Thailand's Building Energy Code (BEC): OTTV, RTTV and LPD Limits — and Whether Factories Are Covered

A common scenario for manufacturers in Thailand: a new administrative building is designed for an existing factory site, drawings are submitted for a construction permit, and the application is returned because no energy assessment report is attached. This article sets out which buildings are captured, the numeric limits they must meet, and what options remain when a design fails one of them.

The regulation and when it took full effect

The instrument is the Ministerial Regulation Prescribing Types or Sizes of Buildings, and Standards, Criteria and Procedures for Designing Buildings for Energy Conservation, B.E. 2563 (2020) — the Building Energy Code, or BEC. It was published in the Royal Gazette on 12 November 2020 and phased in by descending floor area.

What gave the code enforcement teeth was not the Ministerial Regulation alone but the Building Control Committee Notification B.E. 2565, published in the Royal Gazette on 24 January 2023, which brought BEC into the permit process under the Building Control Act. Since 13 March 2023, failing BEC does not cost sustainability points — it blocks the construction permit. DEDE coordinates with the Department of Public Works and Town & Country Planning on this, and assessors must be trained and certified by DEDE.

Nine building types — and why factories are absent

The regulation captures new construction and alterations across nine types: theatres, hotels, entertainment venues, hospitals, educational institutions, offices or workplaces, department stores or shopping centres, condominiums, and public assembly buildings.

Neither production factories nor warehouses appear on that list. Industry is regulated through a separate instrument under the Energy Conservation Promotion Act B.E. 2535 (1992, amended 2007): the Royal Decree on Designated Factories B.E. 2540 (1997) and the Royal Decree on Designated Buildings B.E. 2538 (1995), which impose annual energy management and reporting duties rather than design review before permitting.

The trap industrial owners most often fall into is the site office building. If its combined floor area across all storeys reaches 2,000 m², it is fully captured by BEC as an "office or workplace" — regardless of sitting inside a factory perimeter.

flowchart TD
  A["New build / alteration"] --> B{"One of the 9 types
in the 2020 Regulation ?"}
  B -- "No: production, warehouse" --> N["Outside BEC
see Designated Factory
Decree B.E. 2540"]
  B -- "Yes: e.g. office" --> C{"Combined floor area
in a single building
≥ 2,000 m² ?"}
  C -- No --> N2["Not captured"]
  C -- Yes --> D["Assessment required
before construction permit
fully enforced 13 Mar 2023"]

The numbers: OTTV, RTTV and LPD across all nine types

Limits sit in the Ministry of Energy Notification on Standard Values for Energy Conservation Building Design, B.E. 2564 (2021). OTTV is overall thermal transfer through external walls into air-conditioned space, RTTV the equivalent through the roof, and LPD the maximum lighting power density. All three are ceilings.

The pattern is legible. Types occupied 24 hours — hospitals, hotels, condominiums — get the tightest envelope at OTTV 30 because heat load accumulates around the clock, but the loosest lighting allowance at 12 W/m². Types running roughly eight hours, such as offices and schools, get the loosest envelope at OTTV 50 but the tightest LPD at 10, because lighting dominates consumption during occupied hours.

Note also that RTTV is consistently about five times stricter than OTTV. An office may transfer 50 W/m² through walls but only 10 W/m² through the roof, since tropical roofs receive near-normal solar incidence for most of the day. The practical consequence is that budget spent on roof insulation and roof-sheet reflectance buys more compliance margin than glazing upgrades.

HVAC criteria and the fallback route

Six systems carry minimum criteria in total. Within air conditioning, split units up to 12,000 W must meet at least the current EGAT Label No. 5 seasonal efficiency threshold. Vapour-compression chillers are rated at 7.2 °C leaving chilled water and 32.2 °C leaving condenser water. The clause most often missed is that ancillary components — heat rejection, chilled water distribution and air distribution — must together draw no more than 0.5 kW per ton of refrigeration. That figure forces pump and fan sizing decisions at design stage rather than during installation.

When one or more systems fail their individual criterion, the whole-building energy criterion applies instead: model annual consumption and compare it against a reference building of identical floor area, orientation and per-face envelope areas. Consuming less over a full year constitutes compliance.

flowchart TD
  A["Select method
on form OrPhor.02"] --> B{"All systems pass ?"}
  B -- "Route 1" --> C["Verify 4 parts:
envelope OTTV RTTV
lighting LPD
air conditioning
water heating if present"]
  B -- "Route 2
some systems fail" --> D["Whole-building
energy criterion
vs reference building"]
  D --> E["Deduct solar output
PVE in kWh/year"]
  E --> F{"Annual energy below
reference building ?"}
  F -- Yes --> G["Pass"]
  F -- No --> H["Revise design"]
  C --> G

Route two is where renewables matter. Annual photovoltaic output (kWh/year) may be deducted from total building energy before the comparison, and renewable heat may be converted to an annual electrical equivalent and deducted likewise. Solar thermal air conditioning is exempted from the HVAC efficiency criteria altogether.

A further allowance is daylight-linked lighting. Perimeter zones designed for daylight may be treated as if no luminaires were installed there, directly reducing LPD — provided three conditions are met: separately switched perimeter luminaires, luminaires located no farther from the envelope than 1.5 times the window height, and glazing with an Effective Shading Coefficient of at least 0.3 and a Light to Solar Gain ratio above 1.0.

Practical guidance

1. Count area per building, not per project. The 2,000 m² threshold applies to all storeys within a single building, so massing decisions taken at master-plan stage genuinely change legal status — but they must be taken then, not at submission.
2. Choose the compliance route before construction drawings. The per-system and whole-building routes need different data sets; deciding late means re-collecting equipment specifications.
3. Spend on the roof before the walls. With RTTV roughly five times stricter than OTTV, insulation and roof reflectance return more compliance per unit cost than glazing.
4. Write the 0.5 kW/ton ancillary limit into the MEP tender. Otherwise contractors will bid pumps and fans that clear on price but not on law.
5. If rooftop solar is planned, include it in the original assessment. Output deducted from whole-building energy only helps the permit that cites it; retrofitting later does not reopen an approved application.
6. Verify the assessor holds current DEDE certification, since reports from uncertified assessors are not accepted at permit stage.

Since 13 March 2023, BEC has been a permit condition rather than a voluntary sustainability exercise. For factory owners two points matter: production buildings fall outside the nine types, while site offices of 2,000 m² and above fall squarely inside — and failing a per-system criterion is not a dead end, because the whole-building route allows compensation through better HVAC and on-site solar generation. Sahawatthanakit (1988) works with project owners on the two levers that move these numbers most: roof insulation and roofing materials affecting RTTV, and rooftop solar design whose annual output is deductible from whole-building energy.

Frequently Asked Questions

Are manufacturing plants covered by Thailand's BEC? Not the production building itself — the nine listed types do not include factories. Industrial facilities fall under the Royal Decree on Designated Factories B.E. 2540 within the Energy Conservation Promotion Act B.E. 2535, which requires annual energy management reporting to DEDE rather than design review at permit stage. The exception to watch is the site office: at 2,000 m² combined floor area it is captured by BEC as an office or workplace.

What happens if the envelope fails the OTTV limit? The whole-building energy criterion provides a second route. Annual consumption is compared against a reference building matching the proposal in floor area, orientation and per-face envelope areas; consuming less over a year constitutes compliance. That reference building must itself meet the envelope, lighting and air-conditioning criteria, so the comparison is against a compliant baseline rather than an arbitrary one.

Does rooftop solar help a building pass BEC? Only on the whole-building route, where annual PV output in kWh/year is deducted from total energy before comparison. It does not make OTTV, RTTV or LPD pass on the per-system route, since those are independent criteria — so a project already committed to per-system compliance gains nothing legally by adding solar.

Why is the RTTV limit roughly five times stricter than OTTV? Because tropical roofs receive near-normal solar incidence for most of the day, giving far higher irradiance per unit area than vertical walls. Offices are allowed 50 W/m² through walls against 10 W/m² through the roof, which is why roof insulation and roof-sheet reflectance carry disproportionate weight in achieving compliance.

How does BEC differ from ASHRAE 90.1? ASHRAE 90.1 is a voluntary US standard covering nearly all buildings except low-rise residential, with prescriptive, Energy Cost Budget and Appendix G compliance paths. BEC is Thai law binding only nine building types above a size threshold, and is built on OTTV/RTTV metrics developed for hot, humid climates. Projects pursuing LEED in parallel with permit approval must therefore run two separate calculation sets.