Polycarbonate panels are a popular choice for greenhouse construction due to their combination of light weight, impact resistance, and good thermal insulation properties. Securing these panels correctly is critical for ensuring the structural integrity, longevity, and weather resistance of the greenhouse. Proper attachment methods prevent panels from being lifted by wind and maintain the insulating air pockets within multi-wall sheets, ultimately creating a stable environment for plants. The process involves careful preparation of both the panels and the frame before applying specific fastening and sealing techniques.
Preparing the Panels and Frame
Preparation focuses on protecting the material and accommodating its unique physical properties. For multi-wall panels, the open, fluted edges must be sealed to prevent the entry of dust, moisture, and insects, which reduces light transmission and encourages mold growth. The top edge should be sealed with specialized aluminum foil tape to create a barrier against water and debris. The bottom, or lower-slope, edge requires a vapor-permeable breather tape, allowing internal condensation to escape while filtering out contaminants.
When panels require direct fastening, pre-drilling holes is mandatory to prevent cracking during installation. Holes must be drilled slightly larger than the fastener shaft, typically 2 to 3 millimeters wider, to allow for thermal expansion and contraction. Drill these oversized holes in the solid areas of the panel, ensuring placement between the internal ribs of multi-wall sheets. Maintain a minimum distance of about 1/2 to 1 inch from the panel edges to avoid creating a weak point.
The greenhouse frame requires inspection to ensure structural members are ready to receive the panels. For wood frames, checking for squareness and ensuring the wood is clean provides a strong foundation for sealing and attachment. If joining panels, the frame members at the seam should provide a surface approximately 3 inches wide. This width properly supports necessary joining profiles, such as H-channels or aluminum extrusions.
Specific Fastening Methods Based on Frame Material
The method used to secure polycarbonate panels depends heavily on the frame material, utilizing techniques that either compress the panel edges or fasten through the surface. For aluminum frames, fastening relies on specialized systems that minimize direct drilling. These systems utilize aluminum profiles, glazing bars, and rubber gaskets to hold panel edges in place. In kit greenhouses, panels often slide into channels and are held by spring clips (such as W-clips or Z-clips), which apply light pressure to keep them seated against the frame.
Glazing seals or gaskets are inserted into frame channels before the panel is set to provide a cushioned, watertight seal and prevent rattling. Aluminum capping systems are also used, screwing down over the panel edges and relying on compression against a gasket to secure the panel without piercing its surface. These systems are preferred for aluminum structures because they allow the panel to move slightly, accommodating thermal expansion without stress.
For wood, PVC, or other non-metallic frames, direct fastening with screws is common. Use self-tapping screws (stainless steel or galvanized) with a large-diameter, non-metallic washer. A neoprene or EPDM rubber-backed washer is necessary to spread the load and prevent crushing the polycarbonate. Fasteners must be driven carefully, tightened only enough to secure the panel firmly against the frame, but never overtightened, which restricts thermal movement. Using wood battens or molding strips to sandwich the panel edges provides superior wind resistance by creating a clamping force along the joint.
Addressing Thermal Movement and Sealing
The significant difference in the thermal expansion coefficient between polycarbonate and common frame materials necessitates specific provisions to prevent buckling. Polycarbonate expands and contracts at a much greater rate than aluminum or wood; restricting this movement causes the panel to warp or crack when temperatures change. It is essential to leave a small expansion gap between panels where they butt together and where the panel meets the frame members.
A gap of about 3 to 5 millimeters is recommended between panels to accommodate movement, especially when using joining channels or profiles. Failure to provide these small gaps can lead to panel distortion and structural failure as the material heats up and expands.
Long-term weatherproofing involves sealing joints and connections using compatible sealants. Any sealant used must be a non-acid cure silicone, as acidic or solvent-based sealants degrade polycarbonate over time. Apply sealant along all panel joints, around fixings, and at intersections with the frame to create a moisture-proof barrier. Reinforcing the structure against wind uplift is also important, especially on roof panels, which can be accomplished by adding extra wind-load fasteners or securing edges with additional battens.