Is it true that a home loses 10% of heat through conduction, 25% through convection and 65% through radiation?
BrucePPH
| Posted in General Questions on
I have seen an ad for a product that states these numbers. Is this a proven building science fact? If so why wrap homes in house wrap that is not foil based? Wrap it provide a 1″ gap and side over the wall. It would seem more efficient than super insulated walls without the foil.
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You've already answered your question. You say this "fact" in a commercial advertisement attempting to sell you a product rather than educate you.
The relative percentage of heat loss will, of course, vary tremendously depending on the age, type, and quality of your house and the climate zone it's in. But in a cold climate, most heat loss is by conduction through the thermal envelope. This is why the International Energy Conservation Code specifies high R-values for each element of the envelope.
Secondarily, a large amount of heat is lost in most homes by exfiltration. Thus codes and efficiency programs are beginning to promote air tightness standards - the next IECC will mandate blower door and duct blaster tests.
The only significant heat loss by radiation is between the panes of multi-pane windows, which is why LowE windows are important and have become the industry standard.
Foil-based radiant barriers are useful primarily to prevent heat gain through the roof in hot climates.
Bruce,
Robert is correct. Heat flow through a typical building assembly (a wall, ceiling, or floor) is complicated, as heat is transferred from one component to the next: from drywall to stud, from drywall to air space, from drywall to glass fibers, from one glass fiber to another, from glass fibers to OSB, from OSB to siding, from siding to the outside air.
Moreover, as Robert correctly notes, the typical house loses a lot of heat through exfiltration — air leaks.
Laboratory R-value tests account for all three heat transfer mechanisms: conduction, convection, and radiation. Read more here:
https://www.greenbuildingadvisor.com/blogs/dept/musings/understanding-r-value
Although R-value testing does not tell a builder anything about air leakage, it gives important information on heat flow. Rest assured that a high-R assembly slows heat flow of all types.
Radiant barrier sales people who advocate wrapping a house with aluminum foil are engaged in fraud. Their building science knowledge is incomplete and they are scamming customers. While it is true that aluminum foil (or plastic foil) plus an air space does slow down heat flow — and while it is true that the air space may have a measurable R-value — the cost of building such an air space is always more than the cost of simply installing conventional insulation products.
Richard thanks for your speedy reply and thank you Martin, for the link. I will not count on the 10/25/65 splits as factual. I wonder if there is such a number though? Maybe the answer is it depends.
Thanks again.
Bruce,
The answer is definitely, "It depends."
Radiant heat transfer depends on a difference in temperature. If your wall is well insulated, the siding will be very close to the outdoor air temperature, so there will be very little radiant heat transfer between the siding and the outdoor environment. Radiant heat transfer only becomes significant if a surface or building assembly is uninsulated. That's why radiant heat transfer matters with window glazing.
It can also be significant with uninsulated steel buildings (like some barns and warehouses). If you have significant radiant heat transfer through your walls, you have a big problem -- because it means you forgot to insulate your wall.
The solution to this problem is NOT to wrap your building with aluminum foil. The solution is to insulate the building.
An aluminum foil hat, however, might insulate the brain from the radiant "insulation" industry's misleading propaganda.
As a southerner and a former framer, now architect, having worked in Texas and North Carolina, I have to argue for the value of reducing unwanted heat gain with radiant barriers in southern climates. In multifamily construction projects in NC, we typically use a radiant-barrier faced sheathing (shiny side inward) for roof construction. However, I do this without being able to produce any data to support it. The anecdotal evidence from framers is good. However, the cost has been absorbed easily by the projects, so we continue to do it.
It appears the jury is still out on radiant barriers in roof spaces... in Canada at least.
http://www.ccht-cctr.gc.ca/projects/attic_e.html
And these ought to be of interest:
http://www.ccht-cctr.gc.ca/projects_e.html
http://www.cmhc-schl.gc.ca/odpub/pdf/65295.pdf
http://www.healthyheating.com/Page%2055/Page_55_o_bldg_sys.htm
And if the sheathing showing up on the throng of hundreds of developer tract houses that get thrown up like matchboxes around the greater Toronto area are any indication, I'd say it's yet another cheap way of trying to do something properly.
Hey GBA you need to correct your title- "looses" is not really a word but "loses" is what happens when a baseball team doesn't win, and also what happens when heat escapes a house. The verb is "lose" not "loose". Love your site though! I will be buying the primo membership soon.
Every which way but loose.
I built a house with big oak trees all around...shade in the summer and radiant gain in the winter! ...and insurance to cover damages. Shade makes a wonderful difference!!
Only the Shadow knows.
Alan,
The spelling of words in posted questions (including the titles above the questions) is determined by the person posting the question, not GBA. Sometimes, however, GBA will correct spelling errors in the interests of clarity. In this case, I'll change the title — just so Alan doesn't loose any sleep over the issue.
Or let loose the demons deep inside his head [Iron Maiden, Dream of Mirrors]
I only dream in black and white - nice one Robert - UP THE IRONS!
We just recently began using the radiant barrier roof sheathing and I must say I have been very impressed with its performance. Our HVAC installers love it as well (installing HVAC systems in the attic is pretty common here in NC - I know, a whole another issue). I do agree that there is a lot of misleading information out there - installing a radiant barrier under a slab or on a wall does not do any good. Installing horizontally at the ceiling level is questionable (I would not install this way) but installing at the bottom of the rafters or integrating in the sheathing is a pretty easy and low cost way to improve your home. I quickly reviewed the Canadain studies mentioned above and they all seem to be about horizontal and vertical installations - did not see anything that said installing on the pitch of the roof did not work - in fact, the DOE has done studies on this an recommends it.