Insulation is the most cost-effective way to reduce greenhouse heating costs. Every BTU of heat you prevent from escaping is a BTU you don’t have to pay for. Here are the proven methods, ranked by impact and ease of installation.
The single most effective and affordable insulation upgrade for any greenhouse. Horticultural bubble wrap (UV-stabilized, large-bubble) applied to the interior glazing surface creates a dead-air pocket that dramatically reduces convective heat loss.
A reflective thermal curtain drawn horizontally across the greenhouse at gutter height reduces the heated volume by cutting off the roof space at night. This can lower the volume of air you need to heat by 30–40%. Commercial greenhouses use motorized thermal curtain systems; hobbyists can achieve a similar effect with reflective emergency blankets, reflective bubble insulation, or heavy-duty painter’s drop cloths rigged on a wire system.
Heat loss through the greenhouse base is often overlooked. Rigid foam insulation board (1–2 inch XPS or EPS) buried vertically around the greenhouse perimeter to a depth of 12–18 inches blocks ground-conducted heat loss. For existing greenhouses, an above-ground skirting of insulation board covered by landscape fabric provides a simpler retrofit.
Doors and vents are the biggest sources of air infiltration. Weather stripping on door frames, magnetic door seals, and vent edge gaskets make a noticeable difference in overnight heat retention. Walk through your greenhouse on a cold, windy day and hold your hand near door edges, vent frames, and panel joints—any draft you feel is heat you’re losing.
For poly-covered greenhouses, inflating a double layer of poly film (using a small squirrel-cage blower that creates an insulating air cushion between the layers) roughly doubles the insulation value. The blower runs continuously but uses minimal electricity (10–15 watts). This is one of the highest-impact upgrades for hoop houses and poly tunnel greenhouses.
For an extra layer of protection on the coldest nights, drape floating row cover (frost blanket) directly over plants or over wire hoops on your benches. This “greenhouse within a greenhouse” approach adds 2–8°F of frost protection and costs almost nothing.
For heater sizing and recommendations once your insulation is in place, see our Best Greenhouse Heaters guide and Heating Efficiency guide.