Sun, Shading and Radiant Heat on a House

Why this matters

The call comes in a week after the tree crew left, or a month after new windows went in: one room that used to be fine is now hot every afternoon, and the thermostat in the hall says everything is normal. The HVAC tech finds nothing wrong with the equipment, because nothing is. The load changed. Sun through glass and sun on the roof are the largest swing loads on most houses in summer, and three trades that never touch a duct (window installers, roofers and tree crews) move them more than anyone else. This card is about how big those loads are, which way each trade's work pushes them, and how to read the two numbers that decide most of it. The heat transfer physics itself belongs to Conduction, Convection and Radiation in the Field.

Where the sun hits, and when

Sunlight arrives as a direct beam plus diffuse light scattered from the sky. On a clear day the beam is most of it, and how much of the beam a surface catches depends on the angle between the surface and the sun.

• West glass gets the low afternoon sun nearly head-on, and it gets it in the hours when outdoor air is hottest and the house has been soaking up heat all day. In summer this is usually the worst orientation for overheating.
• East glass catches a similar beam in the morning, but the outdoor air is cooler and the house is starting from its overnight low, so the same gain causes less trouble.
• South glass in summer sees the sun high overhead, so the beam strikes at a steep angle and much of it reflects off; a modest roof overhang shades it at midday. In winter the low sun reaches deep into south rooms, which is useful free heat.
• North glass sees mostly diffuse light in summer and is rarely the cause of an overheating room.

That ordering is for the continental US and similar northern latitudes. Overhangs work on south glass because the summer sun is high; they do almost nothing for west and east glass, where the sun is low enough to come in under any practical overhang.

The window label's solar number

Every rated window carries a National Fenestration Rating Council (NFRC) label. Two numbers on it matter for this subject:

• Solar heat gain coefficient (SHGC) is the fraction of the sunlight striking the window that ends up as heat inside, both transmitted directly and absorbed by the glass and then released inward. It runs from 0 to 1; lower means less solar heat. It is rated under NFRC 200 for the whole window, frame included, so it is applied to the whole window's area.
• U-factor is the conduction rate through the window per degree of temperature difference. It governs the window's heat loss on a cold night and its conducted gain on a hot day, but it says nothing about sunlight.

The label also gives visible transmittance (how much daylight comes through), which is why a low-SHGC window can still look clear. Low-emissivity (low-e) coatings come in versions tuned to block most of the solar heat while passing visible light, and versions that let more solar heat through for cold climates where winter sun is wanted. Two low-e windows can carry very different SHGC numbers, so read the label rather than the sales sheet.

Worked example: two west windows and a tree

A bedroom on the west wall has two windows, each 3 ft by 5 ft, so 15 sq ft each and 30 sq ft together. A large maple shaded them until the tree service took it down. The room is served by supplies totaling about 150 cfm.

Step 1, the incoming sun. On a clear late-July afternoon around 40 degrees north latitude, the sun striking a west-facing vertical surface peaks at roughly 200 Btu/h per sq ft. That is an approximate clear-sky peak; use it to size the problem, not to design equipment (the ASHRAE Handbook, Fundamentals tabulates clear-sky values by orientation and latitude), and expect less on a hazy day and somewhat more farther south.

Step 2, the old windows. The existing clear double-pane units carry an SHGC of about 0.60 (illustrative for that construction; read the actual label where one survives).

• Per window: 15 sq ft x 0.60 x 200 = 1,800 Btu/h
• Both windows: 1,800 x 2 = 3,600 Btu/h

Step 3, what the room's air can carry away. Sensible cooling delivered by supply air is 1.08 x cfm x the temperature difference between room and supply air, where 1.08 holds for air near standard density (close to sea level; it runs lower at altitude). With a 20 F difference:

• 1.08 x 150 x 20 = 3,240 Btu/h

Step 4, compare. Solar gain through the two windows alone, 3,600 Btu/h, exceeds the 3,240 Btu/h the supplies can remove by 360 Btu/h, before counting the walls, the ceiling, the people or a computer. The room cannot hold setpoint during the peak hours, and it was never designed to: with the maple standing, the direct beam on that glass was mostly blocked and the gain was a fraction of this.

Step 5, the alternative the customer is actually choosing between. Replacement windows with an SHGC of 0.25 cut the gain to:

• Per window: 15 x 0.25 x 200 = 750 Btu/h
• Both: 1,500 Btu/h, which is 1,500 / 3,240 = 46 percent of the room's supply capacity

That leaves the supply air headroom for everything else in the room. Exterior shading on the existing windows (an exterior solar screen, an awning, a new tree that will take years) reduces the beam before it reaches the glass; interior blinds help less, because the sun has already passed through the glass and is absorbed by the blind inside the room.

What a tech who skipped the arithmetic would do. Add a supply run, upsize the duct to that room, or tell the customer the unit is undersized. A bigger duct might add 50 or 60 cfm, which at the same 20 F difference is about another 1,100 to 1,300 Btu/h, so the room would just about hold on the hottest days while every other room loses airflow to feed it. The glass is the problem and the glass is where the fix is.

Why a room feels hot even when the air is not

Sun-struck glass and an attic-heated ceiling get warm, and a warm surface radiates heat to anyone near it. A person sitting beside a hot west window feels warm at an air temperature that would be comfortable elsewhere. The thermostat in the hall measures air, not that radiant load, so it reports satisfaction while the occupant reports discomfort. Both are telling the truth. When a customer says a room is hot and your thermometer says it is at setpoint, put your hand near the glass or the ceiling before you conclude they are imagining it.

Roof heat and the attic

A roof in direct sun absorbs most of what hits it, and a dark roof absorbs more than a light one. The deck underneath heats up, and on a sunny summer afternoon the attic air commonly runs well above outdoor air temperature. The deck then radiates heat downward onto the insulation, the ceiling drywall and, in many houses, the ductwork.

That last part is the expensive one. A supply duct in a hot attic gains heat along its whole length before the air reaches the room, so the farthest rooms on the longest runs get the warmest air. A return duct leak in that attic pulls in attic air at its full temperature. See related: Duct Leakage Outside the Envelope.

Radiant barriers are thin, highly reflective, low-emissivity foils, usually installed on the underside of the roof deck or laminated to the sheathing. They work by emitting very little radiant heat from their surface, so the hot deck above passes less heat down across the air space below. Three conditions decide whether one works:

• It must face an air space. A foil pressed tight against insulation or drywall conducts heat across the contact and loses most of its effect.
• It must stay clean. Foil laid face-up on the attic floor collects dust, and dust on a low-emissivity surface raises its emissivity. Deck-mounted foil facing down stays clean.
• It pays most in hot, sunny climates with ducts in the attic. In a cold climate, or in an attic with no ducts and deep insulation, the gain is small.

Cool roofing (shingles or membranes with high rated solar reflectance) attacks the same load at the outer surface. Rated reflectance and emittance are published per product under the Cool Roof Rating Council's rating program; read the product's rating rather than judging by color, since some dark-looking granules are formulated to reflect more of the infrared part of sunlight.

Which trade moves which load

The trade doing the work rarely sees the effect, because it shows up weeks later as an HVAC complaint. A tree crew that tells a customer "the west rooms may run warmer this summer" before cutting, and a window installer who notes the SHGC of the units installed on the invoice, save the next tech a wasted diagnostic visit.

What changes the answer

• The windows face east instead of west. The peak gain lands in the morning, when outdoor air is cooler and the house is at its coolest, so the same 3,600 Btu/h causes a shorter, milder overshoot and may never trigger a complaint.
• The climate is heating-dominated. Cutting SHGC on south glass to fix a summer complaint gives up winter solar heat. A low-SHGC unit on the west and a higher-SHGC unit on the south is a reasonable split; one SHGC for the whole house is not always the best choice.
• The ducts are inside the conditioned space. Then roof heat reaches the rooms only through the insulated ceiling, and radiant barriers and cool roofs have much less to offer.
• The room is on the top floor under a low-slope or cathedral ceiling. There is little or no attic air space, so roof surface temperature drives the ceiling almost directly, and roof color matters more than in a vented attic.

How to verify the diagnosis

• Note the orientation of every window in the complaint room and read the NFRC label if it is still on the frame or the original paperwork exists.
• Measure supply air temperature and room temperature at the peak hour, and estimate the room's sensible capacity the way Step 3 does.
• Check the glass and ceiling surface temperatures with an infrared thermometer at the same hour, and compare them with a north wall in the same room.
• Ask what changed: a tree, a roof, windows, a new skylight. If nothing changed, the problem was always there and the customer just started noticing it.

References

• National Fenestration Rating Council, NFRC 200 (solar heat gain coefficient and visible transmittance rating procedure) and the NFRC label format
• Cool Roof Rating Council, product rating program for solar reflectance and thermal emittance
• ASHRAE Handbook, Fundamentals, chapter on fenestration, for clear-sky solar irradiance by orientation and latitude
• See related: Conduction, Convection and Radiation in the Field; Which Trade Jobs Change House Pressure and Moisture