Do not take R value of solid wood at face value

Blog entry by daltxguy posted 02-05-2009 06:37 AM 48544 reads 0 times favorited 7 comments Add to Favorites Watch

This is a reprint of a recent article on some studies done to show that the straight R-value of solid wood is misleading. Even though it seems worse than other materials, because wood has a lot of mass it actually can store energy, not just prevent the ingress ( or egress) of energy, which is only what R-values measure.

Wood insulation properties studied

Lincoln University researchers have studied the paradox of houses built from solid wood seeming to have greater levels of heat retention and cosiness than the insulation value of the wood wall itself would suggest. On their own, solid wooden walls have a fairly low R-value – meaning that theoretically the walls should not provide a particularly high level of thermal insulation.

“But the R-value is just a measure of the material’s resistance to heat flow,” Research Engineer Dr Bellamy said. “It only says how much heat is passing through, not how much is being stored.” Researchers discovered the answer to this puzzle was wood’s superior thermal mass. Having worked extensively with the concrete industry where concrete’s ability to store heat is well known, Dr Bellamy was not expecting wood to provide as much thermal benefit as it does.

Traditionally brick and concrete were thought to have superior insulation and passive solar heating properties, but the researchers have shown that somewhat surprisingly, weight-for-weight, solid wood actually has over 2.5 times the thermal mass of concrete.

When comparing walls of the same thickness, due to the different densities of the two materials, concrete will still have greater thermal mass, but the little-appreciated thermal retention properties of solid wood walls means they do in fact play a significant role in regulating the interior comfort of a house.

They store the sun’s heat during the day and release it into the house at night. Dr Bellamy used a Danish building simulation model to confirm that when the thermal mass effect was added to the relatively low R value of the external walls, the solid wood house performed very favourably. The thermal performance of the solid wood house was further increased when solid wood internal walls and ceilings were used. “Basically, the more wood you use the better, Dr Bellamy said.” Source: NZ Wood

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View PaBull's profile


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posted 02-06-2009 05:13 PM

Nice study, but I rather have a brick or stone house in the desert than a wooden house. It will insulate me during the day from the heat and heat me at night, because the mass of the brick takes that long to pass through the heat.

The best is to use both, have two brick walls with an insulation blanket between.

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posted 02-07-2009 07:59 AM

My guess would be that the same argument would be true for adobe brick. The insulation value of the brick alone is not significant but the mass of the brick would serve to be both a source and a sink of energy thereby serving the purpose of ‘time shifting’ the energy from the sun/earth.

Gut feel and vernacular building methods would suggest that brick/stone structures would best suit hot climates to keep the heat out during the day while retaining some at night, whereas wood structures would suit cold climates to keep cold out and heat in. This might only be a function of the abundance of local building materials in each of those climates, however.

Having said that, the Anasazi of the southwest actually built structures using logs at first, covered with mud, partially buried in the ground. Clearly, they had access to both for several centuries. Speculation is that at least one of the reasons they disappeared is because of deforestation.

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posted 02-08-2009 05:34 AM

i am completely with you, Daltxguy.

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posted 02-09-2009 06:57 PM

A very interesting article.Thanks for sharing it.

-- Dustygirl Hastings,Ont. Life is too short to sit around doing nothing

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Big Al

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posted 02-10-2009 01:58 AM

I agree with everything said. I found it hard to defend the idea of log building with my new teacher. I did a project on it last year. Apparently they are good for in my area is not only for keeping the heat in in winter, but giving out stored humidity in winter, and if i remember correctly they absorb some of the humidity in the summer. Also they can be reused from older log homes and moved to a new location for reuse.

I thought i’d share this information from the Ontario building code about the R-value of insulation:
(R-value/inch of thickness)

Batt insulation
Glass fibre 3.2
Mineral fibre 3.5

loose fills
Glass fibre 2.9
mineral fibre 3.3
Cellulose fibre 3.6

Polyisocyanurate 6.5
Expanded Polystyrene 3.8 to 4.4
Extruded Polystyrene 5.0
High density Glass fibre 4.2 to 4.5
Polyurethane 6.0
Phenolic 5.0

Spray type
Cellulose fibre 3.5
Isocyanurate 5.0
Polyurethane 6.0

Wood 1.0
Brick 0.2

-- BigAl, Ontario Canada

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posted 02-14-2009 10:40 PM

Hi tourismo206,

Thanks for posting that. It’s obvious to see why an engineer behind a desk looking at pure numbers ( which measures only 1 aspect) might pick some chemical soup solution and state that it is superior to centuries old building materials. Its interesting to see that cellulose fiber, nothing more than shredded paper derived from wood compares favorably with its high energy/petrochemical cousins.

I think I had the same teacher as you did in school. I once wrote an essay on why bicycles are so much better than automobiles, because they use so few resources to produce, get in excess of 1000mpg and have health benefits. He gave me an A on the paper for having done the research, but he doubted bicycles would ever amount to much more than a toy for kids. 35 years later, I still cycle every day to work.

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