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‘Walls Need to Breathe’ and 9 Other Green Building Myths

The Energy Nerd plays Whack-a-Mole with some common misunderstandings in home building

Posted on Aug 20 2010 by Martin Holladay

Just for fun, I’ve rounded up ten oft-repeated statements that are either half-truths or outright falsehoods. I’m sure some readers will disagree with my conclusions; if you’re one of them, don’t hesitate to post a comment.

Green building myth #1. New York City is an environmental nightmare
This myth has been debunked many times, most recently by author David Owen, in his New Yorker article titled “Green Manhattan.” In fact, the average resident of Manhattan uses much less energy, and has a much smaller carbon footprintAmount of carbon dioxide and other greenhouse gases that a person, community, industry, or other entity contributes to the atmosphere through energy use, transportation, and other means. , than the average American. Compared to a resident of New York City, the average suburban American is wearing carbon clown shoes.

Owen wrote, “Most Americans, including most New Yorkers, think of New York City as an ecological nightmare, a wasteland of concrete and garbage and diesel fumes and traffic jams, but in comparison with the rest of America it’s a model of environmental responsibility. By the most significant measures, New York is the greenest community in the United States, and one of the greenest cities in the world. … The average Manhattanite consumes gasoline at a rate that the country as a whole hasn’t matched since the mid-nineteen-twenties, when the most widely owned car in the United States was the Ford Model T. Eighty-two per cent of Manhattan residents travel to work by public transit, by bicycle, or on foot. That’s ten times the rate for Americans in general, and eight times the rate for residents of Los Angeles County.”

In a separate article, Owen explains why the residents of Manhattan are so much greener than Vermonters.

Green building myth #2. Walls have to breathe
Bored readers may move on to the next item; I know that this is a tired old argument. But the “walls have to breathe” statement still keeps popping up, so I’ll take this opportunity to whack it back into its hole.

As I’ve written elsewhere, mammals and birds breathe to oxygenate their blood. Walls don’t have any blood that needs to be oxygenated, so they don’t need to breathe.

Walls have different needs from people. Although walls don’t need to breathe, they do need to be able to dry out when they get wet. And a building’s residents need fresh air.

Most people who are hung up on the “walls have to breathe” idea seem to like straw-bale walls covered with plaster or clay; evidently “walls that breathe” need to be vapor-permeable. (I think. But you had better ask the people who say “walls have to breathe” to be sure.)

But it’s possible to build a high-performance wall that is, for all intents and purposes, vapor-impermeable and airtight. Consider a well-sealed wall that is sheathed with foil-faced polyisocyanurate insulation. Almost no water vapor or air can get through the wall, so most people would agree that the wall doesn’t “breathe.” But it can perform quite well, as long as it can dry on both sides of the polyiso. (This could be accomplished by installing rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. siding on the exterior of the polyiso, and permeable materials on the inside of the polyiso.)

Of course, the occupants of any building need fresh air. That’s why buildings have operable windows and mechanical ventilation systems.

Green building myth #3. Renovation is less expensive than new construction
Whether this is true depends on the extent of the intended renovation. If you’re aiming for a high-performance home, you’re probably embarking on a deep-energy retrofit. My advice: double-check your bank balance, and good luck.

If you live in a drafty house with poorly insulated walls, a poorly insulated ceiling, a damp basement with no insulation, and old windows, all you need is everything.

A remodeling contractor will be glad to help you. He knows how to build what you need, and he’s done it many times. The only problem with your job is that your existing house is in the way. That’s why the work will cost more than new construction.

Green building myth #4. Spray polyurethane foam creates an air barrier
If you install spray polyurethane foam in your wall and ceiling cavities, you don’t have an air barrier; you’ve just got one component of an air barrier.

Here are just a few of the potential air leakage sites that need to be addressed before your foam-insulated house has an air barrier:

  • The crack between the foundation and the mudsill.
  • The crack between the mudsill and the rim joist.
  • The crack between the subfloor and the bottom plates.
  • The crack between each pair of doubled studs.
  • The crack between the top plates and the second-floor rim joist.
  • The cracks around the perimeter of your attic access hatch.

Green building myth #5. Caulking the exterior of a house reduces air leakage
For the last 30 years, newspaper advice columnists have been telling homeowners that a good way to seal air leaks in a wall is to caulk any cracks on the home’s exterior. This is bad advice, for two reasons: first, because the most significant air leaks in a typical home are located elsewhere, and second, because exterior caulk can do more harm than good.

A caulk gun in the hands of an overenthusiastic homeowner can be a dangerous weapon. It’s not unusual to see caulk installed where it doesn’t belong — for example, blocking drainage at the horizontal crack between courses of lap siding, or blocking weep holes in windows.

If you want to improve the airtightness of your house, put away the caulk gun and ladder. Instead, get a few cans of spray foam and head for the basement or attic.

Green building myth #6. R-value tests only measure conductive heat flow
Of the three heat-flow mechanisms — conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., convection, and radiation — radiation is probably the least understood. Sensing an opportunity, some marketers of radiant barriers, reflective insulations, and “ceramic coatings” take advantage of common misconceptions to promote their products.

An oft-repeated falsehood is that “R-value measures only conductive heat transfer.” In fact, R-values include all three heat transfer mechanisms.

The most common method of testing a material’s R-value is ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. C518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. In this test, a technician measures the thermal resistance (resistance to heat flow) of a specimen of insulation placed between a cold plate and a hot plate.

To understand how all three heat transfer mechanisms are involved, consider the flow of heat across a fiberglass batt. Heat wants to flow from the hot side of the fiberglass batt to the cold side. Where individual glass fibers touch each other, heat is transferred from fiber to fiber by conduction. Where fibers are separated by an air space, heat is transferred from a hot fiber to a cooler fiber by radiation and by conduction through the air. In ASTM C518 tests of fiberglass insulation, air movement within the fiberglass batt (convective heat transfer) is rare, although the test captures the phenomenon when it occurs.

For more information on R-value testing, read “Understanding R-Value.”

Green building myth #7. Air conditioned homes don’t need a dehumidifier
In a hot humid climate, air conditioners make a home more comfortable by lowering the temperature of the air (sensible heat removal) and by dehumidifying the air (latent heat removal). When the thermostat detects that the indoor air temperature is too warm, the air conditioner switches on; when the thermostat is satisfied, the air conditioner switches off.

While the equipment is operating, some dehumidification occurs. However, the ratio of latent heat removal to sensible heat removal is a function of equipment design and weather conditions; it is out of the control of the homeowner.

When an air conditioner runs flat out for hours at a time, it’s usually pretty good at dehumidification. But in an energy-efficient house with low-solar-gain windows, the typical air conditioner runs for fewer hours. Although the equipment easily cools the house, it may not lower indoor humidity levels to comfortable levels.

As reported in the January 2003 issue of Energy Design Update, researchers in Houston were called to investigate high levels of indoor humidity plaguing a group of energy-efficient homes participating in the US Department of Energy’s Building America program. They discovered that “improvements in window performance and envelope tightness … lowered the buildings’ sensible cooling loads to the point that existing air conditioners [were] unable to handle the latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates..” The recommended solution: each house needed a stand-alone dehumidifier in addition to a central air conditioner.

As homes continue to be built to higher energy standards, the need for supplemental dehumidification is likely to increase in hot humid climates along the Gulf Coast and elsewhere in the Southeast. Stand-alone dehumidifiers are a fairly inexpensive solution to the problem. Unlike an air conditioner, a stand-alone dehumidifier continues to lower indoor humidity until the desired set point is reached. The downside: a dehumidifier adds heat to the house. As long as the house has a properly sized air conditioner, however, this shouldn’t be a problem.

Green building myth #8. Efficiency rating labels on furnaces account for all types of energy
The Annual Fuel Utilization Efficiency(AFUE) Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. (AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. ) number on a furnace or boiler label does not include any accounting of electrical energy. As a result, an apparently efficient appliance with a high AFUE may still be an electrical hog.

The AFUE number is a laboratory rating of an appliance’s efficiency at burning natural gas, propane, or oil. The calculation accounts for typical chimney losses, jacket losses, and cycling losses, but not electricity use.

A gas furnace has several electrical components, including the furnace fan (by far the biggest electrical load), an igniter, a draft inducer, and controls. Oil furnaces include an oil pump, an oil burner motor, perhaps a power vent unit, and a furnace fan. The AFUE gives no clues concerning the power draw required to run these electrical components, which varies from appliance to appliance.

Most furnace fans draw between 500 and 800 watts, with an annual electricity use that averages about 500 kWh per year. Furnace fans account for 80% of the electricity used by furnaces, so total furnace electricity use averages about 625 kWh per year. If a homeowner operates the furnace fan continuously, either to improve air mixing or to satisfy an electronic air cleaner, annual electricity use is much higher. Since inefficient furnace fans produce waste heat, they are particularly problematic in cooling climates.

If you’re buying a new furnace, look for one with a blower powered by an electronically commutated motor (ECM). Such motors use significantly less electricity than conventional permanent split capacitor (PSC) motors.

Green building myth #9. In-floor radiant heating systems save energy
Proponents of in-floor radiant heating systems often claim that such systems save energy compared to conventional heating systems. The idea is that people living in homes with warm floors are so comfortable that they voluntarily lower their thermostats, thereby saving heat.

The only problem with the theory is that no reputable study has ever shown it to be true, while at least one study has disproved it. Canadian researchers visited 75 homes during the winter to note where the homeowners set their thermostats. The 50 houses with in-floor radiant heating systems had thermostats set at an average of 68.7°F, which was a little bit higher than the thermostat setting at the 25 homes with other types of heat delivery (either forced air or hydronic baseboard), which averaged 67.6°F. Since homes with radiant floors don't have lower thermostat settings, the researchers concluded that “there will generally be no energy savings [attributable] to lower thermostat settings with in-floor heating systems.”

Other radiant floor proponents have suggested that homes with radiant floors can have lower boiler temperatures compared to homes with baseboard units. This factor, however, would be responsible for only very minor energy savings, if any. It has also been suggested that homes with radiant floors might have reduced infiltration compared to homes with forced air heat. While this is certainly possible, high infiltration rates are best solved by addressing air-barrier problems at the time of construction.

Radiant floors, like baseboard radiators, are a heat-distribution system. When it comes to heat distribution, a BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. is a Btu. The overall efficiency of a hydronic heating system is basically governed by the boiler; the distribution equipment plays only a minor role in system efficiency.

Finally, it should be noted that a home with a slab-on-grade radiant floor heating system may have higher heat loss to the ground that would a home with a forced-air heating system — a factor that might lower rather than increase the radiant heating system’s overall efficiency. The best way to counteract this problem would be to increase the thickness of insulation under the slab.

Green building myth #10. Green building helps save the environment
New construction, like virtually all economic activity, has a detrimental effect on the environment. To build a new house in the Northeast, trees must be cut on the building site and a foundation must be dug. More trees are cut to supply lumber, and cement factories must burn fuel to produce the necessary cement. And so on.

Unless you’re engaged in environmental restoration — for example, decontaminating the soil at an abandoned industrial site so that trees can once again be established there — doing nothing is always better for the environment than developing land and building new homes.

If it’s important to build a new building, of course the construction should be done with environmental sensitivity. But I propose we give a green building award every year to any American who didn’t build a house. These people are true green heroes!

Portions of this article first appeared in the Journal of Light Construction.

Last week’s blog: “Saving Energy With Manual J and Manual D.”

Photo credit for photo on home page rotator: Peter Lenardon

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Image Credits:

  1. Daniel Morrison
  2. Wally Gobetz

Aug 20, 2010 11:39 AM ET

Tons of good info
by jeff williams

I'm pretty new to the site so I wasn't aware of all of the myths. I especially enjoyed the in-floor heating one. Do you have the link for the temp comparison study for the in-floor vs forced air? I'd love to see how the 75 homes break down. If they had similar outdoor temps, from the same region, etc.

Aug 20, 2010 11:50 AM ET

Response to Jeff Williams
by Martin Holladay

Jeff Williams,
Here's a link to the Canadian study: Thermostat Settings in Houses with In-Floor Heating.

I'll add the link to the body of the article as well.

Aug 20, 2010 1:33 PM ET

Diminishing "Value" of R-Value ....Myth?
by John Brooks

How about this caption from A FHB article:

"As the thickness of the insulation increases for both open-cell and closed-cell foam, the insulating value of each diminishes drastically."

Many of my friends buy into this "Myth".

Aug 20, 2010 1:38 PM ET

Aug 20, 2010 1:43 PM ET

Response to John Brooks
by Martin Holladay

That's a good one. I could have included it in this list. There's no doubt that the FHB article muddied the waters instead of clarifying the truth on that issue.

As I have often pointed out, the R-value per inch of spray polyurethane foam, like the R-value per inch of all insulation types, does not appreciably change with thickness. If an insulation has an R-value of R-3.5 per inch, then 2 inches of insulation has an R-value of R-7.0, and 10 inches of insulation has an R-value of R-35.

The FHB article should have noted, "As the thickness of the insulation increases for both open-cell and closed-cell foam, the insulating value also increases."

Aug 20, 2010 2:59 PM ET

radiant heat systems
by Brennan Less

Martin, I think you've written off some of the potential savings from radiant heating systems with hot water distribution. I agree with most of your points, but I think that when you say that distribution is not important, we have a point of disagreement. The differences between hot water distribution and forced air distribution are interesting and meaningful. Heat transfer/loss is a function of surface area, thermal conductivity and the temperature rise across the surface (in this case either a duct or a water pipe). Water stores heat much more efficiently than air, meaning that a smaller volume of water can deliver the same Btu's as a much larger volume of air. As a result, you need a much smaller distribution system, usually with MUCH smaller surface areas (think of 3/4" pipe vs 10" air duct). This minimization of distribution surface area certainly increases the overall efficiency of a hyrdronic heating system in comparison to a forced air system of the same combustion efficiency. Of course, the delta temp may be higher, which could offset some of this benefit...When you say: "the distribution equipment plays only a minor role in system efficiency", I believe you are incorrect...unless you're saying that in-floor radiant is equally efficient as baseboard radiators, and you are ignoring the system that brings heat to these devices? I would say that it is clear that distribution equipment (ie. hot water piping, forced air ducts, etc) has a MAJOR impact on overall heating/cooling system efficiency. Of course, all your other points are valid and entertaining, good post.

Aug 20, 2010 3:17 PM ET

Response to Brennan
by Martin Holladay

In most homes in the Northeast, whether heated with a furnace or a boiler, the distribution system is entirely within the home's thermal envelope. (We don't put ducts or pipes in attics here in New England.) Any heat loss from distribution pipes or ducts just heats the interior of the home.

There are exceptions, of course. If ductwork is located between the joists separating the first floor from the second floor in a two-story home, and if the supply ducts are leaky, they can pressurize the joist bays. This pressurization can force heated air outdoors through the rim joist, decreasing the efficiency of the distribution system.

Obviously, to take an extreme example, a hydronic distribution system that is located entirely within a home's thermal envelope will certainly be more efficient than ductwork located in an unconditioned attic. But that's an apples to oranges comparison, and an example of gross stupidity.

The standard heat distribution system in northern New England is hydronic baseboard. I don't see any evidence that in-floor radiant heat distribution is more efficient than hydronic baseboard heat -- and if the hydronic system requires high slab temperatures in a slab-on-grade home, the in-floor system may be less efficient.

If we are comparing in-floor radiant to forced hot air, you are quite right that the in-floor radiant distribution system should be much more efficient than ductwork installed in an unconditioned attic.

You wrote, "This minimization of distribution surface area certainly increases the overall efficiency of a hydronic heating system in comparison to a forced air system of the same combustion efficiency." Why would that be? Again, for apples to apples, let's assume both distribution systems are inside of the home's thermal envelope. Heat is heat. Where is the heat going -- the distribution losses in the forced air system that you allege contribute to forced-air inefficiency?

Aug 20, 2010 3:56 PM ET

Heat is heat, but it is also not where you need it
by Brennan Less

You are right that the location of the distribution system is important, but to say that heat loss/gain in conditioned/semi-conditioned spaces has no impact on system efficiency, I believe is false. At the very least, if a system is dumping large amounts of heat into a basement in the NE, then the delivery temperatures will be lower and the unit will need to run longer in order to change the sensible temperature at the thermostat. Or, the home will become uncomfortable. Heat that is dumped into the basement is not necessarily "lost", but it also is not delivered in the necessary way to a region that requires heat. To say that heat lost along the way within the thermal envelope is the same as heat delivered is untrue. For example, in a basement, the accumulated heat from distribution losses will tend to follow the strongest temperature gradient, which almost certainly is through the foundation walls and basement slab. That heat does not necessarily travel into the useful space upstairs, and if it does, it does so with a time lag, thermal losses along the way, and with uneven mixing. In situations where the heat is lost into interior partition walls or framing chases, then that heat creates thermal drive, turning that cavity into a chimney. Once again, the heat lost along the way, even when in the thermal boundary, more likely escapes directly to the outside, rather than transferring to the conditioned space. Cheers.

Aug 20, 2010 4:47 PM ET

Response to Brennan
by Martin Holladay

As you can imagine, GBA provides advice to builders of green homes. In general, green builders strive for low levels of air leakage through their thermal envelopes, and high levels of insulation.

In such buildings, it's extremely unlikely that radiant-floor heating systems will have distribution systems that are more efficient than other heating systems.

As you probably know, with the trend toward Passivhaus buildings, designers are realizing that it is less and less important where heat is delivered. The better the envelope, the less it matters. Some Passivhaus buildings are heated from a single point source.

You are certainly correct that if a boiler or furnace is located in the basement, some heat will be lost from the ducts of a forced air system, just as some heat will be lost from distribution tubing of a hydronic system. You still haven't demonstrated why the fact that some tubing is stapled up to the subfloor, while other tubing runs through hydronic baseboards, affects efficiency.

If you're losing too much heat through your basement walls, then for heaven's sake, insulate your basement walls!

Obviously, good heat distribution system design requires loads to be calculated for each room, and for distribution systems to deliver the right amount of heat to each room. Distribution ducts or tubing should be insulated to minimized heat loss, unless it is desirable to heat the basement. Bad distribution system design results in a bad outcome -- no matter whether you have an in-floor radiant system or a forced-air system.

Aug 20, 2010 5:02 PM ET

by Paul Brazelton


I'm not sure if I understand #3. Are you trying to say that feature for feature, renovation is more expensive than new construction? Because it seems like even the most rigorous deep energy retrofit is still cheaper than building a whole new house. I understand that you may be saying, "tearing off all of your siding, insulating, sealing and residing an existing house is more expensive than just building an exterior wall on a new house," but it doesn't read that way.

#10 could also have wording to soften #3. Perhaps a renovation would cost more in some ways than new construction, but the environmental impact is radically smaller.

Aug 20, 2010 5:23 PM ET

In-floor heating
by Bryce

In our neck of the woods, in-floor heating tends to INCREASE energy use as 99.99% don't understand the second law of thermo-dynamics. They use "double-bubble" insulation and think they've done a good job until they get their first electric bills. The issue comes from the delta-t....the higher slab temperatures have increased the heat transfer through the slab to the soil below. As they can't increase the r-value of the slab, I recommend they install a forced air system and abandon their slab heating. Amazingly, their energy use goes down.

Aug 20, 2010 5:33 PM ET

Best thing I've read in the last five years
by Steve P

Great column!

I'm always amazed by how many people ignore the laws of physics or attempt to bend them to their will.

Energy can neither be created or destroyed, folks.

The higher the differential/potential, the greater the transfer.

Simply put, you put energy (let's call it heat in this case) into a house. It wants to get out.

The higher the differential between in and out, the quicker it leaves. There are ways to mitigate this. You can seal the house. You can insulate it. And you can turn the thermostat down.

But it doesn't matter if it is hot-water, hot-air, electric baseboard, radiant electric, hydronic or infra-red from the planet Zon - you are going to use X amount of energy to get Y effect.

The biggest benefit in energy savings (after decent building standards) is probably zoned heating with setbacks so that heat is only used where it is needed.

Can we also address the myth (IMHO) that incandescent light bulbs "waste" energy? Where I live (in Maine) we have short days in the winter heating season and long days in the summer. If I turn on a 100 watt light bulb to read on a winter's night, where does that "waste" heat go to?

Aug 20, 2010 5:48 PM ET

Back to Martin
by Brennan Less

I agree that in-floor vs. baseboard systems should be similar, except that most baseboard systems deliver heat at a point on an exterior wall with some thermal drive to the outside. I am approaching this issue more from an existing homes perspective, rather than from a passive house sort of mindset. So, I do agree that with more efficient envelopes, all these issues become less important. BUT the fact of the matter is that, on its face, hydronic heat distribution, in an apples-to-apples comparison, will have less heat loss along the way, likely making it more efficient. The impact of this efficiency gain will be less in an efficient home (both in terms of energy and comfort), but it still seems to me that it will be there. I totally agree that a poorly designed hydronic system will perform badly, but equally well-designed hydronic and forced air systems will not perform the seems to me to be a matter simply of surface area and leakage losses, both of which will be less in hydronic systems. Same length, same insulation level, same location, same fluid temperature; hydronic will lose less heat long the way. Thanks for the banter.

Aug 20, 2010 6:43 PM ET

Do you want to go green? Stop reproducing.
by aj builder, Upstate NY Zone 6a

There is only one way to personally truly impact sustainability
1) Do not have offspring
2) Convince others to not have offspring
3) Vote for those that advocate reducing world populatiion
4) If you have offspring, convince them not to have offspring

Building and renovating homes to any present known way... is not doing what one less birth can do.

Earthly sustainability is directy related to population and little else by several orders of magnitude.

Play with your neighbor's kids and build any home you want. That is living as green as one can.

Or just get off the planet yourself. If you choose this last choice, thank you.

Don't take me wrong.... as I do enjoy redesigning homes to use less energy.... I just know that it has very little impact compared to overpopulation.

Aug 20, 2010 7:01 PM ET

Walls need to breathe
by Ray T.

You sort of have it correct. Healthy walls need to be able to allow water VAPOR, not water, to pass easily. When walls get wet, they dry out by evaporation. Hence the need for water vapor permeable membranes or coatings. Walls can be insulated AND vapor permeable. They are not mutually exclusive. No need for non-biodegradable polyiso this or polyiso that.

Aug 21, 2010 4:08 AM ET

Response to Ray T.
by Martin Holladay

Ray T.,
I'm afraid I still don't agree with you. As my example (a wall with polyiso sheathing) demonstrated, walls don't have to be vapor permeable. They just need to be able to dry on either side of their least permeable layer.

You are correct that walls CAN be vapor permeable. (A straw bale wall performs extremely well, as long as it is protected from rain and has a dry footing.) But they don't HAVE to be vapor permeable. Nor do they have to "breathe," whatever that means.

Aug 21, 2010 4:16 AM ET

Response to Paul Brazelton
by Martin Holladay

You asked, "Are you trying to say that feature for feature, renovation is more expensive than new construction?"

Yes, that's what I'm saying. If you want a new kitchen with an island sink, oak cabinets, granite counterops, Viking range, and Sub Zero refrigerator, the kitchen will be cheaper to install a new home than it will to retrofit into your current home.

You wrote, "It seems like even the most rigorous deep energy retrofit is still cheaper than building a whole new house." Ah, but it isn't -- especially if you mean "the most rigorous deep energy retrofit." Here's the story of one such retrofit:
A Leaky Old House Becomes a Net-Zero Showcase

The owner of a simple ranch house had a simple dream -- and her dream came true, $1,190,000 later.

Aug 21, 2010 10:32 AM ET

by Dan Kolbert

Agreed - as someone who does both new construction and renovations, the latter is far more expensive, illogical though it may sound. May be less material-intensive, though.

Aug 21, 2010 5:31 PM ET

distribution efficiency
by Dan Perunko


Great article I enjoyed it. I am, however, dissapointed to hear you say that distribution efficiency is not very important. We think we are seeing 50% or greater distibution losses in forced air systems in CA. I don't have a handy number to throw around for hydronic floors, but I have seen some really well heated rim joists out here. One of the primary improvements we focus on for occupant comfort and energy consumption is the distribution system, be it forced air or hot water. Maybe the building stock is just much worse in my market? In any case improving the distribution system in the typical CA home can reduce the building load by 30%. That is, with out improving the building itself we can reduce the required capacity of the conditioning equipment by as much as a third when we correct the delivery system.

Aug 22, 2010 4:14 AM ET

Response to Dan Perunko
by Martin Holladay

Dan Perunko,
I am a strong believer in the proper design of heating distribution systems. As you point out, hydronic systems are often very effective at heating rim joists, and forced-air duct systems are often very effective at heating and cooling the great outdoors.

I have little patience, however, for hucksters and salespeople who exaggerate and mix issues to suit their economic purposes. All residential heating systems, whether hydronic or forced air, need good distribution system design. Bad thermal envelopes leak heat -- whether at the rim joist, basement walls, or second-floor ceilings. The solution to bad thermal envelope design is better thermal envelopes. The solution to poorly insulated hydronic tubing or ductwork in unconditioned spaces is to move the tubing or ductwork inside of the conditioned space.

I remain unconvinced that in-floor radiant heating systems save energy compared to other types of heating systems -- which was the point I was trying to make.

Aug 22, 2010 10:14 AM ET

The Greenest Building is... One Already Built
by Kevin Dickson, MSME

This myth is similar to #3 above. Unfortunately, it is being spread as absolute fact by nearly all preservationists.

Preservation is very important and often for the greater good, but the adoption of an unscientific slogan is a classic case of myth-spreading.

Aug 22, 2010 12:52 PM ET

Re: Diminishing "Value" of R-Value ....Myth?
by tim Rowledge

Original problem paragraph - "As the thickness of the insulation increases for both open-cell and closed-cell foam, the insulating value of each diminishes drastically."

I suspect that what might have been intended would be
"As the thickness of the insulation increases the marginal insulation value of each inch diminishes"
Add qualifiers about open and closed cell as needed. If you add 1" to a 2" layer you get a 50% improvement. Add 1" to a 6" layer and you get a 15% improvement. Adding 1" to a 12" layer gains you 8% etc. In general the cost of adding each 1" is about the same (I guess - there are probably places where it cost more when you get to using odd size lumber and stuff) so it is reasonable to consider each increment having 'less value'.

It's all part of the discussion about how valuable it is to achieve actual Passiv-Haus or Net-Zero status. Does it make sense to spend $20,000 to reduce the annual energy bill by $300? How about $50,000 to save the last $300? Can anyone invent a practical way to reduce the payback time? And so on.

Aug 22, 2010 5:24 PM ET

Response to Kevin Dickson
by James Morgan

"The greenest building is the one already built" may be an oversimplification but is perhaps a necessary counter-perspective to the new-construction bias of much of the green building dialectic. Your linked article carefully points out the significant financial considerations in the historic preservation calculus. Many historic preservation battles are fought over locations where the land value has come to greatly exceed that of the building which sits upon it. For many of us laboring in the vineyard of more modest structures the reverse is true and it is simply not an option for our clients to demolish a home with poor energy performance because its bricks, shingles and lumber represent the bulk of their financial investment in the property. If any of them consider building new it will be on a new lot with new infrastructure to assemble and most likely further out of town: meanwhile the old 'gas-guzzler' continues to consume more energy than it should, just as before only with a new owner. No matter how 'green' the new home is it represents a net addition to overall energy and material consumption, just as buying a new Energy Star refrigerator while moving the old one to the garage to keep a few beers chilled results in a net energy-use increase. For that reason alone I will continue to encourage my clients to rehab and upgrade wherever it makes sense for them and the overall condition and arrangement of the home justifies it: I do not even need to mention the cultural and community value of maintaining and improving neighborhoods through upcycling of existing housing stock.

Aug 22, 2010 6:27 PM ET

by Greg Duncan

Great post.

Tim, the value of each additional inch of insulation can be calculated on a case by case basis. I did a very quick rough estimate based on a townhouse in Germany. Increasing the insulation from 25 mm to 275 mm would cost about $10,000 but save about $3000 per year. Increasing the insulation from 275 mm to 525 mm would also cost about $10,000 -- probably more due to additional installation costs -- and only save about $200 per year. Passive House status was achieved for this building with 275 mm of insulation. It seems to me that as a general rule, additional insulation is worth the investment up until the point where the thickness interferes with constructibility and other practical concerns. No, it does not make sense to spend $20,000 to reduce annual energy costs by $300. But spending $10,000 to save $3000 per year does.

Aug 23, 2010 5:24 AM ET

Offsetting of Environmental Penalty
by Interested Onlooker

"...meanwhile the old 'gas-guzzler' continues to consume more energy than it should, just as before only with a new owner. No matter how 'green' the new home is it represents a net addition to overall energy and material consumption..."

Is the environmental cost of the green new-build not offset by the fact that the people who moved into the 'gas guzzler' did not incur the environmental penalty of the house that they didn't build?

Aug 23, 2010 5:59 AM ET

On remodeling costs, new construction ethics, and tear-downs
by Martin Holladay

Retrofit an existing house or build a new superinsulated house? What to do?

1. The U.S. today has too many homes -- more than the market can absorb. This situation will probably last for a decade or more. This is Fact #1: America doesn't need any new homes.

2. Developing undeveloped land is an environmental sin. In some cases it is a small sin, but it is a sin nevertheless.

3. As our country struggles to meet future carbon-reduction goals, we'll need to find cost-effective ways to slash our energy consumption. Eventually, carbon taxes will completely change the economic equation, and many homeowners, unable to afford space heating or cooling, will need to make a choice between a deep-energy retrofit or new construction.

4. For the majority of American homes, a deep-energy retrofit costs more than new construction. For those who are desperate to come up with a solution to this dilemma, the solution will usually involve a bulldozer. So -- in the future, we probably won't be developing much undeveloped land, but we will be doing a lot of tear-downs.

Aug 23, 2010 7:54 AM ET

Radiant in-floor
by DoctorJJ

Overall great article. Lots of good info.
In regards to comments numbered 8 and 9, it seems that an in-floor radiant system would save money due to the energy penalty of distribution with a forced air system. In number 8 you discuss the sometimes quite significant energy usage of a fan blower on a furnace which surely is more significant than the energy usage of a small pump associated with a hydronic system. Even considering that a BTU is a BTU and that both are within the envelope so that no heat in either type of system is lost to the outdoors, it still seems that a radiant in floor system would save energy through the above proposed mechanism. Or also consider the efficiency of an electric floor heating system such as the STEP Warmfloor where there are really zero losses fr having to move warmed air or fluids throughout a home. Again, mentioning the "hidden" energy usage of a furnace because it's fan isn't listed and may not be an energy efficient type in comment number 8 but then failing to account for this in comment number 9 doesn't make a lot of sense. I still loved the article though. Thanks again.

Aug 23, 2010 8:12 AM ET

Response to DoctorJJ
by Martin Holladay

Remember, all of the electricity used for forced-air blowers or circulators during the heating season helps heat the house. Although using electricity to heat a house is expensive, at least the extra thermal energy is useful during the winter. (By the way, using electric resistance coils for a radiant floor is the worst of all possible worlds -- since all of the heat is derived from electric resistance, a very expensive form of heat.)

I wish you were correct that circulators on hydronic systems were not a significant energy load; unfortunately, you're wrong.

In an article I wrote for the June 2007 issue of Energy Design Update ("Near-Zero-Energy in New England"), I reported on the high electrical energy use of circulators in a carefully designed in-floor radiant system installed in a superinsulated house:

"Aldrich [Robb Aldrich, an engineer at Steven Winter Associates, one of the designers of the space heating system] is still not convinced that the in-floor distribution system was a wise choice. “Because of the radiant slab, storing direct solar gain is out of the picture,” he noted. “Having invested so much in the envelope, they could have gone with a really simple, cheaper, lower-cost heating system. In their next project, RDI does not plan to do a radiant floor because it is just too expensive.”

" ... Before deciding on in-floor radiant heat, the system’s “parasitic” energy load -- electricity required for pumping -- must be calculated. “We looked at pumping energy a lot,” said Aldrich. “It’s a big concern of mine. I was fairly rigorous on all the friction and pressure calcs. On the solar thermal
system, we decided to use a PV-powered DC pump. We chose the smallest possible circulators for the radiant ... We ended up with a constant circulation system -- basically, the pumps operate continuously for the entire heating season.” The Colrain heating system has two circulators that together draw 173 watts. Since the circulators will operate for 4,000 to 5,000 hours per year, they are likely to consume between 19% and 23% of the annual output of the home’s PV array. “The radiant pumping energy will really be a significant load, which definitely bothers me,” Aldrich wrote ... One of the little battles I picked (and won) was sizing the circulators as small as they are. Takagi recommends a circulator twice as large for moving water through their water heaters. I did some fairly careful pressure-drop calcs and insisted we could use a smaller pump.” "

Aug 23, 2010 10:03 AM ET

Myth #7
by Art

This is hardly a "green" solution and I think your missing basic problem - the original AC system was oversized so that it is cooling the house too quickly to lower interior humidity. Dehumidifier are basically AC units themselves. Have you actually added up the energy consumpution of (1) running a compressor that is larger than it should be (2) running another compressor inside the building to remove moisture and (3) running the main compressor yet again to remove the heat created by the dehumidifier?

Aug 23, 2010 10:11 AM ET

Response to Art
by Martin Holladay

Of course the green solution is to open the windows and live without air conditioning. However, most people in Houston aren't doing that.

The solution came from Armin Rudd of the Building Science Corp., one of the smartest engineers in the country. No residential AC units are yet available that provide adequate latent cooling for a small, very well insulated house. The best available residential AC equipment still leaves these homes humid; the use of a small stand-alone dehumidifier is a relatively inexpensive solution to the problem.

Aug 23, 2010 11:04 AM ET

1560 sf house
by Armando Cobo

No kidding, Martin.... I just designed a 1560 sf house that requires .9 tons of cooling and 18K btuh, but have to install a 1.5 ton and 40K btuh as minimum units by the manufacturer. It's a model home and the production builder wants to keep prices low on his end. Manufacturers of residential equipment are not doing favors to the industry!!!

Aug 23, 2010 2:43 PM ET

In-floor radiant heating systems
by Jim

A home in the south east US will need cooling more than heating so the air system is there anyway. I suspect that in the not to distant future we will be way more concerned about our carbon output than cost. Would it be reasonable then, to add an in-floor hot water system to take advantage of a solar water heater?

Aug 23, 2010 2:49 PM ET

Response to Jim
by Martin Holladay

There's no simple answer to your question. You wonder whether rising energy prices will make an expensive investment in a solar thermal spacing heating system cost-effective. I don't know. I certainly know that such a space heating system is not now cost-effective.

If I understand correctly, you live in the Southeast corner of the U.S., in home that is already equipped with ductwork. The easiest way to retrofit an active solar thermal space heating system in such a house would probably be to install a hydronic coil in your air handler.

Aug 23, 2010 9:16 PM ET

On remodeling costs, new construction ethics, and tear-downs
by James Morgan


1. - agreed. There are already too many homes and greenies need to consider other strategies than just adding to the inventory.

2. - ditto

3. and 4. - false dichotomy. If my experience is anything to go by most Americans will not choose between deep-energy retrofits and new construction, they will go for the third option of a more modest energy retrofit which will nevertheless result in substantial energy savings.

Financial reality check: in our area a typical older home in a pleasant mature neighborhood goes for about $300,000. The lot value of such a home is about 25% of the total or about 75K. Demolish the home to build new and you have just spent $330,000 (house value + demo and landfill fees) for a $75K lot. While I agree that a super-insulated home is easier and cheaper to build new than to retrofit it's not THAT much cheaper. In nearly twenty years of practice here with many hundreds of clients I have encountered exactly one to whom that seemed like a reasonable option - and as it happened the replacement home this client chose to build (without my help, I'll add) was very far below Passivhaus or zero-energy standards.
Energy savings. It would not be unusual for our typical $300K house to consume about 30-40,000KWh per year. A 'shallow' energy retrofit (sealed crawl, attic insulation upgrade, general air-sealing, new high-performance mechanicals, new Energy Star appliances) can easily reduce that energy use by half, at a cost of perhaps a tenth of the 255K write-down represented by the tear-down option. If we regard the total resource available across the country for residential energy upgrade as a zero-sum game the total aggregate energy and carbon savings represented by millions of low-cost upgrades will far exceed that of a few thousand zero-energy flagships. I'm not fond of the phrase 'the perfect is the enemy of the good' but in this situation I think it describes well the dismissive attitude of many green builders and designers to the rank and file of existing low-performimg homes.

Aug 24, 2010 5:14 AM ET

James, we're in agreement
by Martin Holladay

I agree with most of what you wrote. Of course there won't be a single response to sharply rising energy costs -- there will be a variety of responses. In most cases people will respond as you suggest.

All I'm saying is -- there will be a lot more tear-downs in the future, because many American homes can't be affordably prepared for a future in which energy costs are very high. You look at some homes and you say, "Wow -- fixing all of this home's problems won't be easy." That's when you invite in the bulldozer.

Aug 24, 2010 1:43 PM ET

PV powered minisplits + wood
by Michael Blasnik


I'd like to hear details of your scenario where tearing down homes to build super efficient new ones will make sense compared to the alternatives that already exist. The effective cost of energy to heat a home should not rise above the current cost of PV powered ductless minisplits used to heat conventionally retrofitted homes. The actual cost should be much less than that. Add in the potential for supplemental wood heat and it's really hard to imagine that scenario you envision. I'm not even including the likelihood that people will build lots of nukes if we somehow (unlikely) have a huge carbon tax to make fossil fuels 10x their current cost. .

Aug 24, 2010 2:04 PM ET

Response to Michael
by Martin Holladay

First of all, none of us can predict Americans' responses to steeply rising energy prices, if indeed prices rise steeply. I'll be the first to admit that my crystal ball is cloudy, and your predictive powers may be better than mine.

Heating a home with ductless minisplits balanced by the production of a grid-tied PV array isn't cheap -- especially in an old leaky house. Many New England families are now paying $2,000 a year to heat their houses. If that cost rises to $10,000 or $12,000, what's going to happen?

Poor people will have few choices, and will depend on programs like the WAP for low-income families. Those higher up the income ladder will long for a zero-energy house that is very tight and very well insulated. They may call up a contractor and ask whether retrofit work will convert their leaky old house to a net-zero-energy house, and be appalled at the high cost of the retrofit work. In some communities, old hard-to-retrofit houses will be undesirable and will languish on the market. Those who can afford to will want to buy a new zero-energy home.

If an older home is in bad shape and is hard to retrofit, the bulldozers may be called in. If the lot is in a desirable location close to public transit, it would make sense to build a new home on the lot.

Aug 24, 2010 4:46 PM ET

On remodeling costs, new construction ethics, and tear-downs
by Paul Brazelton

James, thanks for the back-of-the-napkin math you just presented. Only the wealthy can afford to simply destroy an existing house so they can start with a blank slate. Much of the 'wealth' people own is tied directly to their home, and most of it is in reality the balance owed on a mortgage.

Martin, I know you're not an advocate of tear downs, but are attempting to present a realistic view of costs - especially in an energy poor future. But I'm still troubled by the notion that getting to the goal of a low energy home requires the massive use of virgin materials and energy. It sounds like a contradiction of sorts - to survive a low energy future, we must expend an enormous amount of energy. Not to mention the market dynamics of attempting to replace a significant portion of the nation's housing stocks in a short period of time...

Aug 24, 2010 7:49 PM ET

Response to Paul
by Martin Holladay

We're entering uncharted territory as our species releases enough greenhouse gases to change the climate of the entire planet. Predictions are very difficult.

You may well be right. I certainly think that we will not be as materially wealthy in the future as we are now. Clearly, most American families will not be able to afford to move into a new net-zero-energy house. Instead, we may all be living in much smaller quarters -- perhaps heating a single room with a small space heater.

As I've often said, I have a long history of making bad predictions about our energy future. During the late 1970s, I was sure that energy prices would continue to rise steadily. I was certainly wrong on that one.

Aug 24, 2010 9:27 PM ET

by Kevin Dickson


The scenario you describe is happening right now in my neighborhood. I can hardly believe it, but in the last two weeks, eight brick bungalows were demolished. These homes were quite serviceable and decent sized, at 900-1200 sq.ft.

Energy costs aren't the reason, however.

The replacement homes will be at least 2600 sq.ft. The reason the old homes were scraped and not remodeled was economic. The ROI of the completely new home is always a bit higher than a complicated retrofit and expansion of the old home.

Unfortunately, energy costs are important to only a small percentage of people. No one is paying significantly more for a low energy house. And why should they care? The mortgage payment will be $54,000/yr, and energy costs will be $4000. There are at least six things more important to these buyers than saving that $3-4000/yr.

The desirable location is the key driving force. Future energy costs, if higher, will just add to the ROI of the new house, and guys like us will be scratching our heads trying to tighten up homes built to 2010 minimum code specs.

Aug 25, 2010 9:57 AM ET

$10,000 heating bill?
by Michael Blasnik


I really don't see $10,000 heating bills (in today's dollars) happening anytime soon (ever?) except for people who are rich and living in huge homes and can afford it. If oil becomes unaffordable, electric will provide a backstop cost and wood use will skyrocket and people will lower thermostat settings and zone heat. What sort of consumption of what type of energy will lead to this cost you envision?

Even if there are $10,000 heating bills, few people will abandon or tear down their homes -- which they are often paying several times that each year for mortgages and taxes and maintenance. The tear down or deep energy retrofit will still have to compete with other options that are almost always more affordable.

Aug 25, 2010 10:00 AM ET

Michael, let's have a beer in 2025
by Martin Holladay

Let's get together in 15 years or so and see where the planet is at -- and how much we're all paying for energy. You live in Boston; I live in northeast Vermont. If we both get on our bicycles, we can meet in Laconia, New Hampshire. I'll buy the beer.

Aug 25, 2010 5:03 PM ET

tear downs
by Sea Wolf

I agree with Martin, there will be more teardowns in the future. Right now, some people are still doing late-20th-century teardowns. You know, buy an expensive 1500-sq.-ft. Bungalow on a city lot, tear it down, build a 2500-sq.-ft. modern home. Eventually, more and more people will do Martin's version of this -- tear down a leaky, uninspired, 1200-sq.-ft. house from 1920 and replace it with a super-efficient new house of the EXACT SAME SIZE. This almost makes economic sense in parts of Seattle, where a $400K buys you a $250K lot and $150K of beat-up-old house; triple the price of energy, and it will make perfect sense. I suspect we'll also see the opposite of the late-20th-century teardown -- tearing down a ridiculous (and poorly built) new 3500-sq.-ft. beast on a nice urban lot and building a smart 1500-sq.-ft. house in its place. I certainly hope so. Maybe we'll call this new trend house halving.

Aug 25, 2010 7:06 PM ET

Walls that breathe.
by Steve Satow

This email (and all the corresponding posts) have arrived in my mailbox five days late which means that I have not waded through them all, so I apologise if I am repeating something that has been said before, but... when people talk about walls that 'breathe' what they really mean is walls that transpire. As has been noted by Martin, walls do need to transpire (aka breathe) since it is virtually impossible to ensure that no moisture will ever penetrate them. But, my purpose in writing is to point out that, historically, walls that breathe (and therefore transpire) have outlasted building that are highly vapour-impermeable by a factor of four or five (and possibly a lot more). Virtually all the building stock in the UK and Europe built before the war has walls that transpire and many of them are 500 years old. I question whether modern sealed building are likely to last that long? Yes, they are not as energy efficient, but rebuilding every fifty years isn't very 'green' either.

Aug 25, 2010 10:28 PM ET

sensible heat fractions
by David Whte

Hi Martin,

In the Texas study, what sensible heat fractions were encountered in the homes? Conventional air conditioners may be inadequate, but I've been impressed recently looking at capacity tables for mini splits. The lowest SHF I've come across is for the Mitsubishi Halcyon - I found 0.63 in the capacity table for the 9,000 btu model, at 95 outdoor 75DB/63WB indoor (around 50% RH).

Aug 25, 2010 10:40 PM ET

radiant floor heating
by David Whte

if you are generating heating hot water with non-combustion sources such as solar or a heat pump, then a lower supply temperature can make a big difference. best example i've seen is the Balanced Office Building in Aachen, Germany. With very low loads, the heating supply temperature is seventy nine (79) F. they achieved a measured COP of 4.2 (including all plant loads) on their geothermal system because of this. an air system for the same building would need a much higher temp on the water or refrigerant side.

Aug 26, 2010 3:27 AM ET

Response to Steve Satow
by Martin Holladay

So, a wall has to traspire; presumably you mean that a wall must be vapor-permeable.

You note that for many years, humans have been building walls that are vapor-permeable. Well, that's true. We've built walls with stone and timber and plaster, because these materials are available and they work well.

We now have many materials that were unavailable in 1510 or 1610, however. You propose a rule: walls have to breathe (or perhaps, walls have to transpire, or walls have to be vapor-permeable). Whenever a rule is proposed that limits designers' ability to specify materials, I want to know if the rule has a purpose.

So, I ask simply, Why? It isn't good enough to answer, "Because that's the way walls have been built for 500 years."

Here are examples of wall components that aren't vapor-permeable: window glass, structural insulated panels (SIPs), and insulated concrete forms (ICFs). Sure, you could impose a rule to make these components forbidden (although you'd get a lot of resistance if you insisted on building walls without any windows). But I'd ask, Why?

SIPs and ICFs are useful because they insulate very well. Older building materials that are vapor-permeable (solid masonry, timber frames with wattle-and-daub infill) don't insulate as well. If you are going to propose a ban on useful building systems that insulate very well, you are going to have to come up with a reason why we can't build that way.

Aug 26, 2010 4:18 AM ET

500 years old
by Interested Onlooker

Do I sense a deliberate mis-understanding? The point about 500 year-old walls that are vapor-permeable is most definitely not "Because that's the way walls have been built for 500 years", rather it is that walls built that way are 500 years old. Of course, anyone who has read "How Buildings Learn" (which eveyone in this field should read IMO) will know that the building may be 500 years old but many of its constituents may not be. The point is that the building has survived rather than being torn-down and replaced. If we are serious about limiting the environmental impact of our building then this is an important consideration.

BTW If the greenest building advice is "Don't build" do we need a GBA? ;o)
Of course we do - since building will not stop just because it isn't needed it must be guided on ways to limit its deleterious impacts.

Aug 26, 2010 4:38 AM ET

Response to Interested Onlooker
by Martin Holladay

Interested Onlooker,
I don't think there is any controversy concerning your point. I certainly agree that most (perhaps all) 500-year-old walls are vapor-permeable. So what?

I'm still waiting for an answer to the question I asked Steve Satow: Is there any reason to establish a rule that forbids designers from specifying wall components that aren't vapor permeable -- in other words, walls that include window glass, SIPs, or ICFs ?

Aug 26, 2010 6:58 AM ET

Walls or Components
by Interested Onlooker

The argument that walls have to be vapor-permeable does not preclude components in those walls which are not vapor-permeable. The questions are :

Should walls, as a whole, be vapor-permeable and why?

If so - how permeable ?

If so - how much of the wall can be non-permeable?

Are there arrangements of non-permeable components which constitute poor practice?

Is vapor-permeability to be measured for every building or assured by building to agreed standards?

BTW on re-reading Steve Satow's posting I was unable to find a suggestion that designers would be banned from using wall components which were not vapor-permeable.

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