The Air Between: How Multiwall Sheet Insulates

Oct 30, 2026

U-values at a glance

Lower is better, in W/(m²·K).

Build-up	Typical U-value	Note
Single glazing	5.5–6.0	Almost no insulation
8 mm twin-wall	3.0–3.5	Close to double glazing
10 mm twin-wall	2.8–3.2	Common specification
16 mm multiwall	2.2–2.6	Greenhouse walls
25 mm multiwall	1.6–2.0	High insulation demand
PU sandwich panel	0.3–0.5	Cold-store grade; PC cannot match

Typical published ranges; confirm per datasheet. Three conclusions: multiwall matches double glazing at a fraction of the weight; thickness and wall count roughly halve the U-value from 8 mm to 25 mm; and it should not be compared with sandwich panels, since PC delivers light plus insulation while sandwich panels deliver insulation only.

Where heat actually escapes

Panel U-value is only half the ledger. Three details usually explain disappointing performance.

Frame thermal bridging. Aluminium conducts heat extremely well, and a single profile running from inside to outside can lose more than the panel itself. Use thermally broken profiles, or place an insulating pad between profile and sheet.

Unsealed ends. An open multiwall end lets cavity air circulate, which destroys insulation value and admits dust and water. End sealing is part of the insulation system, not an accessory.

Joints and laps. Air leakage at panel joints and edge closures often exceeds conductive loss. Gaskets, closure profiles and compatible tapes all matter.

Three practical benefits

Fuel savings in greenhouses. Night-time heat leaves mainly through the envelope; 16 mm multiwall walls and roofs combined with a thermal screen cut heating demand noticeably. The percentage depends on climate, structure and heating method, so it must be modelled per project.

Less condensation. A warmer inner surface means water vapour condenses less readily — which is why single-skin panels drip and multiwall panels largely do not.

Comfort underneath. Canopies, walkways and grandstand soffits in single-skin sheet are baking in summer and cold in winter; multiwall moderates both.

When insulation is not the priority

Hot climates where shading and ventilation matter more; purely decorative canopies and partitions; and open-sided car parks with free airflow. Where insulation competes with daylight, work out which costs more — in a greenhouse, yield lost to reduced transmission can easily exceed fuel saved.

Three questions to ask

What is the U-value for this thickness and wall count, and is there third-party test data; which profile do you recommend and is it thermally broken; and are end-closure profiles supplied with the sheet. The third is most often forgotten and does most of the real work.

FAQ

Can multiwall serve as a cold-store wall? No. Cold stores need sandwich-panel performance; multiwall is an order of magnitude higher in U-value.

Do more walls always insulate better? At equal thickness, usually yes, but transmission falls too — balance the two.

Why is my greenhouse still cold with multiwall? Check bridging and sealing first. Unbroken frames, open ends and leaky joints typically lose more than the panel.