Insulating and Heating a Garden Office or Summerhouse

‘Get the insulation right and everything else follows’ is a good axiom to follow whether considering a garden office or designing a new house. After six months in my office, one of my few regrets is not using more than the 100mm of insulation which I have in the floor, roof and walls. Its not that I am cold it’s just that, when the temperature is in single figures outside, the heating is continually cycling on and off and I would prefer a more even temperature.

The way I look at it is this; most rooms in a house are surrounded by other rooms, at least on two sides, possibly three as well as having another room above or below. Sitting in a garden office with nothing but walls, roof and floor between me and the elements I reason I must need more insulation than I have in my house. So, how much is that? Without getting too technical, it depends on the thermal performance of the insulating material, usually represented by its u value.

New houses have to be built to meet minimum u values but frankly, compared with the standards demanded in mainland Europe, the UK ones are pretty pathetic so, you would be well advised to insist on u values which exceed those currently listed in the UK Building Regulations. If your supplier doesn’t know about u values he probably doesn’t understand insulation but, as a guide, you should look for at least 100mm (4”) of insulation, preferably more.

You could, of course, have a building with minimal insulation and simply keep the heating on more during the Winter. Aside from the expense and discomfort, this approach falls down in the Summer. As well as keeping heat in, the insulation also keeps heat out. So, when the sun is blazing down, it won’t take very long for a structure with just 40mm of insulation to heat up and stay hot.

As for insulating materials, some environmentalists insist on only ‘natural’ materials but the decision is not entirely clear cut. For example, a natural material which is not as dense as a manufactured material will require a thicker wall and, therefore, more material. Also, although the energy required to manufacture synthetic insulants is a concern this ‘embedded energy’ is usually a lot less than the energy saved during their lifetime.

Heating the space throws up a number of options. It would be very attractive to use renewable energy but, by and large these are simply not economically viable. A wood burning stove is probably the most feasible renewable option but there are few stoves small enough and there is the hassle of firing it up in the morning and feeding it with logs as well as dealing with the ash. Ground or air source heat pumps are another option but are usually designed for much larger spaces and require electricity.

Since most people will need mains electricity to power computers, music centres and the like, the simplest option is electric heating and, of the systems available, underfloor electric is one of the most effective and easiest to install and operate. It is not the cheapest but will give a much more even heat than other point sources. A wind turbine to generate electricity or even photovoltaic roof tiles sounds great but again economies of scale mean that these are really not feasible for a small structure of this type.

The only free source of heat you can take advantage of is the sun. By having at least some windows facing South, you will capture some solar gain when the sun is out, even during the Autumn and Winter months. Finally, body heat and the heat put out by computers, printers and music centres etc. all mounts up especially if the space is well insulated.