Not sure but shouldn't it work the other way around. I think increasing insulation helps prevent heat from the sun from heating up the rest of the house. You would still need controlled ventilation in the attic to help dissipate heat.

Sorry, but your old-timer's logic is totally wrong. Too little or no insulation directly above the living space ceiling in your attic will result in a dramatic increase in the amount of radiant heat entering your living space through the ceiling. Your attic can store an enormous amount of heat during the day, which will slowly dissipate after the sun goes down. Until it dissipates, and without sufficient insulation above your ceiling, heat will radiate into your living space during the day and continue to do so at night until the attic completely cools. Radiant heat from the attic will dramatically increase the load on your air conditioning system.

Sufficient ceiling insulation will block radiant gain through your ceiling and lower your cooling costs (it will also decrease your heat loss in colder months).

Perhaps you misunderstood your old-timer. It's a good thing to have insulation above your living space ceiling, but not good to have insulation in the attic on the underside of the roof. You also want good ventilation in the attic. This allows the attic to cool quickly after the sun goes down.

Putting good insulation directly above your living space ceiling allows the attic to heat up during the summer without radiating heat into your living space, and also allows the attic to remain cool during the winter so that condensation doesn't form up under the cold roof.

Thanks for the input. I figured that the conventional wisdom of "heat always moves upwards" didn't always apply. Heat will radiate downward through a hot ceiling.

Maybe I'm just dense, but I'm still wondering why a well-insulated ceiling, in addition to the benefits listed above, does not also cause at least some worsened retention of heat in the living areas in summer. In winter, you want to block the movement of heat (convection) through the ceiling, and so insulation is layered above the ceiling. But in summer, at least when the attic is cooler, like in the evening, wouldn't the living areas cool faster without the insulation above them? Or is this just a small price to pay for limiting the amount of heat that can radiate downward during the daytime? In other words, in the summer, does radiation during the daytime trump convection at night?

I've asked some "knowledgable" people about this, and most admit they never really thought of it before.

Thanks again.

Dave11 wrote:

Posted by dave11 (My Page) on Sat, Aug 25, 07 at 7:44

Thanks for the input. I figured that the conventional wisdom of "heat always moves upwards" didn't always apply. Heat will radiate downward through a hot ceiling.
Maybe I'm just dense, but I'm still wondering why a well-insulated ceiling, in addition to the benefits listed above, does not also cause at least some worsened retention of heat in the living areas in summer. In winter, you want to block the movement of heat (convection) through the ceiling, and so insulation is layered above the ceiling. But in summer, at least when the attic is cooler, like in the evening, wouldn't the living areas cool faster without the insulation above them? Or is this just a small price to pay for limiting the amount of heat that can radiate downward during the daytime? In other words, in the summer, does radiation during the daytime trump convection at night?
Bottom line: Heat does rise to a point. It will rise to the ceiling of your 2nd floor and stop. Heat flow, by conduction, convection, or radiation will flow from a region of greater heat to one of lesser heat in an attempt to 'even-out' the temps.

Your attic temperature could be upwards of 130 degrees. It will be several hours before the attic is cool enough to attract any heat from the 2nd floor.

Veesubotee

I think the idea is that you don't have heat in the living space anyting close to attic temps, thus nothing to dissipate upward.

"but I'm still wondering why a well-insulated ceiling, in addition to the benefits listed above, does not also cause at least some worsened retention of heat in the living areas in summer."

It probably would, but its retention effects would likely be miniscule compared to the amount of heat that insulation layer keeps OUT of the home on a hot summer day.

"But in summer, at least when the attic is cooler, like in the evening, wouldn't the living areas cool faster without the insulation above them?"

I think you're making an invalid assumption that the heat quickly dissapates when the sun goes down.

As Jamesk alludes, those attic spaces will stay VERY warm well after the sun goes down.

I have my attic fan set to turn off when the attic temperature reaches 90 deg. F. On the hottest, sunniest days, it's not uncommon for the attic fan to run well into the early morning hours (3, 4, or even 5 a.m.).

Given that the fan turns off when the temp hits 90, that means that the attic spaces are still 12 to 15 degrees hotter than my bedroom.

I don't see any advantage to your scenario at all.

the old timer's logic is from back before HVAC systems were common in homes. in the deep south homes were built to cross ventilate. you opened the windows and doors opposite of each otehr to allow the cool evening breeze to pass thru. you also wanted the attic to cool off so that it did not reheat the house.

without HVAC, insulation will keep the house cool longer, BUT it will also keep it hot longer int eh evening and not allow it to cool down.

since you have HVAC, you want to go for max efficiency and that means a well insulated house, attic and all.

back a few years ago we had some equipment in an old refrigerator truck bed tha thad been converted to use as a tower site building. the site owner did not put in AC because he reasoned that as well insulated as it was it would not get too hot. he found out REAL quick that idea only hold for a day or so, then it get real hot. we had the AC go out about a year after he put it in and it got to 155 in the building. of course this ruined our electronics. i was 6 hours away, i had the local store manager put a fan in teh door and prop it open until i got there. 6 hours later, a couple hours after dark, it was still 135 in there.

so yes, he is correct IF you do not have HVAC, but he is incorrect if you do.