Greenhouses consist either of full-height frames glazed or covered with transparent material down to ground level, or else of a glazed frame raised up on low supporting walls of either timber or brick. When they’re designed with this skirting wall they are often called planthouses or vinehouses.
Modern commercial units are almost always full height to admit the maximum amount of illumination per square foot. That is, to get the highest production out per pound invested. These lightweight full-frame greenhouses are also the cheapest to install.
Because farm land is frequently re-purposed, the fact that these constructions are not very long lasting is considered as much an advantage as a disadvantage. Brick buildings would be more expensive to demolish when the wheat market becomes more profitable than cucumbers and cut flowers. In recent years these swings in land use economics have unfortunately been commonplace.
For the home grower, permanence, appearance and multi-functionality are usually more important.
Planthouse style greenhouses have a host of other practical advantages that appeal more to the home gardener (and his or her family) than they do to professional farmers. Appearance is certainly not the least of them – a planthouse is a step closer to a sun-lounge and if tastefully designed will create extra leisure space and add value to your property.
Frames over a moderate area can only have a moderate height without becoming unstable in high wind. Fixed to a dwarf wall they have the stability to support extra headroom – far better for both people and taller or hanging plants. Extra headroom can also prevent the tops of tall plants being scorched on especially hot days.
The headroom provides clear views from windows and allows you to design doors into whichever side you like, instead of being confined to the gable. It becomes practical to fit double doors if you prefer – both more elegant and more convenient for wheelbarrows, lawnmowers and rotavators.
Walls are usually about 2’ and constructed of either timber or brick. Although logically an aluminium frame should be able to keep out pests such as rats, experience suggests you are safer with a brick, or substantial wooden footing. Passing mowers are a particularly good way of taking out a few ground level glass panes, as are boots and tools, and that’s all they need.
Aluminium frames that aren’t seated on a concrete foundation are obviously no defence against burrowing animals (and that’s most of them by the way, even some birds will dig to reach fruit).
Cedar panels at the bottom are a popular choice. Cedar is naturally repellent to most insects and rodents, as well as having good resistance to moisture and rot. Both timber and brick have the useful feature of absorbing heat when it is hot during the day and releasing it slowly in the evenings. Cedar will keep your planthouse an average of something like 3 degrees warmer throughout the year, and brick will likely do better.
In larger planthouses, consider designing drainage into the floor. If you plan to plumb in a tap, a drain beneath it makes sense. If you are wiring in electric switches and sockets, keeping them clear of floors that could flood is an essential safety precaution and dwarf walls make it much easier to do this.=