Why the Top-Floor Office Overheats: Roof, Glazing and AC

The top floor is a different building

Every building has one floor that the facilities team hears about every summer. It is the top floor, or the floor with the west-facing glass, or the one under a row of skylights. People move away from certain desks by mid-afternoon. Meeting rooms are avoided. The AC runs flat out and the compressor trips. Someone asks for the thermostat to be lowered, and nothing improves.

The misconception is that this is an AC problem that more tonnage will fix. It is mostly a heat gain problem. The top floor sits directly under a roof slab that absorbs sun all day, and that heat arrives in the space in ways the floors below never see. If the AC was selected as if it were a typical floor, which is common in speculative office buildings and in fit-outs that copy the floor below, it is undersized by design.

For a business the cost is not only discomfort. It is rent paid for space people avoid, an AC plant running at its limit and failing in the hottest weeks, and money spent on the wrong fix.

Where the heat comes from

Two figures describe how much heat a building element lets through. The U-value of the roof build-up describes how readily heat is conducted through it; an uninsulated concrete slab conducts far more than an insulated one. The solar heat gain coefficient of the glass describes how much of the sun striking it ends up inside the room. Both should be part of any diagnosis.

Why feeling hot is not only air temperature

A hot ceiling makes people feel warm even when the air in the room is at the set point. Comfort depends on the temperature of the surfaces around a person as well as the air, along with humidity, air movement, activity and clothing. ASHRAE 55, the standard for thermal environmental conditions for human occupancy, treats thermal radiation as one of the factors that decide whether a space is comfortable.

That is why the occupants on a top floor often say the AC "is not working" when the thermostat reads normal. It is also why lowering the set point helps less than expected: the air gets colder, but the ceiling is still radiating heat, and the people near the glass are still sitting in the sun.

Measure before spending

The fixes are different for each source, so the first step is to find out which one dominates. A practical diagnosis covers:

• Temperature logging on the top floor and a typical floor through a hot day and into the evening, to see when the gap opens and how long it lasts
• Surface temperatures of the ceiling, the underside of the slab and the glass at their hottest, using an infrared thermometer or thermal camera
• Roof inspection: is there any insulation above the slab, what is the surface colour and condition, and where are the skylights
• Ceiling void inspection: duct insulation, gaps and damaged vapour barriers, and whether the void is used as a return air plenum
• AC performance: supply air temperature and airflow at diffusers, and whether the outdoor units are shaded, spaced and serviced
• A heat load calculation for the actual floor by the MEP engineer, using what was found rather than typical-floor assumptions

This is also the point to check whether the complaint is really about heat. Stuffy afternoons are often a fresh air problem, not a cooling one, and the two need different fixes.

The options, and what each one solves

The order matters. Reducing heat gain first makes any AC change smaller, and an AC upgrade sized before the roof is insulated may end up oversized afterwards.

Roof insulation is usually the most direct fix where the slab is bare. Insulation above the slab, protected by a screed or tiles, keeps the heat out of the concrete altogether. Insulation below the slab is often the only option a tenant has, and it still reduces what the ceiling radiates into the room. The options are covered under wall and roof insulation.

Cool roofs work by reflecting sunlight before it is absorbed. ECBC 2017 includes provisions for cool roofs and roof thermal performance for buildings within its scope; whether they apply to an existing building is for the project's consultants. Reflective products are compared using solar reflectance, thermal emittance and the solar reflectance index, calculated under ASTM E1980. Dust and pollution reduce reflectance over time, so maintenance is part of the decision.

Glazing fixes only address the glass. Where large sun-facing windows or skylights are a major source, film, shading or replacement each have their place; see solar control film versus replacing office glazing.

AC changes close whatever gap remains. That may mean rebalancing airflow so the hottest zones get more supply air, splitting a large zone so the perimeter and the core are controlled separately, or adding capacity. How the system is designed is covered under HVAC systems design. Air conditioning provisions for buildings are in NBC 2016, Part 8, and the design for a specific floor is for the MEP engineer.

Common mistakes

• Adding tonnage first. The room gets colder air and stays uncomfortable under a hot ceiling.
• Copying the typical floor's AC design onto the top floor in a fit-out.
• Lowering the thermostat across the floor to fix three hot desks.
• Ignoring the ceiling void, where bare or damaged duct insulation heats the supply air before it reaches anyone.
• Coating the roof without checking the waterproofing. A coating that traps moisture or is incompatible with the membrane creates a leak.
• Filming every window on a floor whose main problem is the roof.
• Forgetting the evening. The slab keeps releasing heat after sunset, which is when late meetings happen.

What to ask before choosing a fix

• Has anyone measured where the heat is coming from, on a hot day?
• Is there any insulation on or under the roof slab now?
• Was the AC for this floor sized from a heat load for this floor, or copied?
• Are ducts and return air in the ceiling void insulated and sealed?
• Which works need the landlord's consent, and who pays for roof works?
• Will the waterproofing be affected, and who is responsible for it afterwards?
• After the heat gain is reduced, what AC change is still needed?

Standards referenced

Building envelope provisions, including roofs, cool roofs and fenestration, in ECBC 2017; thermal comfort in ASHRAE 55; calculation of solar reflectance index in ASTM E1980; air conditioning and ventilation in NBC 2016, Part 8. Heat load calculations, insulation and roof build-ups, AC design, and any structural check for added weight on the roof must be established by the project's MEP engineer, structural engineer and consultants for the specific building.