How Much Roof or Ceiling Insulation Do I Need? A Thai Home Guide

Why Thai upstairs rooms get so hot

In many Thai houses, the space between the roof tiles and the ceiling heats up during the day. After sunset, that heat keeps coming down through the ceiling into the rooms below. The result is a bedroom that is still hot at 10 p.m., with an aircon that runs flat out.

Insulation over the ceiling, foil under the roof, or both are common ways to slow that heat down. This guide covers working out how much to buy. It can't tell you which product or system suits your house. That depends on the roof's construction, its ventilation and any moisture problems, and needs a site inspection.

Measure the area

Measure the ceiling you want to insulate, in metres. For an L-shaped or broken-up plan, split it into rectangles and add their areas together.

Subtract only areas that won't be insulated, such as an access hatch, or a space you're deliberately leaving clear.

Measure the gaps between ceiling joists as well. The width of a roll or slab has to suit that spacing, even when the total area works out.

Keep insulation clear of light fittings, flues and ventilation openings as the product instructions require. A simple area figure can't tell you whether those details are safe.

Turn the area into packs

Material needed = net area × layers × (1 + cutting allowance ÷ 100), then divide by the m² per pack and round up.

Example: a 2-bedroom upstairs, ceiling 8 × 10 m

Net area = 80 m² (no deductions)

One layer, with 10% for cutting → 88 m²

If the pack label says 5 m² per pack: 88 ÷ 5 = 17.6 → 18 packs

The 5 m² per pack is an example; coverage changes with product and thickness. ROCKWOOL’s Cavity Slab 32 data shows this variation, but that UK cavity-wall product is not a recommendation for a Thai ceiling or roof. Use the exact product and thickness label for your installation.

Before buying

Check the label: confirm the product name, thickness, roll or slab size, and the m² per pack.

Two layers of the same product: the calculator handles this. Calculate different products separately.

Don't squash it: don't compress insulation to make it fit a shallow space unless the manufacturer allows it. Squashed insulation works less well.

Wear protection: wear a mask, gloves and long sleeves when handling fibre insulation, as the instructions specify. A Thai roof space in the afternoon is extremely hot, so work early in the morning.

Ask about other fixes: if a contractor is doing the job, ask about roof ventilation and the existing ceiling's condition as well as the insulation.

FAQ

Will insulation lower my electricity bill? It can help, but the calculator doesn't estimate savings. The result depends on the roof, how you use the aircon and how the job is done.

Does fitting into the space mean the installation is right? No. The depth check in the calculator doesn't assess ventilation, moisture or electrical clearances.