I live in a medium-size co-op apartment building from the 1920s (30+ units, 3 story, in Seattle). As extreme weather events have intensified in recent years, we have gotten more interested in weatherizing the building.

One issue that has remained difficult over time is that intense heat events (>90ºF for us) leave us with a cushion of hot air in the attic due to trapped solar heat, making it difficult to cool the building down, which should be a familiar problem. See diagram for some measurements that I made last summer with an IR thermometer on the roof, in the attic, and in a top-floor unit. There is no building-wide A/C system, only little plug-in units, so in extreme heat the building is very uncomfortable.

diagram of how building gets hot and then cools off over the day Note: all measurements are in ºF. 1:30PM measurement had exterior temp of ~90ºF, 9PM had exterior temp of ~70ºF.

Approximately 20 years ago the roof and attic had significant work done, including blown in insulation the floor of the attic crawlspace (reference picture for a general idea of what it looks like). I don't know the R value offhand for this insulation. The attic is (theoretically) passively ventilated, and others have told me that there is rigid insulation in the roof assembly as well.

Other ongoing retrofits include upgrading windows etc., but they can't address the problem of the attic keeping top floor units uncomfortable all night, and may actually make matters worse by holding heat in when the air outside is cooler.

Picture of crawlspace with loose insulation

Does anyone have any suggestions for simple, low-cost fixes that can make this a bit more liveable? An architect who lives in the building seems to strongly believe there's nothing to be done without spending a lot of money, but that seems unlikely to me.

For example:

  • answers to this question and this question suggest that we might be able to just add a little more insulation to the attic in a low tech fashion.
  • I could imagine just putting a box fan up in the attic to circulate hot air for cool air through the vents at night, given that the passive ventilation isn't helping much.
  • Very speculatively, we could arrange reflective material on the roof to redirect incoming solar radiation on the hottest days.

Do any of those ideas have any merit?


In response to an answer below, here is a look at the roof color (according to google maps photo view):

cropped roof color photo, light gray

Our building roof

It is notably off-white, comparing to satellite photos of other nearby buildings it appears slightly darker but not worlds different:

cropped other building roof color, off-white

nearby building roof

So I doubt that roof albedo alone can be corrected easily enough to make a big difference here. That said, I don't know what the paint or whatever is, I haven't directly measured reflectivity or albedo.

  • 1
    Better ventilation, more insulation on the ceiling and maybe having the roof a light(white) colour should help, without breaking the bank. Better insulation in the outside walls will help also(probably better than the newspaper), but that will start to add up.
    – crip659
    Commented Feb 17, 2023 at 23:09
  • 1
    Have you considered thermostatically controlled power attic ventilators?
    – RetiredATC
    Commented Feb 20, 2023 at 0:49

2 Answers 2

  1. Additional blown cellulose insulation is VERY cheap and based on the pic, it should be done.
  2. I don't recommend a box fan. Increase the ventilation and use proper fans that are designed for that space. This is not terribly expensive generally.
  3. It is possible to put reflective material on the roof to reflect the heat.

I would start with the blown insulation as that is a weekend project. Easy and hard to mess up; just don't block soffit vents. Adding additional ventilation is the next step I would take; if you can move enough air, you can keep the attic quite close to outdoor temperature.

The reflective roof idea is not bad, but what you can do and how depends on exactly how your roof is designed; Hard to say if it is even a good idea without knowing far more information about the roof. However, I would price check solar panels that are mounted above the roof; generating electricity and blocking sunlight seems like a good idea and it should pay for themselves anyway as the payback is 6-10 years and they last 25 years.

  • The solar panels are a good idea. They provide shade for the roof and a bit of income/reduced expenses.
    – crip659
    Commented Feb 18, 2023 at 0:07
  • Solar panels will cost you $30,000 or more
    – Traveler
    Commented Feb 18, 2023 at 3:11
  • 4
    Cost of solar depends on the size of the installation and some technology choices, and in some areas subsidies are available. My system, about a decade ago, cost me about $8k after group buy discount and rebates.
    – keshlam
    Commented Feb 18, 2023 at 4:19
  • I once painted the roof of a cheap tin garage with some white/cream paint I had spare, and it knocked the temperature inside down by 5 degrees. And that was one coat of left-over indoor paint totally unsuited for the task. Most roof surfaces need protective layers anyway, so make the next one as light-coloured as possible.
    – Criggie
    Commented Feb 18, 2023 at 21:27
  • Thanks for this quite informative answer! We are quite limited spending wise as the community is very cost averse, but other than solar panels (which I would love but just won't be on the table), it seems like all of these ideas are pretty viable. Re: reflectivity, we are ok (but not great) on the roof albedo so maybe it's not worth it, but I just keep looking at that exposed vs. shaded roof temp. There's gotta be a way to get that roof temp better! Commented Feb 18, 2023 at 21:47

First... what color is your roof?

All your suffering is coming from a thing called solar gain. About 100 watts (341 BTU) of solar energy falls on every square foot of surface that is square on to the sun. How much is reflected by the sun is called reflectivity and emissivity.

My paint supplier tells me "Snow white has the highest reflectance value with a score of 82.6, Cloud White is the 2nd runner us with 80.7". This is top tier LPU paint and these are rather white colors. With the Snow White we're rejecting 80% of solar heat. Since we're now down to 20 watts (76 BTU) per square foot), the roof doesn't get that hot, and so emissivity doesn't play a role.

Any darker than that and it goes off a cliff. Like I say "cloud white" is down 2 points, and it's barely any darker than Snow White. By the time you're into true off-whitew you're down in the 50s, and for anything normal people call a roof LOL you're sub 10% reflectivity. This is where emissivity comes in. Now that the roof is hot as funk, you start shedding some via radiant energy. Day late and a dollar short for a roof, but think about a shade structure sat high enough above your roof to have good airflow below it. Emissivity there works just fine.

If you want to make the shade structure out of silicon and have wires coming out of it, bully on you!

The same goes for walls, though shade structures are more awkward there.

Heat energy in the building is accumulated over the day.

Think about winter. The shortest day of the year is December 21, but that's only deemed the beginning of winter. If there was no thermal mass, then 2 months prior (Oct. 21) would be exactly as cold as 2 months hence (Feb. 21). But it's not. Fall is more temperate because the earth's surface spent all summer heating up, and it 'coasts' on that stored energy for months, making fall temperate. By December that stored energy is depleted, and even though we're getting more solarization every day, it actually gets colder from Dec 21 to. Jan 21.

Well, the same thing happens with the mass of your building. Solar gain 2 hours after sunrise is identical to solar gain 2 hours before sunset. Yet nobody's dying at 10 AM, it's 6 PM that is miserable. Like the earth, the building's thermal mass is causing a time delay to heat effects.

In other words, your house is a battery.

This thermal mass issue can be greatly gamified. Like the fella in the video who runs the A/C entirely from 11 pm to 7 am, completely ignoring the "conventional wisdom" of holding a house at a constant temperature.

For instance you might not put ventilators in the crawlspace, you might put heat pumps in there - no large vent penetrations needed. And here's the important part, run them when the sun is blasting, not when the heat has already reached painful temperatures! That means the power is absolutely free coming off your solar panels, the disease is the cure! They are pre-chilling the crawlspace so it never gets a chance to get hot.

  • Thanks for your answer, it agrees pretty well with my intuition but it's great to have some numbers here. We don't have the capital to rig solar and heat pumps right now (if only!), but something like a large scale shade structure is an interesting possibility (I rigged something like this on a very small scale to make the shaded roof measurement above.) Commented Feb 19, 2023 at 20:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.