blocking heat gain through the roof
mmoogie
| Posted in Energy, Heating & Insulation on
Anyone got any thoughts (backed up by data) as to the effectiveness of various insulations at blocking heat gain? (ie: through a cathedral ceiling in the summer)
Is it strictly a function of r-value, or are some materials better at blocking heat gain at the same theoretical R-value than others?
Steve
Replies
Light colored shingle help a great deal. But if you have no plans to replace your roof any time soon then yes more R value better = better performance. If I were you I would consider doing a hot roof app.
http://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates/?searchterm=hot%20roof
I am biased towards hot roofs. I just don't think venting gets you much more than a few years more on the shingles at best.But my question, whether hot or cold assembly, is if there is a substantive difference in the insulative materials themselves when it comes to blocking solar gain. ie: is R-30 of foam better or worse at reducing solar gain than R-30 of Cellulose or R-30 of fiberglass (ignoring all of fiberglasses other shortcomings).Steve
Light colored shingle help a great deal.
I disagree. I read an article in Energy Design Update once that pointed out a study ... white shingles are like only maybe 5% better (i.e. more reflective than any other colors). Due to the texture of [asphalt] shingles, I think they don't promote reflectivity comensurate w/ their base color. This is a litle along the lines of an old theory a professor of mine once coined the "hot white, cool black" principle. He designed a solar collector w/ high reflective aluminum plate ... that worked very well at ... collecting solar energy.
I surmise that ultimately, the emissivity of white shingles is VERY low because of the nature of the materials; in spite of highly reflective white color, the effect is not very dramatic. A shiny pocket knife blade also has low emissivity ... and will get very hot in the sun in spite of its highly reflective surface.
Need to take care making assumptions like this. White shingles do little and I wouldn't choose them over a better color (aesthetically speaking) because of their energy performance.
I think someone makes sheathing with a heat-reflective layer...
someone makes sheathing with a heat-reflective layer...
They do, except that the shiny material is low-emissive, not reflective. And that emissivity is negated where it is in rafter contact. So, only the bay areas in between are low-emissive. They still have the same thermal mass, that mass is still firmly attached to the rafters, and thus into the framing envelope of the house, which has a great deal of mass to take up 15-18 hours of summer daylight gain.
Largest deltaT in our houses is versu the attic, not the outdoors. If the ambient is 98º and desired indoor temp is 78º, that's 20º of deltaT; if yout attic is only 128, that's a forty degree deltaT to overcome. Which is not a nice place to leave duct work even with R6 maybe R7 blankets--you are still pumping a shot of 110+ air out the registers when the air handler kicks on.
Right now, the airport shows 97º 32%RH 62ºDP for a lovely 98ºHI; my north porch shows and even 100º' 101º on the south porch; the attic reports -11 & 99%--but it does that over 121º--it's probably around 130 with our July-like temps today.Occupational hazard of my occupation not being around (sorry Bubba)
I just blew 10 inches of cellulose in my attic today. Yesterday Ire- wired the overhead fixtures.
101* new record from pensacola. My unit probably hasn't kicked off in a couple of weeks. If it has, its been late at night.
I just blew 10 inches of cellulose in my attic today.
catfish
Why did you stop at 10 inches???? Was that on top of something else?
If you have the room, 15 inches or more would block more radiant heat from reaching your ceilings.
Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?
Thats actually all I could get in there. Thats a 4/12 roof. Eventually I will put some more where I can. I'm just trying to get by until I get more money. My wife won some money on lottery so I spent that on the insulation.
I'm trying to seal air leakage as I go. This is a 1948 house so its hard. I was fine with the 5 1/2 of fiberglass even though it was about 2 inches thick. But hurricane Ivan blew down a huge tree and I have been fighting the heat ever since. Been running a window AC in additon to the unit.
If you are home and it is brutally hot, you can hose down the roof with some water every now and then. It helps a bit. Unless of course the FL water police are watching.
Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?
"My wife won some money on lottery so I spent that on the insulation."
Are you absolutely sure you aren't Canadian?
Unless the border has moved to the Red River, I still an Okie. That was her win for the year. Florida Lotto, Live in Pensacola.
Well with a move like that I think it is safe to give you honorary Canadian citizenship.
I've seen OSB w/ a reflective surface. Better than nothing. Don't know about the cost increase. But to RELY on the performance in any significant way is probably not a good idea. As with radiant barrier technology, in practice it doesn't work nearly as well as it does in the laboratory or in theory.
Andy E had a lot to say on that and ref to an article. I think you could find it searching the term Emissivity here, or combine that with radiant barrier
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I used to live in Toronto where everyone had A/C running non stop in the summer. We put a thermostatically controlled fan in the roof. We could barely breath it was so hot putting it in. Once we got it running we never turned the A/C on again.
mmoogie;
Any wood will conduct heat into the space. The best system for a cathedral roof is SIP's because they are a solid block of foam seperated by 2 pieces of OSB.
Yesterday it was 97 degree here, my 5500 sq.ft. house with a cathederal ceiling going up to 28 feet. is cooled by tiny room air conditioners (2) I paid $179.00 for It's was around 68 degrees during the day. The reason so little A/C works on such a large house is because I have 12 inches on the roof and 6 inches on the walls.. Oh and 107 windows!
If I had a more normal 25-30 windows, I could probably cool it with one little, cheap A/C unit
The solar heat gain (SHG) through a roof with incident solar radiation is calculated as follows:
per unit area, SHR x SRR X SAR = SHG
SHR is related to the angle of the sun, ranges from 150 at 8 am to 280 at noon. SAR is related to the color, with black being 1.0 white being 0.4. SRR is empirical data that relates the R-value (or actually U, the inverse of R) to how much heat passes through, ther greater the R, the lower the SRR.
Then there is conduction due to temperature difference, directly related to R-value.
Is it strictly a function of R-Value? No. Some structures are better at resisting heat gain with the same composite R-Value than others.
ABF
Anything but fiberglass.
Dense-packed cells, foam, etc.
The only time I ever took measurements was when I finished my own attic many years ago. Wide open space, about 46' by 19', 5' kneewalls with the ridge board up at 11'.
Had fiberglass batts in the rafter bay, 6-mil poly vapor barrier on the rafter faces, the attic temperature at the peak was 127 degrees measured with my wife's kitchen thermometer.
I faced the bottom edges of the rafters with 2" thick polyiso. I put 1" poliso on the gable end walls. The temp dropped to 77 degrees at the peak.
I ended up not even putting heat in the attic. If the attic is closed in the winter it drops to about 66 degrees. Turn the lights on and get a couple of bodies up there and it warms up quickly.
In the summer it hits about 76-77 degrees with no air.
Since the air handlers are behind the kneewalls, I did pull a couple of runs off of the second floor air handler to move the attic air in the summer to keep it from getting humid and stagnant.
So lab-tested? No.
What you want to do is prevent the attic roof from becoming a huge radiant panel. Foam good. Dense-packed cells good. FG as a stand-alone? Not as good.
My opinion.
Don't forget, there's R-value and there's effective R-value.
Hi Mongo,So in your instance you left the FG there, then added another R-14 in polyiso. What I'm wondering is if you had added another r-14 of FG would the results have been the same?My gut tells me no, and in the past I've heard claims that cellulose is better at blocking radiant heat transfer, but I'm wondering if the only thing that would make a difference (from the underside of the roof deck inward, anyway) would be more R-value or a radiant barrier with an airspace. And since I don't think a radiant barrier would stay clean for very long, I don't put much stock in that.So for me, it boils down to what's better at blocking heat gain from the roof: R-40 of cellulose or R-40 of foam or R-40 of fiberglass, all other factors being the same (ventilation, air-tightness, thermal bridging, etc.)Steve
No. In other houses I've added more FG, then after seeing no substantive change, pulled it ALL out and added less R-value in foam and had drastic improvements in performance and comfort. Foil-faced radiant barriers work well when they are correctly installed and later changes or evolutions in environmental conditions don't reduce their performance.A thickness of foam, or a thickness of dense-packed cells in and of itself will absorb and block the transfer of energy better than a thickness of FG batts.And it's not just "blocking radiant." It's handling the energy gain and/or transfer as a whole, as it changes from conductive to convective to radiant to...etc, and how well or how poorly that energy transfers through a material when it is conductive or convective or radiant, etc.In these discussions there's a balance between theory and practical application. The engineer and math and physics major in me can tell you that theoretically a lot of this "insulation versus energy transfer" talk is mumbo-jumbo bunk. R-value is R-value, right? But the builder in me can tell you that in practical application, a lot of the theory doesn't take into account theoretical R-value versus effective R-value.
Well your experience confirms my gut feeling. Is there a theoretical basis for your practical observations? Something that can be boiled down to understandable information for clients? I would rather not ever install FG again. (I've managed not to put any in anywhere for quite a few years now, but I want to be able back up my assertions about cellulose and foam being better. The air-sealing is a no brainer, but I thing the summer heat blocking aspects are an advantage as well, and would like to be able to back it up.Steve
There is, but I'm running out of town for a few days. I'll look to post when I return.
I live in the desert region of Southern Calif. The reflective material folks are talking about is called a Radiant Barrier. The material works really well. If your cathedral ceiling has much of an attic space, you can actually purchase the rolls of the RB and install to the underside of the rafters. If you choose this method be sure and leave an inch of air space between the sheathing and the RB. In this area when the temps reach 100 degrees, an uninsulated garage can be quite comfortable compared to the outside.