Frequently Asked Questions
On this page we aim to answer all your questions, from product selection and configuration to delivery, installation and maintenance.
Can't find your question here?
Who manufactures Janssens greenhouses?
All Janssens greenhouses are produced by our own craftspeople at our production facility in Lier. With more than 75 years of experience, we rank among the leading hobby greenhouse manufacturers in Europe. Our greenhouses are exported worldwide. Find your nearest dealer for more information.
Can I order a custom-made greenhouse from Janssens?
Our dealers often display only our Action Models, greenhouses in fixed configurations. However, we also offer Custom Models: greenhouses that you configure yourself from a set of modular components. Looking for a greenhouse with non-standard dimensions or alternative features? Ask your local dealer!
Why are your greenhouses made from aluminium?
Aluminium is a lightweight, ultra-strong and durable material. It does not rust and requires minimal maintenance. Because aluminium is so strong even in slender profiles, we can make the glazing bars exceptionally narrow, allowing maximum light into the greenhouse.
The material is also 100% recyclable and therefore fully circular.
How do I choose the size of my greenhouse?
The size of your greenhouse naturally determines how much you can grow inside. A larger greenhouse not only provides more space, it also creates a more stable environment for your plants, the temperature is more consistent, and you’ll notice the difference in your growing results.
You’ll also benefit from more room to move and greater comfort, making crop rotation much easier to manage.
Can I add extra doors or windows to my greenhouse?
With our Custom Models, you choose the position, quantity and type of doors and windows yourself. The options depend on the model and size of your greenhouse.
Our Action Models are standard kits. A limited number of options can be added, but fitting additional roof windows or side windows is not possible
Should I choose untreated aluminium or powder coating?
Aluminium doesn’t rust, but it does oxidise. This means you won’t see the familiar orange-brown residue of rusted steel, but rather a white deposit. This mild oxidation is a natural process and has no effect on the strength or durability of the profiles.
The surface finish is therefore purely about the appearance of the greenhouse. The coating we offer at Janssens is always a high-quality powder coat finish, an electrostatic process in which the powder is cured in an oven.
How can I maintain my greenhouse?
Give your greenhouse a thorough clean twice a year. Make sure to avoid aggressive cleaning products.
Moving parts such as rollers and hinges may benefit from occasional oiling.
Close the roof windows during storms to prevent them from being forced open.
Do you have automatic openers? The wax cylinder can freeze in sub-zero temperatures, be sure to remove it when temperatures drop below freezing.
In the event of very heavy snowfall (rare in our climate), clear the roof of snow.
Can I use a greenhouse as a garden room or conservatory?
Greenhouses are increasingly being used as a second living space. If you want to set up your greenhouse as a lounge area, bear in mind the internal climate: significant temperature fluctuations, strong sunlight that can cause fading, high humidity in the shoulder seasons, draughts, condensation and water ingress at the base and in the roof. These are all characteristics inherent to a growing greenhouse.
Which glazing should I choose? Horticultural glass, safety glass or polycarbonate?
Janssens greenhouses are supplied as standard with 4 mm toughened safety glass. We also offer the option to choose polycarbonate.
Cheaper brands typically use traditional horticultural glass of 3 or 4 mm. This type of glass is more affordable, but also less strong, and when it breaks, it shatters into sharp shards.
The safety glass used by Janssens is up to seven times stronger. In the unlikely event it does break, it crumbles into small, harmless pieces.
Want to eliminate the risk of glass breakage entirely while also improving insulation? Then polycarbonate is a good choice. These plastic panels consist of clear twin-wall sheets. The material is extremely strong and resistant to impact from hailstones or a football, for example.
The twin-wall polycarbonate insulates better than glass, keeping temperatures more stable, ideal if you want to heat the greenhouse in winter.
The downside of polycarbonate? It is sometimes perceived as ‘cheap’ because the material is not fully transparent. On the other hand, it does diffuse incoming sunlight.
How do I prepare my garden for installation?
Think carefully about the location of the greenhouse, its orientation and the position of the door and windows. Please note: for lean-to greenhouses, the supporting wall must be straight and square. If you’re having the greenhouse installed by a professional team, ensure the ground is clear and level before the team arrives. Also ask about the necessary foundation work in advance (see the separate topic on this).
What should I do after my greenhouse is delivered?
If you don’t start assembly immediately, make sure the packages and the glass pallet can be stored in a dry location. Avoid at all costs any moisture getting into the packaging or between the glass panes: when heated by the sun or through evaporation, moisture can cause condensation on the glass or profiles, leaving a “haze” that cannot be removed.
For example: a glass pallet left for several days on damp ground (wet grass) in direct sunlight is not a good idea! The moisture that evaporates under the plastic packaging has nowhere to escape and also seeps between the glass panes.
I can see a white “haze” on the glass. How do I remove it?
You can try an intensive cleaning, but the chances are that what you are seeing is “glass corrosion”, and it cannot be removed.
When glass is stored in such a way that moisture (rain, condensation, humidity) gets into the packaging between the tightly stacked panes, the glass surface comes into contact with that moisture.
Please note: even when glass is covered, moisture can get into the plastic packaging depending on weather conditions or from below (ground, soil, grass, etc.). This means the moisture between the panes has a very high sodium concentration, forming a strong alkaline solution (Na⁺ OH⁻). At high pH levels, the glass network is attacked and sodium silicate is formed. This process can be accelerated by heat (when the sun warms the box of glass wrapped in plastic film). If this process continues within the packaging, the glass is damaged and the panes become bonded together by the sodium silicate (water glass).
This is not covered by the warranty; we therefore strongly advise storing packaging and glass in a dry place, protected from the sun, until the day of installation.
Do I need to set my greenhouse perfectly level?
The greenhouse can in principle be installed completely level. To encourage the flow of rainwater and meltwater towards the drainage pipes, you may give the greenhouse a very slight fall along its length, but do not overdo this (maximum 2–3 mm per metre).
Do I need planning permission to install a Janssens greenhouse?
Regulations regarding the installation of greenhouses and outbuildings vary by region. Always check with your local planning authority before purchasing a greenhouse.
Do I need a foundation?
The greenhouse must be placed on a sufficiently load-bearing (concrete) foundation that extends down to solid, frost-free ground. The dimensions given for our aluminium structures always refer to the outer dimensions of the construction. We recommend a concrete ring beam foundation matching the outer dimensions of the greenhouse. This must be perfectly level and smoothly finished so that the foundation frame can be mounted on it. The type of foundation (width, depth, composition) depends on the site conditions, ground type, region and the practices of your contractor. We recommend seeking detailed advice from your builder or contractor.
Our greenhouses are supplied as standard with an aluminium foundation frame including a ground anchor at each corner. In consultation with your contractor, it may be possible in certain cases (smaller greenhouses) to anchor only the 4 corners of the greenhouse, however, in that case the option of a low threshold at the doors is NOT possible.
I have white marks on my untreated aluminium profiles.
What you are seeing is oxidation of the aluminium.
When aluminium is exposed to water and oxygen, it oxidises very quickly. Most people recognise rusted steel by its familiar orange colour. Aluminium corrosion, by contrast, appears white. This process is known as oxidation, and the resulting aluminium oxide forms a thin, hard layer that actually protects the metal from further corrosion. It is completely harmless and has no effect on the strength or quality of the profile.
Why does Janssens use rubber seals with an anti-stretch cord?
Traditional rubber seals are stretched during installation, which means they inevitably shrink over time. To counter this, Janssens integrates an anti-stretch cord into the rubber, which helps prevent stretching during installation and significantly reduces subsequent shrinkage. However, some shrinkage cannot be entirely avoided. We therefore recommend pushing the rubber slightly inward during installation. This may initially cause small “bubbles” or “waves” in the seal flanges, but these will gradually disappear as the rubber settles and shrinks slightly over time.
Please note: shrinkage of the rubber is inherent to the material and has no negative effect on its performance or ability to hold the glass in place. It is purely a cosmetic issue and does not constitute a valid reason for rejection, claims or warranty.
I want polycarbonate panels instead of glass. What about condensation and moss growth?
For quality and durability, our 4 mm toughened safety glass is the best choice. If you want better insulation or more protection from the sun, you can opt for 10 mm twin-wall polycarbonate panels.
To drain condensation from the channels or cells of polycarbonate roof panels and regulate air circulation within the panel, we supply them as standard with AntiDust tape: sealed at the top, perforated at the bottom.
How do I choose the orientation of my greenhouse?
To prevent overheating, a southerly orientation is best, with the entrance of the greenhouse facing south. This way, your greenhouse receives maximum sunlight both in the morning and the evening, minimising the risk of overheating.
If you want to grow heat-loving plants such as cacti or succulents, a westerly orientation is preferable to give them maximum sun in the afternoon. The greenhouse will also be slightly warmer around midday in winter.
How do I ensure optimal ventilation in the greenhouse?
Ventilating your greenhouse is extremely important. Especially during the summer months, temperatures can rise significantly. Many of our models are fitted with roof windows and side windows/top-hung windows. An automatic opener is an affordable option that regulates airflow on warm days without any intervention on your part.
Can I heat my greenhouse in winter?
The average hobby greenhouse is primarily intended for growing plant species that cannot thrive outdoors in our climate: peppers, tomatoes, aubergines, sweet peppers, melons…
If you want to use the greenhouse to overwinter plants such as fuchsias and citrus trees, you will most likely need to heat it using a gas or electric heating system.
Heating a greenhouse is not inexpensive. Insulate the greenhouse using, for example, bubble wrap insulation. Alternatively, choose a greenhouse with twin-wall polycarbonate, which insulates better than single glass. Install a thermostat (positioned low to the ground) to maintain a constant temperature and reduce energy consumption.
How is my greenhouse delivered?
For deliveries from Janssens in Belgium and the Netherlands only!
After purchase and payment, you will receive an email order confirmation. Within a few working days, a member of our team will contact you to arrange a delivery date. You will then receive a confirmation of the appointment by email.
The delivery point must always be accessible by truck. If this is not the case, an alternative unloading point must be provided. Surcharges may apply for obstructions, pedestrian zones, narrow alleyways, etc.
Delivery is always unloaded next to the truck. The driver is not able to move the greenhouse further, carry it into the garden or take away packaging materials. If the driver has a crane, it can be used within reach (10 m). The driver is NOT permitted to drive on unpaved roads, fields or driveways — not even at the customer’s own risk.
Janssens does not carry out installations; please arrange this separately with your dealer or installer.
A typical delivery consists of:
- A standing glass pallet, 90 x 90 x 220 cm, 500 kg
- A package the width of the greenhouse x 25 x 15 cm
- A package the length of the greenhouse x 25 x 15 cm
What warranties do I have?
You benefit from the following warranty periods:
- 10 years on the quality and alloy of the aluminium
- 10 years on the paint finish (powder coating) in case of delamination, peeling, flaking, corrosion, chalking, discolouration and loss of gloss where the pre-defined tolerances are exceeded, in accordance with the Qualicoat or Qualinod specifications
- 10 years on the adhesion and tension of the glazing rubbers
- 10 years on production defects in the glass
- 5 years on PVC components and their UV resistance
The following damage is not covered by the warranty:
- Damage caused by failure to follow the installation and operating instructions
- Damage resulting from use other than the intended normal use
- Damage due to lack of maintenance
- Normal wear and tear
- Materials and items used at the customer’s specific request
- Structures not part of the Janssens range but manufactured at the express request of the customer or distributor
Our greenhouses are intended for growing vegetables, fruit, flowers, etc. The following characteristics are inherent and vary depending on the situation, environment and use:
- Condensation on glass or profiles
- Draughts at doors, windows or gaps
- Roof windows, roof connections and gutters are not 100% watertight
- No waterproof barrier beneath the foundation frame
Can I buy directly from you?
Our greenhouses are available exclusively through a network of authorised dealers. They guide you from advice and design through to purchase and installation.
How can I visit the Janssens showroom?
A visit to our factory showroom is possible, but by appointment only. This allows us to give you a personal welcome and dedicate enough time to walk you through our range.
Janssens also has a very extensive dealer network, so there is always a dealer near you where you can discover our greenhouses.
Visit our dealer page to find your nearest stockist.
My greenhouse gets too hot on sunny days, what can I do?
When the temperature in your greenhouse rises rapidly, it can be harmful to your plants. An automatic opener offers the solution: it opens and closes your roof ventilation or side window/top-hung window automatically based on the temperature, without any need for you to intervene.
How long does a Janssens greenhouse actually last?
A Janssens greenhouse lasts for decades, and you can see it in every detail: a self-supporting structure that never sags, materials that never rust or need painting, seals and drainage systems that always direct water outwards. A greenhouse that stands up to wind and weather, ready to be passed on to the next generation of gardeners.
How does an automatic opener work?
The opener works with a temperature-sensitive wax cylinder. As it gets warmer, the cylinder pushes the vent open. When the temperature drops, the vent closes again automatically. It responds purely to temperature changes, making it reliable and energy-efficient.