I have been using 30 degrees higher than the outside air as my rule of thumb. Are most of you measuring the air temp or rafter/truss?
Most of our inspectors measure air temp:
- 25-35 degree difference - acceptable
- 36-44 degree difference - marginal/monitor
- 45+ degree difference - ventilation needs improvement
You may want to check with your local code enforcement official for verification in Ohio. Hope this helps…
I asked this question some time back and received the same amount of varying answers.
Many of the answers steered me back to the formula of 1 sq ft vent per 300 sq ft attic space with vapor barrier - per 150 sq ft w/o vapor barrier (of which I already knew). Regardless of code requirements, you can have this ratio of vent to attic space and still not have adequate ventilation for various reasons (mostly substandard building practices).
After much research, I have arrived at an answer to my question of “what the temperature differential between attic air and ambient (outside) air should be to indicate adequate ventilation.”
Here is the rule of thumb I have implemented:
- Attics should be very drafty areas during all seasons
- Summer attic temps should be within 10 to 15 degrees of outside temps.
- Winter attic temps should be same as outdoor but there is some lenience here.
Anything outside of these is an indication of inadequate ventilation whether it be poorly placed vents, blocked vents, not enough vents, no vents, etc.
Essentially, an ideal and properly working roof/attic ventilation system should provide the above parameters. Any thing else is not working properly or as intended.
This is my humble and researched opinion. Your mileage may vary.
Has anyone come across this product yet and know how it’s performance factor is. Seems interesting and somewhat an ingredient to lower attic temperatures in the South.
Marcel:) 
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Not sure what those panels cost but I think it is one of those products that may not make much difference. Attic ventilation is needed which in turn will lower the temperature. That product could make someone think ventilation does not matter.
Some people from up north use that in the south and do other strange things like build deep walls to install R25 insulation on a brick house. Then they turn around and build on a crawlspace and allow floor insulation to be compressed in place which drops the R value considerably. Much of the cooling loss is through the floor in the south. If you want an energy efficient house in a cooling climate, build on a slab or put more floor insulation in, don’t worry about the walls unless you are using vinyl siding. Also use R38 in the attic and install powered attic ventilators.
I found those panels in a house a couple of months ago. I asked the seller about it and he claimed that the attic fan doesn’t come on as much since he installed them, and that his utility bills have decreased. I told my Client to get the receipts for the work and inquire about any warranty on the panels, and to get copies of utility bills if the seller were willing to provide them. Everyone worked together and it did appear that the utility bills dropped by about 10% beginning at the time the panels had been installed.
Thanks for the input RRay;
Never seen the product before and was curious.
Thanks;
Marcel
Radiant barrier do work especially in dry hot climates like central California and Arizona. The aluminum metal film absorbs the heat energy from the sun. A property of aluminum is that it has terrible emisivity properties which means that it can radiate the stored energy energy outward into the attic areas. That means it’s much cooler in the attic and the house stays cooler.
In California, we have energy regulations commonly known as “Title 24”. If you use radiant barriers in the attic area, you get extra credits for it in the energy analysis which you can trade off for more glass area.
If you’re in the attic and see radiant barrier sheathing, it also suppose to be on the side walls of the attic too. Lots of time, it’s not there. Not a biggie, but something to note in the report.
Dwight;
Thanks for the information, I guessed that it had to be of some use somewheres and the logic behind the system only seemed to make sense. Reflect heat, attic stays cooler or not much hotter than the ambient temperature.
Thanks
Marcel
In 38 years of inspecting, building, and HVAC work I’ve never measured the attic temp unless out of curiosity as to how hot it is so I could tell everybody I was in a 145 degree attic today. Never met anybody in my area that did so OR talked about it if they did???
Your post reminded me that I keep meaning to ask ART what the hottest temperatures were that we’ve ever found in an attic.
Now that it’s quiet, with no one bothering me, I can ask ART (he never sleeps). It looks like it is 176°F earlier this summer during the heat wave, and 158° two years before that.
The only time we measure the attic temperature is if a wall of heat falls down on us when we move the attic access cover.
Heat can overcome one very quickly, which is why I like having multiple inspectors at houses with large, hot attics.
When it’s really hot and I have to go into the attic, I generally carry around a small towel that I soak in water. I then wrap it around my neck. It helps keep me cool for long enough to get through the attic inspection.
The only time that this trick didn’t work was when the 2 furnaces were both in the attic and I had to go back and forth to the thermostats checking the systems out.
BTW, I did a inspection on a beautiful house a couple of days ago. It had a whole house fan next to the furnace. If you see the same setup, be sure to note the whole house fan as a possible safety issue since the fans generally don’t have any sort of fan guards.
I started to measure the temp this year Dan. I would often be asked “How hot is it when you inspect attics?”
“Friggin way hot” did not seem to be a professional reply.
Common temp here in the summer between 130 to 145 F.
That was a statement not a question :mrgreen: from these persons ;).
Better now?
In my life as a firefighter I would often see attic temps pushing 1200F… I usually recommend some form of attic ventilation at that temp…
It depends on numerous factors: Geographic Location, Typical weather, Type of Roof, NS or EW orientation of home, shade factors, home construction type. But to answer your question it can vary from 20 degrees to 40+ degrees. Minimal Insulation can allow significant heat transfer to the interior of the home radiating from ceiling.