As home energy professionals, we all need to know the best way to prevent attics from overheating in the summer. My 2800 SF 2 story has a standard man made 1 inch thick shingle roof over simple sub roof and an attic just high enough to stand under the peak. The attic has gable vents at the ends and 3 inch holes drilled in threes in the soffits every 6 feet or so. Over the space of a hot day my ceilings get to over 90 degrees and heat my rooms.
On a RESNET discussion some insulation experts agreed that insulation will not prevent hot attic air from heating ceilings. I am looking at a foil radiant barrier for my roof. What should I look for?
You can find some information on Radiant Control Coatings in the group listed below.
You mentioned "On a RESNET discussion some insulation experts agreed that insulation will not prevent hot attic air from heating ceilings." I suspect there is more to that discussion as that statement isn't entirely correct. Most experts I read say that sufficient insulation combined with good air sealing and standard ventilation will certainly minimize the heat flow through your ceiling.
But first, 90° in an attic is not that hot. It sounds like you are not in a southern Texas climate, so approximately where are you? In the deep south a RB can be useful. In fact they have foil faced sheathing that can be installed with the foil surface facing the attic. But one of the prime reasons for their use of RB's in the attic is because their ac systems are up there. As you move north the benefits of a RB give way to just more insulation and good ventilation, because most homes do not install their hvac in the hottest zone in a house.
Question, where are you and are there ducts or hvac equipment installed in that attic?
Hal, I was hoping you would tell us what YOU used.
Bud, I did not say my attic was 90. I said my ceiling was 90 in the west facing bedroom at 4 PM in Temecula CA when the outside air was about 100. The attic was far hotter. In an earlier comment I pointed out that when I was installing RB it was 11 AM in July on the east facing roof and it measured 102 F on the bottom of the roof.
Nearly all new homes here have AC in the attic.
I am a home inspector and have been in over 1,400 attics. I have not seen an attic where RB would not help.
You might re read your comment. It is not inconsistent with my observation. "sufficient insulation combined with good air sealing and standard ventilation will certainly minimize the heat flow through your ceiling"
I was not discussing heat flow. I was discussing the temperature of the ceiling. The experts at RESNET agreed that the hot air heats my ceiling in spite of insulation. And that hot ceiling heats my rooms just like radiant heating ceilings do.
I can point you to one in private message but not on the open forum. We are not allowed to promote any products.
You can email me at firstname.lastname@example.org or you can call me at 209 556 0764. I w2ill be here for around 45 minutes or so.
LOL, I have a one track mind and every once in awhile it jumps the track. The 90° makes more sense now.
"I was not discussing heat flow." Heat flow is what determines the ceiling temperature.
I'll stand by my statement that a well insulated ceiling, with air sealing and ventilation will keep the inside ceiling temperature at a comfortable level. I'm an energy auditor, so let's look at the delta T. A 120° attic and a 75° house is only 55°. In my very cold climate, just r-19 will limit the surface temperature change to about 4° and your attic should have closer to R-50 (not sure what your code is, but code is a minimum). And that 4° is with a greater delta T.
And there is more. In a cooling climate, with the hot air above the cooler ceiling there will be less convection, thus less heat transfer to the ceiling. Yes a RB can be beneficial in your climate and more so if you have fiberglass insulation. We use a lot of blown-in cellulose up here so far less susceptible to IR penetration.
With the AC up there all ducts need to be sealed and insulated and more so than just the typical foil wrap the ac guys like to use.
How much and what type of insulation is currently up there.
What is your total NFA (net free area) of ventilation, high and low. At 2,800 sq ft you should have somewhere between 18 and 36 sq ft of NFA divided approximately half high and half low. If well air sealed use the lower number.
One of the many things I like about these forums is that I always learn something I did not know.
I am working to wrap my mind around insulation and heat flow in my home.
Your calculation "limit the surface temperature change to about 4 deg" flies in the face of my observation. Specifically, right above my head in my largest room upstairs is a walkable attic that has cellulose insulation that WAS about 12 inches think, smashed down under 1/2 inch plywood sheets covered by carpets to make it easy to work up there putting in RB.
I have a FLIR B2 camera and measure the ceiling from time to time and it always gets to 85 by the end of the day. Where does that heat come from? My rooms get to 85 as well. This heat cannot be coming from downstairs. It is about 77 down there max.
Now, about HVAC ducts, I am having a difficult time understanding how they contribute to ANY heating of my ceiling and rooms. Why would I need to modify them??
It is the other way around for the ducts in the attic. When the ac is running they actually cool the attic at a cost.
Of course any heat that finds its way into the house will always end up at that ceiling. But there is another effect that contributes directly to the heat up there and it is (for a lack of a better term) referred to as "reverse stack effect". Winter stack effect is the cold air pushing in through the lower leaks of our homes and forcing the warm air up and out the high leaks. In the summer the "in at the low and out at the high" reverses which means the heavier cold air inside the house pours out the lower leaks and effectively pulls in the lighter hot air from the attic and other high leaks. Recessed lights, electrical boxes, plumbing and electrical penetrations, and more all contribute.
A supply duct that leaks air to the outside will also depressurize the house and pull outside air in as a replacement. Unbalanced supply and return pressures can do the same.
If you have cellulose insulation in the ceiling in question, then radiant energy is not filtering down through it like it does with fiberglass. If you have air leaks between the attic and the house that hot air up there is an issue, but is the best solution to reduce the heat in the attic or reduce the air leakage and also improve the insulation.
Yes a RB will help in a hot climate, but you should make sure the ventilation is sufficient. If the RB was a foil glued to the bottom of the roof sheathing then you definitely need that ventilation as the bottom of the foil will become hotter than the current roof deck. Being a RB it just wont jump the sir gap to the attic floor as directly.
"It is the other way around for the ducts in the attic. When the ac is running they actually cool the attic at a cost".
Got to pick nits here. Heat energy travels towards cold. Cold does not travel, only heat energy travels. The ducts do not cool the warmer attic, they absorb the heat from the attic by radiation, conduction and radiation.
If I may here; When the AC is running, the colder ducts absorb heat enrgy and that heat energy is absorbed by the cold air going through the ducts. The cold air enters the living area warmer than when it left the AC unit. It takes longer for that air to satisfy the thermostat, your AC unit runs longer. That is what you pay for. It is referred to as a 'heat gain loss'.
A good RCC applied to the attic AC unit, plenum and metal or flex ducts can virtually eliminate that heat gain and allow the air to enter the living space much colder.
Anything done to raise the roofing off the sheathing works to reduce the high temperatures in the attic above ambient but has limits.
So, to deal with bad situations like my attic being 160F daily in Phoenix a lot of summer requires reducing conduction into the attic of overheated [or cooled] roofing.
A remodel routine would be to remove the roofing, add furring, insulation board calked & sized to the load, then coat that with a RCC in hot sunny places, furring, stringers & reapply a roofing. [AFAIK hemp-mortar can replace using insulation board, it's fireproof & you can use fasteners into it; for at-risk locations worth a thought.]
This is a thermal solution, it cuts most of the conduction of anything overheated by the roofing because of the board which removes the multiplier effect of the roof surface on the attic heating.
For most here that's easy to estimate for job cost, the result is to remove the reason for the big AC & heating bills by isolating the attic from the extremes of the surface roofing layer so it remains at ambient. In colder climates treating the attic as interior space thermally should be worth it to reduce living space losses after a roof is done.
That's my suggestion for a direct reduction with least labor to the source of the large thermal loads, and, for off-grid-net-zero if one stores the excess heat or cold when available in thermal-mass the roof is a great collector.
I just posted a statement and pictures of a home in Yuma AZ. This is one I did 10 years ago. I just spoke to the owner. The biggest electric bill he has seen is $160.00 on a 110 to 120 degree month. His bills average $70.00 per month.
He is selling the house now and using his electric bills as a selling point because everyone else is paying $550.00 to $650.00 per month.
It is a discussion in the RCC Classroom.
I would like to see that article. Where is it? And what
kind of radiant barrier could cost so much and do so
I am installing what I think will be the perfect solution for
my 2800 sf 2 story. First a ridge vent on the 45 foot
long clean ridge. Then atticfoil.com radiant barrier
between every rafter with gaps at bottom and top. Last
3 inch round holes in the eaves between every set of
rafters. So outside air will flow up under the roof to
minimize heating below the roof and the radiant barrier
will minimize the heating of the attic air. I am confident
this will radically reduce the heating of my upper floor
ceiling. I will be posting the results on my web sites,
wholehousefanguy dot com and invisco dot com Whole
House Fan Company.
"On a RESNET discussion some insulation experts agreed that insulation will not prevent hot attic air from heating ceilings." I suspect there is more to that discussion as that statement isn't entirely correct. Most experts I read say that sufficient insulation combined with good air sealing and standard ventilation will certainly minimize the heat flow through your ceiling."
With all due respect to those insulation experts, I have to assume they are referring to attic heat gain coming tbhrough the roof and radiating down. That heat energy is absorbed by everything in the attic. Convected air then heats the attic to whatever temperature. I think that is a simple case of dealing with the effects of the heat gain from the roof and not with the cause.
A good RCC applied to the roof underdecking stop[s the heat energy from the roof from entering the attic space at all.
A good RCC does not permit the heat energy to radiate into the attic.
Better yet, apply the RCC to the roof exterior and dont let the heat energy even enter the rfoof. Put the insulation on the outside where it should be.