Upstairs too hot? Start in the attic

Why is the upstairs too hot? Two Bay Areas in July

Ask someone in Daly City and someone in Livermore whether their upstairs gets hot, and you'll get two different answers. The California Energy Commission splits the cities we serve into four building climate zones, and summer is where they part ways.

• The fog side (climate zone 3). San Francisco, Daly City, Oakland, Berkeley, Alameda, San Mateo, Redwood City and most cities along the Bay. Marine air keeps many afternoons mild. A hot upstairs here usually shows up on a few heat-wave days a year, and the attic is one suspect among several.
• The inland valleys (climate zones 4 and 12). San Jose, Santa Clara, Sunnyvale and Cupertino in the Santa Clara Valley; Concord, Walnut Creek, Dublin, Pleasanton and Livermore over the hills to the east. Here the sun works on the roof for hours, the attic turns into an oven by mid-afternoon, and the upstairs bedrooms are still warm at 10 p.m. This is where the attic is usually the main cause.
• The North Bay (climate zone 2). Napa and Santa Rosa have hot summer days and cooler evenings. How much heat the attic pushed into the bedrooms through the afternoon decides how long they take to catch up.

The city name is only a starting point. Some Marin and Sonoma addresses fall in different zones, and a house on a ridge in the Oakland hills behaves differently from one by the Bay. The Energy Commission publishes its zones by ZIP code.

How hot does an attic get?

Hotter than the outdoors, and for longer. A U.S. Department of Energy Building America paper on duct placement talks about ducts running through 130°F attics as the problem it is trying to solve. Your attic's number depends on roof color, shade, ventilation and climate zone, and you don't have to guess. Leave the probe of a cheap indoor-outdoor thermometer in the attic for a week in July and read it at 4 p.m. If the attic is far hotter than the street, everything below it is fighting that heat all afternoon.

Why the top floor takes the hit

The upstairs is the floor that shares a ceiling with that attic. Heat moves through the ceiling drywall into the rooms, and it keeps coming after sunset, because the attic stays hot for hours after the air outside cools.

Two things inside the house make it worse. Warm air rises, so whatever heat the house holds collects upstairs. And in many two-story homes the only thermostat is in the downstairs hallway. When the ground floor reaches the set temperature, the system switches off while the bedrooms above are still several degrees warmer. ENERGY STAR lists "hot and cold rooms" and "uneven temperature between rooms" among the classic signs of an attic or duct problem, and a two-story house shows them more than any other.

The diagnosis, in order

When we look at a hot upstairs, we go through five things, roughly from most common to least. The order matters, because fixing the fourth before the first is how people spend money and stay hot.

First: how much insulation is up there

Look in through the hatch with a flashlight. If you can see the tops of the ceiling joists, the insulation is a few inches deep at most, well short of what California's Energy Code now asks for when an attic is insulated. Look for bare spots near the hatch and at the eaves, and for insulation crushed flat by storage boxes. Our guide on how to check attic insulation walks you through measuring it. If the attic is thin, blown-in insulation is usually the first fix.

Second: air leaking through the ceiling

Insulation slows heat; it doesn't stop air. The tops of interior walls (the top plates), wiring holes, plumbing stacks and older recessed "can" lights all leave gaps between the rooms and the attic. On a summer afternoon those gaps let attic air drift down, and the cooled air you paid for drift up. The Department of Energy's guidance for existing attics is to seal attic floor penetrations first, then insulate. That's the job of air sealing, and it is usually done at the same visit as the insulation.

Third: cooling ducts in the attic

Plenty of Bay Area homes have the furnace or air handler and their ducts in the attic. Cold air leaves the unit at a comfortable temperature, then travels through a hot attic before it reaches the bedrooms. The Department of Energy puts the penalty for ducts in a vented attic at about 10% more energy for heating and cooling, and says duct leakage "has been measured to commonly exceed 20% of conditioned air flow." If the vents upstairs blow lukewarm air or barely blow at all, the ductwork is a prime suspect: crushed runs, disconnected boots and torn insulation on the ducts are common finds.

Fourth: radiant heat from a sun-side roof

A big west- or south-facing roof slope, a dark roof and no tree shade turn the underside of the roof deck into a radiator. It beams heat down onto the insulation and onto any ducts. In the inland zones, once the floor is properly insulated and sealed, a radiant barrier under the roof deck can be the next step. On the fog side it rarely makes the list.

Fifth: blocked eaves and a stuffy attic

When old insulation drifts into the eaves, the vents there stop pulling in fresh air, and the attic holds its heat longer. Baffles that hold the eave channels open are a routine part of a proper insulation job. What we don't do is reach for a powered fan first: our attic ventilation page explains when fans help and when they hurt, and the short version is that a fan pulling on a leaky ceiling can suck cooled air out of the house.

Does attic insulation help in summer?

Yes, and it's a common question from inland homeowners who assume insulation is a winter thing. Insulation slows heat flow in both directions. In January it keeps your furnace's heat in the house; in July it slows the attic's heat on its way into the bedrooms. It doesn't cool anything by itself, and it does more paired with air sealing, but it's the piece that changes how fast the upstairs warms up through the afternoon.

How much it lowers the bill is a different question, and we only quote numbers from people who publish their method. The ENERGY STAR figure we use, and whether the work pays off for your house, are on is attic insulation worth it.

When it isn't the attic

Worth knowing before you book an inspection: a hot upstairs can also come from:

• Big west-facing windows with no shade. Late-afternoon sun through glass heats a room fast, attic or no attic.
• A thermostat in the wrong place, or one system serving two floors with no way to balance them.
• Too few return vents upstairs, so warm air has nowhere to go back to the unit.
• An air conditioner that isn't working well. A refrigerant or airflow problem is an HVAC technician's call.

If the attic is deep, sealed and the ducts are fine, we'll tell you so.

What a fix usually looks like

For a typical inland two-story with a thin attic, the job is sealing the ceiling, getting the ducts sealed or replaced where they're damaged, clearing the eaves and blowing insulation to the depth on the quote. A radiant barrier comes later, if at all. On the fog side, the work is usually smaller: topping up a thin attic and sealing the worst gaps.

Before you book a free inspection, note which rooms get hottest and at what time of day; it tells us where to look first.