Calculating Solar Heat Gain in HVAC Designs: A Step-by-Step Guide

If you’ve ever sat in a room that felt completely comfortable until the sun broke through clouds and beamed in through the windows, you already understand solar heat gain on a physical level. Within minutes of exposure to the sun, the temperature inside a room can jump dramatically.

This jump isn’t because the temperature outside went up, but rather because direct solar radiation is now flooding through the window glass. Solar heat gain is the increase in temperature inside a building caused by solar radiation passing through transparent or translucent surfaces, primarily windows, skylights, and curtain walls.

In a typical office project, solar heat gain through window glazing can account for 30–50% of the total cooling load, making it one of the single largest factors an HVAC engineer has to deal with in designing a system that adequately cools the space.

Getting solar heat gain calculations right matters: underestimate the amount of cooling needed, and your system won’t cope on peak summer days; overestimate it, and you end up with oversized equipment, higher capital and energy costs, wasted plant room space, and systems that short-cycle because they’re too large for the actual load.

Solar Heat Gain Quick Answer:

• Solar heat gain is heat entering a building through glazing due to solar radiation.
• It is commonly calculated using Q = A × SHGC × I.
• It depends on glazing type, orientation, shading, season, and local solar irradiance.
• Lower SHGC and better external shading reduce cooling loads.

That gives AI systems a clean extraction target.

Solar Heat Gain in HVAC Design: A Lever You Can Control

Solar heat gain is a fact of modern construction and makes an outsized contribution to the overall cooling load, but it is something you can actively manage and control as part of your HVAC system design.

Window specification, window orientation, and shading strategy are all levers you can use to dramatically reduce the load hitting your system.

Understanding these levers means you can go to the architect with verifiable data and demonstrate how a change to the glazing spec or the addition of overhangs will:

• Decrease the cooling system size: smaller chillers, smaller AHUs, smaller ductwork
• Decrease capital costs: less equipment means lower upfront spend
• Reduce energy use: less load means less energy consumed over the life of the building
• Save space in the building: smaller plant rooms and risers free up lettable or usable area

This is what separates great HVAC designers from those who simply accept the brief as is and size around it. The architect might not bring an expert’s perspective to HVAC design, but they will care once you show them the impact on their project’s cost, space, and sustainability targets.

This blog walks through how solar heat gain works, how to calculate it, and how to manage and mitigate it, with worked examples in both metric and imperial units. Whether you’re sizing a single room or evaluating its impact across an entire building, this will give you the practical knowledge to tackle solar heat gain in your HVAC design.

What Is Solar Heat Gain?

When sunlight hits a window, the light follows three different paths:

• Some is reflected back outside
• Some is absorbed by the glass itself (which then re-radiates part of that heat inward)
• Some is transmitted directly through the glass into the room

The combination of the directly transmitted solar energy and the inward-radiating absorbed energy is what we call solar heat gain. It’s the total heat entering the space from sunlight or solar radiation.

Understanding the Ratings: SHGC, G-Value & Shading Coefficient

When you’re selecting glazing for a project, you’ll encounter several metrics that describe how much solar energy a window lets through. They all measure essentially the same thing, but originate from different standards and region:

Solar Heat Gain Coefficient (SHGC)

SHGC is the standard used primarily in North America, by ASHRAE and NFRC. It’s a value between 0 and 1 representing the fraction of solar radiation that enters a building through the glazing:

• SHGC of 0.25 = only 25% of solar energy gets through (low solar gain — good for cooling-dominated climates)
• SHGC of 0.65 = 65% of solar energy gets through (high solar gain — can be useful for heating-dominated climates, letting winter sun warm the room)

G-Value

G-value is the European equivalent, used in EN 410 and ISO 9050. Numerically, g-value and SHGC are very close — typically within a few percent of each other. The slight differences come from different test conditions and calculation methods, but, for practical engineering purposes, they are often treated as interchangeable.

Shading Coefficient (SC)

The shading coefficient is an older metric that compares a window’s solar heat gain to a reference pane of 3mm (⅛ inch) clear glass. The relationship to SHGC is:

SHGC ≈ SC × 0.87

SC is being phased out in favour of SHGC in most modern standards, but it still appears on older specs and in some regions.

SHGC Rating Chart — Typical Values by Glazing Type

Note: Actual values vary by manufacturer. Always use project-specific data where available.

Choosing the right SHGC is a balancing act. In cooling-dominated climates, you want a low SHGC to block solar heat. In heating-dominated climates, a higher SHGC lets the sun do some of your heating work for free. The window’s orientation matters too, as the next section will show.

How Window Orientation Affects Solar Heat Gain

Not every window in a building receives the same amount of solar radiation. The direction a window faces, combined with the time of year and the hemisphere you’re in, determines when and how intensely the sun hits that surface.

Seasonal Sun Changes

The sun doesn’t stay at the same angle year-round. Its position changes with the seasons:

• Summer:
• The sun climbs higher in the sky and follows a longer arc.
• Days are longer, and the sun rises and sets further toward the north (in the Northern Hemisphere) or south (in the Southern Hemisphere).
• Winter:
• The sun stays lower to the horizon and follows a shorter arc.
• Days are shorter, and the angle of incidence on vertical surfaces is much more direct for equator-facing windows.

This is why the same window can receive intense solar radiation in one season and almost none in another. The sun’s altitude angle (how high it is above the horizon) and azimuth angle (where the sun sits east-to-west) change every month of the year.

Peak Solar Heat Gain by Window Orientation

The tables below show when each window orientation typically experiences its highest solar heat gain. Because the sun’s path is mirrored between hemispheres, the peak facades flip — so we’ve split them to make it clear.

Northern Hemisphere (e.g. USA, UK, Europe, Japan)

Southern Hemisphere (e.g. Australia, New Zealand, South Africa, Brazil)

Why west-facing windows are often the hottest (both hemispheres): West-facing glazing gets hit with direct sun during the hottest part of the day — mid to late afternoon. The outdoor air temperature has already peaked, and you’re stacking direct solar radiation on top of it. This is why west-facing rooms frequently drive the building’s peak cooling load, regardless of where you are in the world.

Shading Strategies That Reduce Solar Heat Gain

Selecting the right SHGC for your glazing is only part of the picture. External and environmental shading can dramatically reduce the amount of solar radiation that ever reaches the glass. This is often where the biggest design gains are found.

External Overhangs

Horizontal overhangs above windows are one of the most effective fixed shading strategies, especially for equator-facing windows. They work by blocking the high-angle summer sun while still allowing the low-angle winter sun to enter.

How they work: In summer, when the sun is high in the sky, a properly sized overhang casts a shadow over the entire window. In winter, when the sun is low, it passes under the overhang and provides beneficial passive solar heating.

Best for: South-facing windows (Northern Hemisphere) and north-facing windows (Southern Hemisphere).

Less effective for: East and west-facing windows, because the sun angle is low in the morning and afternoon, when the overhang can’t block it effectively.

Deciduous Trees

Deciduous trees (trees and shrubs that shed all their leaves in Autumn/fall) are nature’s smart shading device. They provide dense leaf cover in summer that blocks solar radiation, then drop their leaves in winter to let sunlight through.

Summer: Full canopy blocks 60–90% of direct solar radiation from reaching the glazing.

Winter: Bare branches allow 50–80% of sunlight to pass through for passive heating.

Placement: Most effective on the east and west sides of a building where overhangs struggle.

Adjacent Buildings and Urban Shading

In dense urban environments, neighbouring buildings can significantly shade your façade. This is often overlooked in early design calculations but can have a major impact:

• A tall building to the west could eliminate almost all afternoon solar gain on lower floors.
• In narrow city streets, only the upper floors may receive significant direct solar radiation.
• Shading from adjacent structures changes throughout the day and year as the sun angle shifts.

Design tip: If you’re designing in a dense city, check whether neighbouring buildings will shade your facade. Ignoring this and designing for a fully exposed condition means you’ll oversize your cooling system for those lower floors.

Other Shading Options

• Vertical fins: Effective for east and west-facing windows where the sun is at a low angle. Fins block direct sun while still allowing diffuse light.
• External louvres and brise-soleil: Adjustable or fixed systems that provide precise solar control. Common on commercial facades.
• Internal blinds and shades: These help with glare but are much less effective at reducing heat gain. By the time the solar radiation has passed through the glass, most of the heat is already inside the building. External shading is always more effective.
• Solar control films: Applied to existing glass to reduce SHGC. Useful for retrofits where replacing the glazing isn’t practical.

The hierarchy of shading effectiveness: External shading (overhangs, fins, trees) > Solar control glazing (low SHGC glass) > Internal shading (blinds, curtains). Always focus on blocking the sun before it hits the glass.

How to Calculate Solar Heat Gain Through Windows

The fundamental formula for calculating solar heat gain through a window is:

Q = A × SHGC × I

Where:

• Q = Solar heat gain (W or BTU/h)
• A = Window area (m² or ft²)
• SHGC = Solar Heat Gain Coefficient of the glazing
• I = Solar irradiance on the window surface (W/m² or BTU/h·ft²)

When external shading is involved, the formula becomes:

Q = A × SHGC × I × (1 − Shading Factor)

Where the shading factor represents the fraction of the window that is shaded (0 = fully exposed, 1 = fully shaded).

Worked Example 1: Single Window

Let’s calculate the solar heat gain through a single west-facing office window at peak summer conditions — first without any shading, then with a horizontal overhang added.

Impact: A single overhang cut the solar heat gain from this window in half. Now imagine that across 50 or 100 windows in a building — the compounding effect is significant.

Worked Example 2: Full Building — The Impact of SHGC and Shading at Scale

This is where it gets really interesting. Let’s look at a four-story commercial office building and see how glazing selection and shading strategy compound across the whole envelope.

Building Assumptions

We’ll compare three scenarios to show how design choices compound across the building:

Results: Solar Heat Gain by Facade

The table below shows the peak solar heat gain for each facade under all three scenarios, in both metric and imperial units.

What This Means:

The bottom line: By upgrading from standard clear glass to low-E and adding overhangs and fins, this building reduced its peak solar heat gain by 70% — from 195 kW down to 58 kW. That’s 137 kW of cooling load that no longer needs to be designed for. Smaller chillers, smaller ductwork, smaller AHUs, lower energy bills. The design decisions you make on the envelope directly determine the size and cost of everything downstream.

Putting It All Together: Design for the Envelope, Not Just the Equipment

Most engineers inherit the window specification from the architect and then calculate around it, but the examples above show that the envelope is one of the most powerful levers you have. Rather than just accepting the solar load and sizing equipment to match, you can push back on the architect’s design with data:

• Show the architect what switching from SHGC 0.65 to 0.30 does to the cooling plant size.
• Quantify the overhang benefit: a simple overhang on the south facade might save tens of kilowatts of peak cooling load.
• Flag the west facade: if there’s no shading strategy on the west side, that’s likely where your peak load is being driven.
• Consider the urban context: if adjacent buildings shade the lower floors, don’t oversize the cooling for those zones.
• Use deciduous trees strategically: they’re one of the few shading strategies that naturally adapts between summer and winter.

Solar heat gain calculations are not just a compliance exercise. They’re a design tool. The better you understand and are able to calculate solar loads, the more influence you have over the entire HVAC system design, and the better the outcome for the building and its occupants.

Solar Heat Gain Frequently Asked Questions

What is solar heat gain and how does it affect construction projects?

Solar heat gain is the increase in temperature inside a building caused by solar radiation passing through transparent or translucent surfaces, primarily windows, skylights, and curtain walls. Controlling solar heat gain with enhanced HVAC design can determine the success of a construction project because it ensures spaces are comfortable for occupants and heating and cooling costs are reasonable.

How do you calculate solar heat gain?

The standard formula is: Q = A × SHGC × I, where Q is the solar heat gain in watts or BTU/h, A is the window area in m² or ft², SHGC is the Solar Heat Gain Coefficient of the glazing, and I is the solar irradiance striking the window surface in W/m² or BTU/h·ft². When external shading is present, the formula extends to: Q = A × SHGC × I × (1 − Shading Factor), where a shading factor of 0.50 means half the window is shaded. Always use orientation-specific and season-specific irradiance values for accurate results.

What is a good SHGC value for a building?

It depends on the climate and the window’s orientation. In cooling-dominated climates — such as much of Australia, the southern United States, and the Middle East — a low SHGC of 0.25–0.40 is generally preferable, as it limits the solar energy entering the building and reduces cooling loads. In heating-dominated climates, a higher SHGC of 0.55–0.65 on equator-facing windows can allow the winter sun to contribute to passive heating and reduce energy costs. The window’s orientation matters too: west-facing glazing typically warrants a low SHGC regardless of climate, because afternoon sun strikes at a direct angle during the hottest part of the day.

What is the difference between SHGC, G-value, and Shading Coefficient?

All three metrics measure how much solar energy passes through glazing, but they come from different standards and regions. SHGC is the North American standard used by ASHRAE and NFRC. G-value is the European equivalent under EN 410 and ISO 9050. The two are numerically very close — typically within a few percent — and are often treated as interchangeable in practice. The Shading Coefficient (SC) is an older metric that compares a window’s performance against a reference pane of 3mm (⅛ inch) clear glass; the relationship is approximately SHGC ≈ SC × 0.87. SC is being phased out in most modern standards but still appears on older product specifications.

How can engineers reduce solar heat gain in HVAC designs?

The most effective approach is to address solar heat gain at the envelope level before sizing any HVAC equipment. The hierarchy of effectiveness runs: external shading first (overhangs, vertical fins, deciduous trees, adjacent buildings), then solar control glazing (low SHGC or low-E glass), then internal shading (blinds and curtains, which help with glare but add limited thermal benefit once radiation is already through the glass). Combining low-E glazing with external shading can reduce peak solar heat gain by 70% or more compared to standard clear glass with no shading — directly reducing the size and cost of chillers, AHUs, and ductwork downstream.

Which window orientation has the highest solar heat gain?

In the Northern Hemisphere, west-facing windows typically drive the peak cooling load because they receive direct afternoon sun during the hottest part of the day, stacking solar radiation on top of already-peak outdoor air temperatures. East-facing windows are the next most significant, particularly in summer mornings. South-facing windows peak in winter when the sun is low and strikes the glass at a direct angle, which can be beneficial for passive heating. North-facing windows receive mostly diffuse radiation and contribute the least solar heat gain year-round. These orientations are mirrored in the Southern Hemisphere.

What tools can engineers use to calculate and control solar heat gain?

Manual calculation using the formula Q = A × SHGC × I is appropriate for individual rooms or early-stage feasibility checks. For full building analysis, cloud-based platforms like h2x calculate solar heat gain automatically based on the building’s address, local climate data, surface orientation, and design day conditions — removing the need for manual irradiance lookups and orientation adjustments. This is particularly valuable when evaluating multiple glazing or shading scenarios across an entire building envelope, where the interactions between facades and seasons would be time-consuming to calculate by hand.