Metal Roof Heat in Malaysia: What the Roof U-Value Rule Asks For

What does the roof U-value rule actually say?

It names every building. By-law 38A(2) applies to the roof of all buildings, residential and non-residential, and caps thermal transmittance at 0.4 W/m²K for a light weight roof under 50 kg/m² and 0.6 W/m²K for a heavy weight roof above it, unless other shading or cooling means are provided. No floor area or air-conditioning threshold appears in it.

"The roof for all buildings (residential and non residential) shall not have a thermal transmittance (U-value) greater than — (a) 0.4 W/m2K for Light (under 50 kg/m2) weight roof; and (b) 0.6 W/m2K for Heavy (above 50 kg/m2) weight roof, unless provided with other shading or cooling means."

That instrument is a Selangor one, made under the Street, Drainage and Building Act 1974. Other states adopt the Uniform Building By-Laws separately, so a project outside Selangor should confirm the wording with the authority that will process its submission. Sub-paragraph (2) is broader on its face than sub-paragraph (1) beside it: the OTTV and RTTV duty in (1) is limited to large air-conditioned buildings, while the roof U-value duty in (2) names every building.

For a building owner the practical reading is short. A roof specification is not finished when the sheet profile and colour are chosen; it is finished when someone can show what U-value the whole build-up achieves, or that other shading or cooling means have been provided instead.

Is a metal roof a light weight or a heavy weight roof?

A metal roof falls in the light weight class. The by-law splits the two classes at 50 kg per square metre of roof: below that is light weight and carries the 0.4 W/m²K limit, above it is heavy weight and carries 0.6 W/m²K. The Melaka Green Seal Reference Guide glosses the same split in plain terms, as a light roof being a roof structure not from concrete and a heavy roof being a roof structure from concrete.

The split follows ordinary building physics. A heavy roof stores heat in its own mass and releases it slowly, so the same U-value produces a gentler indoor swing; a light roof has almost no thermal mass, so heat arriving at midday is heat arriving indoors at midday. The lighter roof carries the tighter of the two limits.

This matters when a building changes class. Replacing a concrete roof with a metal deck, or a tiled roof with a light gauge steel truss and profiled sheet, moves the roof from the 0.6 W/m²K bracket into the 0.4 W/m²K bracket, so the target tightens even though nothing else about the building has changed. Our guide on whether to repair, reroof or replace a metal roof covers how that decision is usually reached.

How does heat get through a metal roof?

In three stages. The sheet absorbs solar radiation and heats up, it conducts that heat through its thickness almost instantly, and its hot underside then radiates to whatever is below it. A steel sheet has very little thermal resistance of its own, so the resistance in the calculation has to come from what is installed under it.

The driver is relentless rather than extreme. The Malaysian Meteorological Department records that Malaysia receives about 6 hours of sunshine per day on average. A roof here is not coping with an occasional heatwave; it is absorbing solar radiation for several hours of nearly every day of the year.

The third stage is the one most often ignored. Radiant heat from the hot underside of the sheet crosses an air space without needing to warm the air first, which is why a ventilated roof space on its own does not settle the question, and why a reflective surface facing that air space changes the result more than its thickness suggests. The same mechanism is why choosing a metal deck roofing system is a decision about the whole assembly rather than about the sheet.

Does the colour of the roof sheet matter?

Colour decides how much of the sun's energy the sheet absorbs in the first place. The MS 1525 solar absorption factor table runs from 0.12 for a polished aluminium sheet and 0.25 for white semi-gloss paint through 0.47 for green paint to 0.88 for red brick or dark brown paint, on a scale topped by black glass at 0.99. A dark roof therefore takes in several times the solar energy of a pale one before any insulation is involved.

These values are published as Table 3 of MS 1525 (2019 revision) and reproduced in the Melaka Green Seal Reference Guide, second edition 2021, issued by Melaka Green Technology Corporation. That guide is a state green-building reference restating MS 1525, not the standard itself: for a submission, the governing figures are MS 1525's own.

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Colour and U-value answer different questions, so neither substitutes for the other. The solar absorption factor decides how much heat reaches the sheet; the U-value decides how much of it reaches the room. Both belong in the specification, and only one of them is named by the by-law.

What gets a metal roof's U-value down to 0.4 W/m²K?

The layers under the sheet. A roof's U-value is the inverse of the total thermal resistance of its build-up, so it is calculated as U = 1 / ΣR: add the thermal resistance of every layer from outside to inside, plus the surface resistances, and divide one by the total. Lowering the U-value means raising that total, and the sheet contributes almost nothing to it.

In a typical Malaysian metal roof there are only a few terms in that sum:

• The insulation layer. Its thermal resistance is the largest single term in most metal roof build-ups, and it is a property of the material and its installed thickness, not of its brand.
• The facing on that layer. A reflective facing addresses the radiant stage described above. It works by what it faces, so it needs an air space to face into and it needs to stay clean and uncompressed.
• The air space. An enclosed air space between the roof and the ceiling carries its own resistance, which is why a sheet fixed directly to a ceiling performs differently from the same sheet above a roof space.
• The ceiling. The board closing the roof space is the last layer in the build-up and counts in the sum like any other.

Because the number belongs to the build-up, a specification naming only the sheet cannot be checked against the by-law at all. Two roofs using the same profiled sheet can land on either side of 0.4 W/m²K depending on what sits beneath it, which is why the calculation, and not the product list, is the thing to ask for. The layer resistances themselves come from current manufacturer documentation, and the calculation for a given project belongs to its own engineer or energy consultant.

When do OTTV and RTTV apply as well?

To a narrower class of building. By-law 38A(1) requires new or renovated non-residential buildings with air-conditioned space exceeding 4,000 square metres to be designed to meet the requirements of MS 1525 with regard to the Overall Thermal Transfer Value and the Roof Thermal Transfer Value, and to be provided with an Energy Management System. Below that threshold, only the roof U-value limit in 38A(2) applies.

Where the threshold is crossed, the Melaka guide records the limits it restates from MS 1525 as an OTTV below 50 W/m² and an RTTV not exceeding 25 W/m². RTTV is described there as a measure of the heat transfer, or solar thermal load, transmitted through the roof of the building, so it accounts for rooflights and orientation in a way a single U-value figure does not.

The distinction is worth settling early, because it changes who has to produce what. A factory or warehouse with a small air-conditioned office is governed by the roof U-value alone; the same building fully air conditioned above 4,000 square metres brings a second set of calculations and an Energy Management System with it.

What should a roofing quotation tell you?

Enough to check the by-law without a second opinion. A roofing quotation should name the whole build-up rather than only the sheet, state the U-value that build-up achieves, and identify the roof as light weight or heavy weight so the right limit is being applied. Anything less leaves the compliance question open for someone else to answer later.

1. The full build-up, layer by layer: sheet profile and thickness, insulation type and installed thickness, any facing, the roof space, and the ceiling.
2. The calculated U-value for that build-up, in W/m²K, with the layer resistances it was worked out from.
3. Which class the roof falls in, light weight or heavy weight, so the applicable limit is explicit.
4. The sheet colour and its solar absorption factor, so the choice can be compared against the MS 1525 table above.
5. Whether OTTV and RTTV are in scope for the building, and if so who is producing those calculations.
6. How the insulation is held in place over purlins, since insulation that sags, compresses or tears loses the resistance the calculation assumed.

Ducero supplies light gauge steel roof trusses and carries out metal deck roofing, reroofing, strip ceiling, party-wall capping and wall cladding work. Roofing work is quoted from your drawings, so what comes back is the build-up proposed layer by layer and the sheet and colour it assumes; the U-value calculation for the submission, and any OTTV or RTTV work, belongs to the project's own engineer or energy consultant. Send the roof plan and the section through the enquiry form, and say whether the building is air conditioned and how large that space is.

Sources

• Government of Selangor Gazette — Selangor Uniform Building (Amendment) (No. 2) By-Laws 2012, Sel. P.U. 142, 27 December 2012, by-law 38A(1) and (2)
• Melaka Green Technology Corporation — Melaka Green Seal Reference Guide, second edition 2021, section 1.1 External Building Thermal Envelope (roof U-value, OTTV and RTTV limits; MS 1525 Table 3 (2019) solar absorption factors)
• Malaysian Meteorological Department — General Climate of Malaysia (average hours of sunshine, daily temperature range)
• Ducero — roofing scope, as listed in the company's own published project archive