In this day and age of inflation, supply chain disruption and labor shortages, anything is possible.
Q: Have you ever heated food in a wooden (bowl, pan, griddle, vessel, etc.) on a (gas, electric, infrared, induction, etc.) cooktop?
A: No, because that would be stupid. If the wood doesn’t begin to actually burn, it will not transfer heat from the source to the food very efficiently. (You may be OK in a microwave, but that method of increasing an object’s kinetic energy may invoke other concerns not germane to this particular topic.)
Q: Ahh, so what you are saying is that wood is a better insulator than conductor?
Q: Then why are builders and developers so quick to substitute metal studs for wood studs when the price of lumber jumps?
A: Because they don’t know as much as they should about building science. Keep reading, and you’ll know more than those builders.
I think it’s OK, even preferred, to explain a concept in one’s own words, regardless of how many times the same concept has been explained by others using the words of others. “Concepts,” like the laws of physics, can’t be patented or trademarked.
Q: What in the heck are you talking about?
A: I am talking about the emphasis that wood is a better insulator than conductor. And that metal is a better conductor than insulator. Uncle Joe Lstiburek’s (www.buildingscience.com) classic phrase is, “I know this because I’ve never seen a wood wire or a wood frying pan.”
I think the wooden pan and wire examples are spot on. I was trying to contrive my own example with other items that are typically made of metal, such as batteries and solar panels. However, a brief review of contemporary literature reveals that batteries and solar panels may indeed be made from wood. See the references below. You learn something every day.
Q: What does this have to do with your flagship topics?
A: Hang on.
Metal is a good conductor. It conducts heat from the stove top through the pan to your egg. It conducts electrons from your service panel to your receptacle where you have something plugged in. There is little resistance to the flow of heat or electrons. (Actually, the flow of heat also involves the flow of electrons.)
I have seen, in the real estate development world, several (even prominent and well known) builders and developers, pontificate: “I don’t care if the cost of lumber remains sky-high. I’ll simply build my home/office/whatever with metal studs.”
Your walls serve several essential functions: they contribute to supporting the dead loads of the structure as well as bracing its live and lateral loads. They also help keep the outside out and the inside in.
The measure by which they keep the outside out and the inside in is called the R-value. “R” stands for resistance; specifically, resistance to the flow of heat. Remember, cold is not a thing and things that are not a thing cannot flow. (What?)
There is no “cold.” There are only differing degrees of heat. (Yeah, I know, it’s winter in Michigan right now.) If in doubt about my bold assertion, revisit my earlier article, “You can’t make Pizza in a Freezer” here.
If you want a deeper dive into R values, it will come next week.
Q: If something is a good conductor of electricity, does that automatically mean it will be a good conductor of heat?
A: If the “something” in question is a metal, then the answer is yes. If the “something” is a non-metal, then the answer is that it typically won’t conduct electricity (but may conduct heat).
Q: If something is a good conductor of heat, does that automatically mean that it will be a good conductor of electricity?
A: If the “something” in question is a metal, then the answer is yes. If the “something” is a non-metal, then the answer is maybe, but usually no.
There are complicated relationships that further describe the connection between the flow of electricity and the flow of heat through a substance, such as the Wiedemann-Franz law, involving the Lorenz constant (2.45×10-8 Volt2 /K2 ).
(Yes, it’s over my head, too. Generalizations are fine here; none of us will be any better off by heading further into the weeds.)
Q: What kind of impact can metal studs make if substituted for wood without any forethought?
A: Metal is approximately five hundred times more conductive than wood. That’s why it feels cold to the touch – it is transporting heat from your fingertips at a rate 500 times greater than wood. Remember, “cold” is not a “thing” and once again, things that are not a “thing” can’t be transported. Your body simply interprets the exit of heat from it as “cold.”
Let’s say that a wall assembly specified on your construction plans is 5.5” thick and contains cavity insulation. The exterior is sheathed with wood, then there is cladding (wood siding, brick, etc.). If you simply substitute metal for wood without any other adjustments, the metal will conduct heat away from the interior of your wall to the exterior, 500 times faster. Your effective R-value of your wall will decrease by ⅔.
A: But wait. Wood studs can also decrease your R value due to a concept called thermal bridging, but only by 10-20%.
Q: How do you get 500x when the comparison is ⅔ compared to 10-20%?
A: ⅔ is 67%. 67% is 5x greater than a number such as, say, 13%. 5x is certainly not 500x. Perhaps the answer is, the conductivity of a material refers to all directions, while the flow of heat is directionally from the inside out. Or perhaps due to the lower thermal mass of the metal stud (compared to the solid wood studs), the conductivity property of the material is diminished in actual practice. Those are my best guesses. Remember – take home message here – metal stud substitution without any other planning will decimate your wall’s R-value. (And don’t forget, R-value also works in reverse in cooling climates and in mixed climates during cooling season. The heat that you don’t want to drive in, drives in.)
Q: Is there a way to use metal studs and not lose a wall assembly’s insulating value?
A: Yes – it’s called continuous exterior insulation: You design the wall so that some, most, or maybe all of the insulation on the outside of the assembly so that the heat can’t be conducted away as fast.
(Note the key word: design.)
Continuous exterior insulation helps build a very well-insulated, low-energy house. Even a house that produces more energy than it consumes (these concepts lead to a discussion on Passive Houses and Net Zero Houses, the latter of which I have studied extensively). Exterior insulation thickness may vary, but it’s typically a minimum of 1” thick (which would reduce thermal bridging and conductivity and thus the downward drag of those items on your R-value, but it would not in and of itself add a whole lot to your R-value. To conceptualize continuous exterior insulation, think in thicknesses of 2” or greater. For example, I constructed a couple of commercial buildings in which the occupants are completely happy with the thermal comfort obtained with 1” exterior insulation combined with 5.5” of cavity insulation, but in my own home, I’m going for 3” of exterior insulation.
Q: But that seems like it would change the wall thickness fairly substantially, and let’s say I planned to clad with brick – I may not have room for the brick to sit on the brick ledge!
A: Correct. (Back to the operative word: design.) And what if you have specified a ventilated rainscreen between your insulation and cladding (this typically adds another ¾”)? Now do you see why these things need to be planned in advance and not changed on a whim?
Q: You seem to be against metal studs. Why, and are there any other reasons we should be careful with them?
A: 1. I love metal studs. They don’t warp; they stay straight and true. They are pre-drilled for wires. But I love them for interior partition walls, not exterior walls (unless you are following robust design practices and have robust continuous exterior insulation.)
2. In the unfortunate event of a fire, metal studs fail catastrophically. That is, they look OK up to a certain point, but then they totally give way. In the case of a wood stud, it’s either burned or not…and the fire has to get to the wood stud to burn it. With a metal stud, the presence of heat generated by a fire can cause failure, even if the fire hasn’t reached that exact location within the enclosure.
Q: A great many of your readers are doctors. These concepts may not seem sexy or interesting to a healthcare professional. Even beneath him/her. What is the connection and why should these people care?
A: I have helped doctors and other health care professionals design, construct, and remodel their facilities. If a doctor is in the middle of a construction project on his or her own building, investment property, or personal residence, and due to current factors the budget becomes strained, and then the doctor gets a call from the builder needing an immediate answer to “Hey, if we simply substitute metal studs for wood studs, we will be within budget again”, at least now the doctor will know that changing one thing really leads to a cascade of other required thought processes.
Or as I say, “Doctors, know thy buildings.” (Don’t abdicate responsibility because you assume the other person knows more than you do about a certain topic.) The other person will not have to live with the consequences of that decision for the next several years or decades.
There you have it… Heating food and different types of studs.
Generally, not favorable cocktail party topics (unless you’ve been to a cocktail party with me, then you know all topics are fair game.) But nonetheless important in structure design. As a forethought, not an afterthought due to budget constraints. Because why would you want the thermal performance of your enclosure to suffer forever as a consequence of an immediate decision based on a price increase of one component compounded by lack of understanding of building science?
Whether you live in the structure or your business does, or you rent the structure to other occupants, thermal comfort will always remain one of the most important pieces of indoor environmental quality.
Heck, ASHRAE (The American Society of Heating, Refrigerating, and Air Conditioning Engineers) even wrote a standard about it – ASHRAE Standard 55.
As you think about that, also consider other decisions within the building design process that are typically delegated to “others.” What else comes to mind?
Never forget that “others” don’t always have your best interests in mind.
And “by others” can sometimes be the two most dangerous words in the English language.
I invite your questions and feedback about this topic or any of my other topics – send an email to Lee@DrLeeNewton.com.
Until next time,
Dr. Lee Newton
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