Frame material is the greenhouse decision most often made on appearance, and that is a more defensible basis than it sounds, because the functional differences are smaller than the marketing on either side suggests. What genuinely differs is maintenance, how you attach things, and how the structure behaves over decades.
The thermal argument is usually overstated. Aluminium is an excellent conductor and a frame member does carry heat straight from inside to outside, which is a real thermal bridge. But the frame is typically only 5 to 12 percent of the glazed envelope area on a hobby house, so the whole-house effect is modest compared with the glazing choice or with sealing the door.
The comparison
| Measure | Aluminium | Timber |
|---|---|---|
| Maintenance | Effectively none | Treatment or paint every few years |
| Thermal bridging | Significant at the member, modest overall | Very low, timber is a poor conductor |
| Weight | Very light | Heavy, which helps stability |
| Fixings | Only where the profile allows, usually a channel | A screw anywhere along any member |
| Dimensional stability | Excellent | Moves with moisture, so glazing needs allowance |
| Service life | Decades, will outlast the glazing | Decades if maintained, much less if not |
| Rot and insect risk | None | Real at ground contact unless a durable species or treated |
| Cost | Lower for the same size | Higher, particularly in cedar |
| Assembly | Fiddly, many small fixings | Heavier but usually simpler |
| Repairability | Awkward, profiles are proprietary | Straightforward with ordinary timber |
The fixings argument, which is the strongest one
The practical difference growers notice most is not thermal and not cosmetic. It is that timber takes a screw anywhere.
Every greenhouse accumulates fittings: shelf brackets, hooks for hanging pots, a wire for string training tomatoes, a batten for shade cloth, a hook for a thermometer, a bracket for a fan. On a timber frame such as a U-MAX 6x8 Wood-Frame Greenhouse ($449.99), each of those is a screw wherever it is wanted. On an aluminium frame, each one depends on finding a channel, a proprietary bolt, or a clamp.
This sounds minor and it shapes how a greenhouse gets used over years. String training in particular, which is the single change that most increases tomato yield per square foot, needs an overhead wire carrying real load, and that is trivial in timber and a project in aluminium.
Where timber genuinely needs care
Timber in a greenhouse is in the worst environment for it: warm, permanently humid, splashed with water, and often in ground contact at the base. That is a rot specification rather than a storage one.
- Ground contact is the failure point. The base rail and the bottom few inches of any upright are what rot first. Standing the frame on a masonry or treated kerb rather than directly on soil addresses most of it, and it is far easier at build time.
- Species matters more than treatment. Cedar and other naturally durable timbers last far longer untreated than a softwood does treated, which is why cold frames such as the Backyard Discovery Aggie 4 x 2 Cedar Cold Frame ($349.00) are usually cedar.
- Treatment must be plant-safe. Anything applied inside a structure growing edible crops needs to be a product suitable for that use, applied and cured according to its label. This is a case where reading the label is not a formality.
- Movement affects glazing. Timber swells and shrinks with moisture, so glazing needs allowance for that movement rather than being clamped tight, in the same way polycarbonate needs allowance for thermal movement.
Where aluminium genuinely wins
- Nothing to do, ever. Powder-coated or mill-finish aluminium in a garden needs no attention across the life of the glazing it holds.
- Precision and squareness. An extruded profile is dimensionally exact, which means glazing channels are consistent and panels seat properly. A timber frame built slightly out of square makes rigid glazing a fight.
- Cost for the same footprint. Most kits at a given size are cheaper in aluminium, sometimes substantially, which is why the great majority of hobby kits are aluminium.
- Weight during assembly. One person can build most aluminium kits. A timber frame of the same size usually cannot be handled alone.
- No rot at the base, which removes the single most common structural failure of timber greenhouses.
The thermal bridging question, honestly
Aluminium conducts heat roughly a thousand times better than timber, so a bare aluminium member genuinely is a thermal bridge from inside to outside. The reason this matters less than it sounds is area.
On a small hobby house the frame is typically somewhere between 5 and 12 percent of the glazed envelope, and heat loss is dominated by the other 88 to 95 percent. Improving the frame from aluminium to timber therefore changes the whole-house figure by a few percent, while sealing the door properly can change it by up to 25 percent and moving from single-wall to twin-wall glazing changes it by nearly half.
The practical conclusion is to spend the attention on the glazing and the sealing, covered in insulating a greenhouse, and to treat frame material as a decision about ownership rather than about energy. If aluminium condensation running down the inside of the frame is a nuisance, a strip of bubble insulation on the inner face of the member addresses it locally.
A note on resin and steel
Two other frame materials appear at hobby scale. Resin frames, used on houses such as the Palram Canopia EcoGrow 6 x 8 Greenhouse ($1,049.99), are dimensionally stable, need no maintenance and do not conduct heat the way aluminium does, at the cost of some stiffness. Galvanised steel appears on tunnels and hoop houses, where it provides the strength a film structure needs at low cost, and it is not usually offered as an alternative to aluminium in rigid houses.
Neither changes the conclusion. Frame material determines maintenance, fixings and looks. Glazing, sealing and the systems inside determine what the greenhouse can grow.
Frames at both ends
Whichever frame you choose, what determines whether it stays standing is anchoring rather than material, and that is covered in anchoring a greenhouse against wind.