly possible).

Over the past 25 years we have used similar designs of housing from a few meters square up to structures 30M X 12M, and in every case, the larger units performed the best, however, even the smaller units offered considerable improvements in temperature compared to what could be achieved without them. These design concepts are very flexible, and can be easily adapted to different locations and situations. We have created near desert condition in wet and windy Wales, UK, and humid tropical conditions in the semi-deserts of Southern Spain… it really is as straightforward as understanding the principles involved and applying them imaginatively.

Arid habitat created in Wales, UK

A warm, arid habitat created in one of the wettest places in Europe without artificial heat or light.

The new UV-B transmitting flexible plastic sheet in combination with these designs is capable of transforming our expectations of how to keep reptiles in captivity. Few would dispute that where possible, some form of outdoor maintenance is best – and these designs and concepts greatly extend what is possible, even in colder, challenging climates. The less we have to rely on artificial light and heat sources the better too, not only for the animals subjected to them, but also from the point of view of our own environment. This takes us much closer to a genuinely 'naturalistic' method of husbandry using the best possible light and heat source out there for reptiles – the sun itself.

Thanks and acknowledgements

Special thanks to everyone at the Tortoise Trust Forum ( for testing various components of this design and for collecting very important data from such a wide range of climates and locations. Thanks also to Frances Baines for her comments, support and diligent testing of the UV-B properties of materials. The Tortoise Trust Forum also hosts a discussion area that concentrates upon practical housing methods and construction of terraria.

Practical construction - photo guide

                film to frame

                tools required

                  stapled to wooden frame

                  stapled to frame



Covers in

                frame latches

                fixing latch

                fixing latch

                frames with elastic tie

Thanks to Big Joe for construction photos.

License to use this design

This design is based upon original work carried out by A. C. Highfield and the Tortoise Trust. Keepers may use it freely. Any commercial use or sale of terraria based upon this design is prohibited without prior content in writing, however. If you wish to use this design in a commercial context us, please contact us for terms and conditions.


Copyright Tortoise Trust 2015. All rights reserved.