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Old 12-09-13, 01:39 AM   #371
AC_Hacker
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Quote:
Originally Posted by jeff5may View Post
I believe you're missing the point here...
.

jeff5may, I think the real crux here is that he keeps talking about "still air", and it simply does not exist... not until absolute zero.

Since his imagined model is based on still air, his imagined advantages do not exist either.

-AC

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Old 12-09-13, 02:54 PM   #372
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Originally Posted by AC_Hacker View Post
In the link that michael pointed to, there was the sentence, "The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact."

That material was designed for use in space, where, except for conduction thermal radiation is the only mode of heat gain or loss.

The nature of thermal radiation on earth is the same, but heat loss will have the additional mode of convection. Convection depends on the flow of fluids (air can be considered a fluid, since it flows) and fluid flow is inhibited by friction.

So, if the gap between layers, radiant insulation layers that is, is very small, the fluid friction will inhibit convection losses. The larger the space, the lower the fluid friction, the greater the convective losses.

So, the "at least 3/4 inch" idea might have a problem.

Also, regarding wavelengths, check this out:



So, a long wavelength is 1mm.

If your wavelength idea is correct, you need a maximum space of 1mm to allow the reflective surface to do it's reflecting magic. As your space becomes larger than that, fluid resistance drops, convective currents form... the larger the space, the greater the convection.

I believe I have heard this referred to as "convective pumping".

Best,

-AC
Well spoken, AC! In covering the general case of friction's affect on natural convection heat transfer. Later in this thread we discuss the special case of controlling downward heat flow,
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Old 12-09-13, 04:15 PM   #373
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I too was first schooled (several decades ago) on the virtues of insulating the attic, walls and letting the sub-floor slide with minimal insulation. We have learned just how wrong that advice was . . . .

In particular, I also am a BIG supporter of slab insulation of the periphery - up the foundation and stem wall until you reach the level where the wall insulation exists. I saw, very clearly, just how much heat would "wick" out of a slab or concrete wall. One house in Michigan had bulbs sprouting in February right next to the concrete stem wall. The soil there never froze due to thermal wicking.

In particular, within ten feet of a corner, I double the insulation (to 2 inches of closed cell foam) in those areas. Where winter degree days F are > 6000, I suggest three inches of closed cell foam up the stem walls.

Yes, heat does rise, but moisture in crawl spaces, drafty conditions and convection down there just sucks heat out of the floor above. There are areas (heating dd < 1500) where some of this is not economical, but insulation is CHEAP to put in at the time of construction and labor intensive to put in later.

One trick I have found is to use 5/8" "R-max" 4x8 foam sheeting underneath floor attached with hangers or such to give about 12 inches of insulation space. Before decking the floor, I blow in about 8 inches of insulation. It doesn't have to be perfect and a bit of insulation leaks out between sheets.

If I were REALLY fastidious, I would seal each sheet with aluminized page to prevent this. I will have to play around with bubble foam insulation as some of the gap ideas are intriguing.

The difference in floor "foot comfort" with sub floor insulation is amazing. Having grown up on a frigid cold winter slab floor in New England, I HATE cold feet.

I still do feel that a greater R value should be in the attic vs. floor; perhaps 2:1 ratio, but I think we REALLY messed up by not encouraging more sub-slab insulation, below floor insulation and periphery insulation.

Steve
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Old 12-09-13, 04:37 PM   #374
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Quote:
Originally Posted by jeff5may View Post
I believe you're missing the point here. Even though the fiberglass batting does conduct a miniscule amount of heat, it offers a surface for your floor to radiate its heat into. Once the radiated heat is spent on the top surface of the batting, the underlying batting prevents this heat from travelling anywhere. It merely compresses the stratification effect to near the source. At the bottom of the batting, the surface temperature is much closer to ambient, preventing much of the radiant heat from heating the air below.
Jeff, You have a point, being that inasmuch as that the upward facing 'visable surface' of the bat will absorb downward radiant energy from the subfloor and, because of its low mass, will quickly rise in temperature approaching that of the radiating subfloor, thus reducing radiant heat transfer (being the difference of the 4th power of the absolute temps of the radiating bodies) from the subfloor. But the black poly film on the crawl space floor sees the radiating subfloor much the same and may introduce convection currents heating the crawl space air.

You say that "the underlying batting prevents this heat from travelling anywhere". Not so! The conductivity of the batting is higher than that of the
crawl space air. Again, batting, bulk insulation's only purpose is to prevent
natural convection heat transfer! And we we have agreed that there is no natural convection heat transfer process in the downward direction.

Radiant energy passing thru air does not heat the air!

Last edited by berniebenz; 12-09-13 at 04:49 PM..
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Old 12-09-13, 10:07 PM   #375
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Yes it does! In your case especially, the radiant energy is low-grade, long-wave, far infrared heat. Unless we are in a vacuum, there is matter in the air whose atoms will be heated by this radiation. Extra water vapor in the air acts as a sponge for infrared. The further the journey, the more heat is lost along the way. Even if 1/2 of 1 percent of the radiated heat goes into the air, you are still setting up eddy currents within the air. The other 99.5 percent that may or may not make it to a far surface is heating your crawl space by all 3 mechanisms from the bottom and sides.

In this situation, you are heating your whole crawl space with the heat loss through the bottom of your floor. The radiated energy that is not drained away by the walls and ground is causing the convective heating of your crawl space that you ignore.

Lets compare no insulation to 4 inches of insulation below your floor.

With no insulation, and with your "warm-air-buoyancy" acting as a gradient layer of resistance to conduction and convection within this 4 inches at a value of Q=0.5 BTU/SqFt per hour per DegF, at a delta t of 30 DegF, this layer would lose heat at a rate of 1500 BTU per hour per 100 square feet to the crawl space below it. At the boundary of your floor joists, the air drops 5 degF and nature takes its course from there. Somewhere in the middle of the crawl space, the air has a mean value of 17.5 degrees below floor temp.

With the 4" of insulation, lets say it cut that Q value to 0.05 and in that 4 inches, the temperature drops 22 degF. In this case, the layer would lose heat at only 150 BTU per hr per 100 SqFt downwards, the rest of which quickly heats the spaces in the floor between your heat pipes. Somewhere in the crawl space, the air has a mean value of 26 degrees below your floor temp.

More importantly, the insulation trapping and spreading this lost heat around gives the upper surface of the floor more effective means to heat the space above it.

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Old 12-09-13, 11:20 PM   #376
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It's pretty enjoyable reading the under-floor insulation scuffle; lots of good thoughts there, and I'll take them to heart. Today we finished installing all the PEX tubing, and we'll pour the concrete on Thursday barring a new rain storm. Here are a couple photos of the work. It will be good to have this phase wrapped up. Still stressing a bit over the report of low quality in the manifold.
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Old 12-09-13, 11:26 PM   #377
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Quote:
Originally Posted by jeff5may View Post
Yes it does!
Here are a couple of conflicting quotes from Wikipedia in the article titled "Radiant Energy":

"Radiant energy is one of the mechanisms by which energy can enter or leave an open system. Such a system can be man-made, such as a solar energy collector, or natural, such as the Earth's atmosphere. In geophysics, most atmospheric gases, including the greenhouse gases, allow the Sun's short-wavelength radiant energy to pass through to the Earth's surface, heating the ground and oceans. The absorbed solar energy is partly re-emitted as longer wavelength radiation (chiefly infrared radiation), some of which is absorbed by the atmospheric greenhouse gases."

and

"Radiant energy is used for radiant heating.[9] It can be generated electrically by infrared lamps, or can be absorbed from sunlight and used to heat water. The heat energy is emitted from a warm element (floor, wall, overhead panel) and warms people and other objects in rooms rather than directly heating the air. Because of this, the air temperature may be lower than in a conventionally heated building, even though the room appears just as comfortable."

I'd say the jury is still out! Seems to me that most of the energy radiated from a heated floor, whether it's radiated up or down, is going to be infrared, and atmospheric gasses be under my house as well as over it. Probably, because I can't stand the thought of not doing it, I'll be insulating my floor, but that's a ways off, and my mind is still open. mm

Last edited by michael; 12-09-13 at 11:30 PM..
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Old 12-09-13, 11:53 PM   #378
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Quote:
Originally Posted by jeff5may View Post
Yes it does!
With out a quote, what does this profound statement relate to??????????
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Old 12-10-13, 12:17 AM   #379
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Hi Berniebenz, I think you can figure it out! mm
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Old 12-10-13, 12:31 AM   #380
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Hi Berniebenz, I think you can figure it out! mm
Why would you assume that such would be worth my effort?

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