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Thermal resistance of plywood

"Jim Thompson" wrote

I hope you can see this post as I've never gotten any acknowledgement on any other post I've made in this group.

From:

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For most practical purposes it is neither necessary nor feasible to determine the actual species makeup of the plywood panel. For determining the overall coefficient of heat transmission (U value) of a construction assembly, APA publications use k =

0.80 for softwood, as listed by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). Use of this single value simplifies computations, and produces only insignificant differences in resulting design heat losses.

The table below shows thermal resistance, R, for several plywood panel thicknesses, based on k = 0.80. Thermal resistance represents the ability of the material to retard heat flow and is the reciprocal of k, adjusted for actual material thickness. (for a 3/4" panel R = 0.94)

The definitive authority on wood properties is R. Bruce Hoadley and his book, "Understanding wood: a craftsman's guide to wood technology", is considered by many to be the holy bible on wood properties.

I hope this helps. Art

Reply to
Artemus
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dude, DUDE, m stands for meters

Reply to
gearhead

Except by those who know enough physics to know what units of heat conductivity are - power per (area cubed * temperature difference / linear distance), which simplifies to power per (linear distance unit * temperature difference). People who know this and metric units as used in physics are familiar with watts per meter-kelvin and recognize that w/mK stated as a unit of thermal conductivity meant this.

It appears to me that you went quite a ways on a spelling flame.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Meters makes less sense; thermal conductivity might be in terms of such-and-such per square. Seems a little more specificity is needed.

Reply to
Robert Baer

Oh, but that is exactly what i said; convention is that the first letter in a sentence is capitalized; but you should know that?

Reply to
Robert Baer

Excellent question; i do not know - but the manufacturers data sheet should disclose that. Also, there are (web) sites that give thermal conductivity and R-values for a large group of different materials from cement to steel (and may include plywood; do not remember since it was over a year ago and only looked one and bypassed plywood if it was listed).

Reply to
Robert Baer

Ahh.. that is what someone else implied and did *not* specify. But since i am not into thermal conductivity and/or insulation, the "meter" does not make sense to me in this context. Would have to dig into this usage and the units.

Reply to
Robert Baer

Neeeerp! Conductivity is proportional to AREA and inversely proportional to THICKNESS.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

Thermal conductivity, in SI units, is specified in watts per meter-Kelvin.

Aluminum is about 240 w/m-K.

So if you had a 1-meter cube of the stuff, and forced a 1 degree C difference across opposite faces, 240 watts of heat would flow.

The thermal resistance would be the reciprocal, about 0.004 degrees C per watt.

Starting from there, it's easy to scale for area and thickness.

Fun fact: the thermal resistance of copper is about 150,000 degs C per watt per ohm.

John

Reply to
John Larkin

It also stands for milli.

John

Reply to
John Larkin

Thermal conductivity is proportional to area (m^2) and inversely proportional to thickness (m) which leaves you with just m.

0.13 W/mK is 0.13 W/K across a 1m cube.
Reply to
nospam

I very much doubt that any pine tree forests were cleared in the making of my home or shop -- here in western Oregon it's all Doug Fir.

Environmentalists are so dippy sometimes.

--
www.wescottdesign.com
Reply to
Tim Wescott

Surely you could find the numbers in an architectural handbook somewhere and do the math?

For the same box in aluminum the biggest limiting factor is airflow against the inside and outside surfaces -- IIRC from past projects something with about that much surface area and internal (but not external) stirring fans can dissipate 50-100 watts with a 30C temperature rise.

But I'm sure that your plywood will significantly increase the thermal resistance.

--
www.wescottdesign.com
Reply to
Tim Wescott

It's probably simpler to measure it. That also covers the wood-air connection and things like input from the sun.

But since you asked...

Conduction along the grain is about 2.5 times higher than across the grain.

For across the grain:

k = G (1.39 + 0.028 MC) + 0.165

k is thermal conductivity in units of Btu in / hr ft ft F

G is specific gravity MC is moisture content (%)

--
These are my opinions, not necessarily my employer\'s.  I hate spam.
Reply to
Hal Murray

NymNoNuts is dippy 100% of the time.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

I learned R-1 = 1 inch of dry wood. R-12 = 12 inches of wood. Plywood has glue so its less.

I learned in my basement, I used two sheets of foil/foam/foil and I get a lot more insulation than what the stuff specifies. It about 3/16 inch thick. On a cinderblock wall, which itself is R 3.5, I lay a sheet against the wall. Out from the wall about 1 inch I put another layer of the foiled sheet. Its almost as good as 2 inches of fiberglass.

I am getting an insultion value of over double what the foil specifies. The foil adds almost R1.5 for the reflective addition, but I am seeing more.

In the garage, I stapled porous aluminum on polyethelene sheeting to the joists on the ceiling. The results are far better than the R value. You cannot feel any radiated energy from the roof compared to before. Even in the winter its helps insulate mostly from reflective energy. The higher the differential the more reflective coatings play a role in heat transfer.

greg

Reply to
GregS

I'm sorry, but the only 2x4s in MOST of the nation are PINE.

Pine is the predominate construction media in the US.

Pine grows at a wide spacing, at two feet per year. Hemp grows, even tightly spaced, at two feet per month.

An acre of hemp can produce far more material than an acre of ANY tree species.

The fibers are also more manageable, and there are other products that can be derived as well.

Reply to
Archimedes' Lever

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Hope This Helps! Rich

Reply to
Rich Grise

Have you tried google?

Good Luck! Rich

Reply to
Rich Grise

I mentioned it right at the start of the thread, and was even derided for it.

I guess some folks will have to come fool circle to realize they were wrong.

Reply to
Archimedes' Lever

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