More Insulation or High Efficiency Heating & Cooling System?
Earlier this week, my HVAC guru, David Butler, made a statement in a LinkedIn discussion group that’s obvious to those of us who know building science and high performance homes, but it’s something that I think may not be clear to a lot of people who are building or remodeling their homes. His comment answered the question in the title of this article – Should you spend your money on a better building enclosure (air barrier and insulation) or on high efficiency air conditioners, heat pumps, or furnaces?
Earlier this week, my HVAC guru, David Butler, made a statement in a LinkedIn discussion group that’s obvious to those of us who know building science and high performance homes, but it’s something that I think may not be clear to a lot of people who are building or remodeling their homes. His comment answered the question in the title of this article – Should you spend your money on a better building enclosure (air barrier and insulation) or on high efficiency air conditioners, heat pumps, or furnaces?
Back in the days before I spent most of my waking hours thinking about heat transfer, the refrigeration cycle, and pressure testing, I might’ve been a bit confused had someone asked me that question. It’s all stuff to help cut down your energy bills, right? True, but there’s definitely a preferred
order (when you have the option). As David said, “Ironically, the more efficient the shell, the harder it is justify expensive high-end HVAC systems.”
Here’s the way to look at this.
- The building envelope reduces the amount of heating and cooling your home needs.
- High efficiency HVAC equipment reduces the amount of fuel you need to meet the heating and cooling needs.
Think of your house as a basketball game. The HVAC system is your offense. The building envelope is your defense. You might have an amazing offense, but if you keep getting clobbered on defense, you’re going to wear yourself out on the offensive side just trying to stay in the game. The better your defense, the less you have to rely on a high-powered offense. And you know what they say: “Offense sells tickets; Defense wins championships.”
In David’s quote above, he used the word ‘ironically’ because it might seem that if you want to build a high performance home, you have to put in an expensive, 
high-end, super-duper, brag-to-your-neighbors-about heating and cooling system. Right? Well, maybe not.
When we start talking about Deep Energy Retrofits, Net Zero Energy Homes, and homes that meet the Passive House standards, the building enclosure reduces the amount of heat flow between inside and outside to such a degree that the house needs little heating and cooling. I remember an article a few years ago about a Passive House in Illinois that claimed the house was so efficient, it could be heated with a blow dryer.
If you read my article about electric resistance heat, you know that using a blow dryer, which is electric resistance heat, isn’t the most efficient way to heat your home. But it doesn’t matter in that case. This was a home in Illinois, and if I’m remembering correctly, the heating bill for the whole winter was about $38. This is a clear case of energy conservation trumping energy efficiency.
So, as odd as it sounds, the more energy efficient the home is, the less sense it makes to put in high efficiency HVAC systems. Do as much as you can with the building envelope before you tackle the HVAC side of the equation. If you start with the heating and cooling equipment, you’ve probably lost the battle before you’ve even begun.
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This is very analogous to
This is very analogous to fuel savings via living close to work, vs. living a long way away and having a hybrid car.
Federal requirements probably don’t hurt the situation, but there are other paths to the agreed-upon goal. I have always marveled that an AC in Maine for example, is used so few hours and subject to the same energy efficiency standards as one in Texas. Conversely the higher efficiency furnaces make far less economic sense in Texas.
I was troubled however by
I was troubled however by your use of the words “more insulation” to stand in for energy efficiency. I am aware of many inefficient houses for which “insulation” is practically just a placebo. If a house has air leakage problems in a hot-humid environment, especially if that leakage is in ductwork in an unconditioned attic… more insulation will do little to solve the problem. But it has an image in consumer’s minds as a PRODUCT to buy for efficiency and savings, not a SOLUTION to a problem which is properly diagnosed.
This article starts out from
This article starts out from the wrong premise, though it ends in the right place. Faulty premise: that building envelope and HVAC systems are two separate decisions. The right conclusion: invest first in a tight envelope, then pick and HVAC system to supply the right amount of heating/cooling. I would guess that the builder/designer/engineer instead looked more holistically at the project. Electric heat is relatively cheap to install — so it made sense here to use the $10,000 — maybe more — it would take for a tradition system/ducting/etc for the building envelope. $38 worth of electricity is at most 400kwh per year, maybe 1.5 million BTUs and probably in the neighborhood of 500 pounds of CO2. Would a high efficiency gas furnace deliver that number of BTUs with lower carbon ouput? My back of the envelope calculation says maybe it would cut it in half. But with a number already this low, there’s no need.
Allison,
Allison,
You described envelope improvements such as insulation as conservation, and the used efficient to describe mechanical systems.
That seems to be a differential based on cost.
I would pose that efficient regards ‘how things work’ and conservation regards ‘how people work’. If you change how things work, the home owner / building operator does not have to make frequent decisions to do something as would be the case if you change how people work.
A programmed programmable thermostat changes how things work. Putting on a sweater and turning down the thermostat, or changing the program, is changing how people work.
Both have to work together to achieve the best savings. I find that home owners are pleased to know they can increase their comfort in some ways without making all the sacrifices.
M. Johnson
M. Johnson: The title was meant to meet homeowners where they’re at. You’re absolutely right that insulation by itself isn’t the answer, even though it’s all too often that that’s what people get sold. See my article, Don’t Insulate Your Attic!
Jim N.: Yes, you’re right, Jim. The house is a system, and cannot look at the components in isolation. I probably could have done a better job presenting it, but as I said to Mark above, I was aiming at the starting point of where a lot of homeowners come into this discussion. Regarding that furnace in the PassivHaus, if the heating bill for the whole winter was only $38, it would be a tremendous waste of resources to put in a furnace at all because there’s no way you could ever find one anywhere near small enough. That’s a big problem with a lot of homes today. See the guest posts by David Butler about this:
Just Say No to Furnaces in High Performance Homes
Heat Pumps and Hydronics – A Great Team for High Performance Homes
John N.:
John N.: See the article that I linked to with the phrase “energy conservation trumping energy efficiency.” It explains how I see the difference between conservation and efficiency.
Get the envelope performing
Get the envelope performing well – and that includes the ducts buried in insulation or inside the envelope.
Then install the most basic 13 seer system. Just size it right, like 5% below an aggressive Man J and S.
The AC system will be affordable, and easy to maintain and repair.
Christopher, my understanding
Christopher, my understanding is that we do not want to bury the ductwork in insulation (at least, not in a hot humid environment). Ductwork lying below the top of the required insulation depth leaves a hole in the blanket. Also, dew points in the low 70’s can cause some condensation on the ductwork when you only have R-6 duct insulation. This can be evaporated by the natural air movement in the attic space, but if the ducts are buried, the moisture remains in the insulation.
I’m not an “expert”, but I have been building and designing for many decades, and that is what I have been led to believe. Please correct me if I am in error.
Walter: Good points, and
Walter: Good points, and definitively things to consider before a retrofit or install.
That makes me wonder about ducts inside the thermal boundary in humid climates with surface dew. Maybe since the boundary is being dehumidified it is not such an issue. Or R8 duct insulation is needed with a good air barrier outside the insulation.
I am in no way any kind of expert on humid climates, and I am a dry climate expert.
As far as the “hole in the blanket” I usually handle the inspectors by getting them to see that the inside of the duct is already the inside of the thermal boundary. So the overall delta t, and heat transfer is markedly improved, by getting the ducts out of the attic and closer to the inside of the thermal boundary.
Thanks for elevating this
Thanks for elevating this important point, Allison. One of the problems we face as an industry is the proliferation of dumbed down prescriptive programs that wrongly emphasize product-based solutions, including high efficiency HVAC. This makes for an easy check-off on a form, rather than attacking more fundamental house-as-a-system issues.
I have nothing against high efficiency HVAC equipment, except that the money would nearly always be better spent on getting the basics right. I constantly tell builders they and their clients would be better off by putting an extra $1k into a first class duct system than upgrading to a two-stage variable speed HVAC system.
To Jim Nail: I think you’re missing the point with your carbon analysis. Budgets are never infinite. Presumably, if a $10k forced air system had been installed, it would have traded off against the envelope, and thus the higher loads would have produced much more than 500 lbs of carbon.
This is very simple, guys.
This is very simple, guys. Insulation works 24/7 but high efficiency HVAC does not. The high HVAC is only 15% more efficient than the standard one!
Putting the ductwork within
Putting the ductwork within the insulation envelope is the most sensible way to achieve optimal performance after sealing and insulating the ducts to prevent leakage. Since air conditioning lowers the dew point, condensation within the ductwork is, theoretically, never an issue, and, since the duct insulation is typically a foil-faced mylar, it functions as a zero-perm vapor barrier, so that ambient humidity within the insulated envelope never should touch the ducts, and condense. The condensate drain at the air-handler should take care of any residual problem with humidity condensing. If it doesn’t, then either the system is not well designed, or well executed, or there is too much ambient humidity within the living space.
@WJParker: Since Allison didn
@WJParker: Since Allison didn’t bite, I’ll throw in my 2 cents. The condensation issue you spoke of does not occur INSIDE the duct but on the surface of the outer jacket. This is especially a problem for an undersized system due to very long run times. In areas of the southeast where dew points may remain above 70F for a week or more, it’s not hard to imagine how a duct carrying a continuous stream of 55F supply air could eventually end up with a 70F jacket, dripping with water.
Burying ducts in the ceiling insulation is a bad idea in cold climates as well since the duct duct displaces ceiling insulation. You can easily see the impact with an IR camera.
I’ve seen some builders construct dams with sections of corrugated cardboard. I seriously doubt this is worth the hand labor required. If there’s no way to avoid ducts in attic, the best strategy is to support ducts just above insulation, thus keeping them out of the hottest parts of the attic. The worst possible configuration is an upflow with ducts supported from rafters.
Well, here be my two (2) bits
Well, here be my two (2) bits on Hi-E furns. I have two (2), a 20+ yr Lennox-pulse and a nine (9) yr old Trane. I have spent approx $300 in svc cost on the old Lennox and over a grande on the Trane. I say….too bad the Len was discontinued…but it DID make lotsa HVAC people…..VERY RICH!!!