Rainwater HOG

Ground HOG: a unique combination of water storage and effective thermal mass

The Ground HOG is simply a Rainwater HOG used within a building structure for thermal mass. We just thought Ground HOG was such a great name we needed to use it! Like the Rainwater HOG, the Ground HOG is potable, and can be connected to reticulate and circulate the stored water within a structure. Alternatively, if you are simply seeking the highly effective thermal mass of Ground HOG, each HOG can be filled and permanently sealed with a threaded cap. An added bonus — just because you cap a HOG off now, doesn’t mean it can’t be uncapped later and reused for rainwater storage. It’s all part of the HOG modular, reusable design concept!

As a thermal mass unit, Ground HOG outperforms concrete of the same area by at least 10%. The Ground HOG is most effective when installed between floor joists immediately below, and ideally in contact with, the exposed internal floor surface. Insulating the underside of the HOG ensures that the stored heat or cool temperature doesn’t escape to the outside.

Ground HOGs can also be installed on their long side, around the perimeter of the floor area, and act as a thermal bridge from the internal temperature to the ground temperature beneath.?) Again, the key is effectively insulating those areas of the HOG which you are not using for thermal gain. Talk to us about what you are planning and we can help you explore new uses for the HOG.

Ground HOG Units

rainwaterhog dimensions

The modular nature of the Ground HOG means that you can have an installation ranging from one unit to over 100 units, depending on the size of the building. Each Ground HOG consists of the plastic tank unit which can be manufactured in any color (dependent upon volume ordered) and the connections between the tanks. Once the tank is installed and has been filled with water, the HOG is capped off with the provided plug. The Ground HOG will weigh approximately 440 lbs (200 kg) when full. Each Australian Ground HOG tank has a capacity of approximately 180 liters, or about 51 gallons for the American tank. The external dimensions of a Ground HOG are the same as the Rainwater HOG: Australian tanks 1800mm x 500mm x 241mm American tanks 71″ x 20″ x 9.5″

So I can drink from Ground HOGs?

Of course you can! The plastic is potable, and FDA approved. However, if you only want to use the HOG for thermal mass, talk to us about adding a recycled plastic component for added LEED points.

I want one now!

Check out our list of suppliers to find a local supplier who can meet your Rainwater HOG needs.

Ground HOG Thermal Performance

HOG Works asked Sequoien CIC to investigate the thermal mass performance of the Ground HOG as a low-embodied energy alternative to concrete.

Comparison of Ground HOG with concrete and lightweight insulation

…the Ground HOGs outperformed concrete by around 10% in the analysis and it can be concluded that they would do so to some extent under any circumstances. This is great news due to the low embodied energy of the full Ground HOGs, and leads to a great option for green builders to improve the environmental credentials of their design with a ready-made water tank solution that can be fitted discreetly in any number of locations.

Learn more about the principles of effective thermal mass in building

The Details:


The Rainwater HOG modular tank is rotomolded in virgin polyethylene which complies with AS2070. The HOG is molded of food grade plastic which complies with FDA and HPB regulatory standards for food contact.  However, Australian and USA law state that rainwater should not be used for drinking water. The water from your roof can be contaminated with animal and vegetable matter which needs to be filtered from stored water before it is fit for consumption. The fact that off grid communities and rural areas do use rainwater for drinking purposes demonstrates that it is possible if the right collection and filtration measures are taken. Generally in suburban and urban areas it is not permitted to harvest rainwater for drinking or other potable use apart from toilet flush and laundry wash. The tank material itself is 100 % recyclable and has been designed to last you a lifetime.


Each Rainwater HOG tank manufactured in Australia has a capacity of 180 liters, and the following dimensions: 500mm x 220mm x 1800mm. The HOG weighs approximately 18 kg when empty and 200 kg when full.

Each Rainwater HOG tank manufactured in the US has a capacity of 51 gallons and the following dimensions: 20” x 9.5” x 71”. The HOG weighs approximately 40 lbs when empty and 440 lbs when full.


The Rainwater HOG is currently available in the standard tank color of Bronze Olive Green, but can be produced in almost any other color for orders of 20 or more units. To name a few exciting color options, we have produced red, orange, black, white, and lime green HOGs. Please be advised that we do not paint the HOGs to make them different colors — the Rainwater HOG is manufactured by our factory in the chosen color.

Mix ‘n’ Match

The Rainwater HOG is designed to work with conventional connection components which are individually guaranteed by the manufacturers. But since our connection points are a brass 1 “ NPT /24mm thread, you can work with our suggested parts — or simply buy your own. Go to The Accessories page for more information.