Like many other owner/builders planning an energy-efficient home, RedFalcon6 is well aware that careful air-sealing is considered a cornerstone of high-performance building. But do the numbers really add up?
“I have to confess,” RedFalcon writes in a recent Q&A post, “I’ve never sat down and done ‘the math’ on the payback for increasing airtightness from ‘OK’ to ‘Pretty Good’ to ‘Awesome.'”
All else being equal, RedFalcon asks, what’s the financial value of reducing air leakage by two thirds—from 3 air changes per hour at 50 pascals of pressure (3ACH50) to 1ACH50?
RedFalcon points to a 2013 California study by the Lawrence Berkeley National Laboratory that suggests the financial return isn’t dramatic. Researchers found that the average value for a California home where leaks had been reduced to Passive House levels (0.6ACH50) amounted to about 2000 kwh of electricity per year.
“So going all the way to passive standards only saves an incremental $200 bucks on an average home?” RedFalcon asks. “Seems not worth it!”
“Prove me wrong!” he adds. “Am I an idiot or just misreading this study?”
Is airtightness worth as much as it’s cracked up to be? Or, as the title of RedFalcon’s query asks, is this revered benchmark “…mostly BS”? That’s the topic for this Q&A Spotlight.
It depends on where you live
Several GBA readers say the answer depends in part on where you live—not only from a climate perspective, but also taking the local cost of energy into account.
Russell Miller, for example, says he is a builder in a poor state where energy costs are not a burden, at least not now.
“I build for a living,” he says, “and I can state for a fact, it’s hard to justify extreme insulation and airtightness to…
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I just completed a blower door test of a new home with an air leakage rate of 4.24ACH50. I use the Tectite software from The Energy Conservatory which will estimate the costs of the air leakage. The home I tested is heated by an electric hot water boiler on a duel fuel program rate of $.0615 per kWh, half of what the normal electricity rate is. The software estimated the cost of air leakage at $523 per year. The cost of the heating fuel is key, the cost would double on the standard electrical rate. LP gas is another option in the area (no natural gas option). Over the last year, the cost has varied from $1 to $1.75 per gallon and a few years ago, we hit $4 per gallon. The ever changing cost of gas can drastically change the payback. The other issue with this home was the humidity level, 15% (lowest humidity of a new home I have ever seen). I checked with 2 different gauges and both had similar readings. There is an HRV which has been off for the past month. I'm assuming the air leaks are the cause for the uncomfortable humidity level. As the article suggests, air seal is not BS.
Thanks for the thoughtful comment Randy. I'm curious, did you recommend a course of action for the homeowners? If so, what's the plan to make their home more comfortable?
I did have some suggestions. There were several areas at or near the ceiling with fairly substantial air leaks that needed sealing. The homeowner wanted to use a couple attic knee wall as storage, problem was that knee wall was directly connected to a vented attic. My hope is sealing these large air leaks would tighten the home enough to increase RH. Thermal image is of knee wall access with blower door operating.
A electric boiler and an ach50 of 4.24‽ Why not just heat the driveway while they're at it? I doubt they'd notice the difference in their bill. O_o
Blower door tests are a comparative technique, building A is 15% tighter than building B. Both can be under-insulated and blower door tests don't flag the problem. "Air sealing" is as critical as quality insulation. Unfortunately, a lot of nonsense is passed off as air sealing. One Youtube video demonstrates foaming the seam between drywall and the interior wall top plate when retrofitting insulation. The applicator fails to recognize the drywalled wall is mudded/taped with the ceiling drywall, airtight. Years of dust guarantees the foam won't adhere. The practice is a waste of time and money and reveals insulators who don't understand heat loss and construction anatomy. Air sealing is about closing genuine structural gaps (penetrations), not pretending air leaks exist where they cannot. Myths like this are perpetuated by rebate program administrators. Apparently, if it's on Youtube, it's science.
I think we mentally make the assumption when conversing about air leakage in a building, that it is like a finely perforated structure, where high leakage is evenly spread out and we incrementally make the perforations smaller and smaller. Actual homes are built by humans, who start installing a window and then go to lunch. Or who get sick and the guy who replaces them isn't quite so good at using that sealing tape. Or the tradesman who cuts a giant hole in something right before the sheetrock goes up.
Then as the heating plant size/reach decrease due to higher insulation, you get a room that runs cold[amusingly like a crappy old house]
Good points Keith,
I recently visited a builder to check out his method for installing European-style windows (article coming later this month) and one of the things he noted is that he tends to put one of his employees on certain details (he specifically mentioned using air sealing tapes) because the work is tricky and takes finesse and he feels that you need to get your method down and get in a groove to do the work consistently well. That was something I hadn't thought much about, carpenters tend to be expected to be generalists and to be able to jump from one thing to another with ease, but it makes sense as a quality control measure.
I think having framers involved in any spect of air-sealing is a mistake. It's a vestigial role from before the task was seen as important and plays against their strengths. All the framers I know are in the trade because they like framing and wood. That's in part why I continue to champion the use of poly and interior air-sealing that dominates up here in Canada. It's not the best approach for many reasons, but it does involve a specialized trade, and can be tested and inspected all at once. Hopefully a similarly specialized trade evolves to deal with more contemporary air-sealing strategies.
That's smart Malcolm. Builders should play to the strengths of their employees, subs, and the local ways of doing things. Forcing change and expecting trades to do things their not familiar with will only expensive, yet poorly performing houses.
I think process is the issue. The framers generally build the shell, or weather it in, and then go home. That usually includes installing the housewrap, windows, siding and trim. Maybe roofing.
To have someone else do it means bringing in another crew at the correct point in time so the work gets done before the trim goes on.
My last house addition was 2000 sq ft under the roof, and two guys shelled it in 6 weeks.[after foundation and some steel]
I am pretty sure that if I had insisted on them waiting for a sub to come install the windows I would never have seen them again.
Not saying its right, just is
Around here there are two models. The crew that works on a single project and always has at least one person on the site as sub-trades come and go, and the general contractor overseeing multiple projects, who tries to make the subs be responsible for everything. Both can work, but the balance is weighted towards the former for quality. I think the second model is why so many owner/contractors get into trouble. They don't understand the scope of work subs will be responsible for, or what the gaps are.
This is the best local framer's work. Do I really want him flashing penetrations and installing window sill-pans?
It wasn’t mentioned, but I’d expect certain air sealing measures to help stop insects, such as cockroaches, from entering and nesting in the walls. For the small PGH I’m planning in SW New Mexico the “bug barrier” is near the top of my list of reasons to thoughtfully and carefully detail an exterior rigid air barrier, i.e., taped sheathing and gaskets/fluid applied flashing to concrete foundation wall.
I just finished completely gutting and renovating a 1970's bungalow in Ontario, Canada. It had literally thousands of houseflies that previous homeowners had tried and tried to get rid of. The windows would be coated with flies during the summer. When we gutted it in the winter, we found the flies all nicely nested up behind every singe insulation batt. Walls were black with flies. They couldn't move because of the cold, so we just vacuumed them all up. New insulation and careful airsealing was completed, one year later there is barely a fly in the house. Whether that is because we removed the existing population almost entirely or if it is because there is no longer any entry paths into the house due to the air sealing, I can't say with certainty. We will see after a few more years but my money is on the air sealing. That alone was worth the airsealing efforts.
I did a seismic / thermal retrofit about five years ago and haven't had an ant problem since (several invasions / year before). Part of the seismic was adding sheathing and I was pretty careful caulking every joint. It's been a nice improvement that I didn't really think about in advance. Still get a few cockroaches / year. I am guessing that they come in through the doors when they are open or infest packages before they come in, but don't really know. There are many less cockroaches than before., though.
Here in Newfoundland we have infestations of European earwigs. Harmless but creepy. Haven’t had one inside our passive house. I saw a ladybug once. We’ve been here now for almost two years. Ontop of the savings in energy it saves us 250$ per year in whole house earwig control!
We had them in the states when i was a kid. Awful looking creatures aren't they!
Yup, hate them! Haven’t had to deal with them now for several years!
Insects and roaches won't nest in cellulose-insulated walls. Still, air sealing is important for comfort and energy conservation.
Hard to put a value on even temperatures and comfort throughout a home. You know it when comfort is there and when it is not. I would say for retrofits, 3 ACH50 is a good target with high and low leaks sealed first. New construction can achieve 1.5 ACH50 without any extraordinary measures.
To see new homes testing above 3 ACH50 is disappointing, a leaky new home is a poorly built home. In my ice dam remediation project for the neighbor I have been monitoring gas usage this winter compared to before the retrofit. Gas usage so far is 24% less than before bringing the cathedral and flat ceilings up to R-50. We have not done a followup blower door test yet but the 24% savings are far higher than the added ceiling R-value would indicate. Reduced air infiltration on this project is showing itself in reduced gas usage, I will be interested to do another blower door test to apportion the energy savings.
Ironic that I just had a debate today with someone who believes "houses need to leak" air through the walls, roof, windows, etc. That building tight homes is bad as it creates mold issues and poor indoor air quality. Always anecdotal evidence is given, not scientific studies and white papers, just anecdotal stories from others.
Some people will not be won over and they will continue to pound the drum of houses need to leak air. Science be damned.
you can't effectively debate an idiot.
Good article, points, and discussion, thanks Scott and to everyone who has commented. It's always good to question assumptions. I agree with most of what's been said and often question whether or not "lower is always better" with respect to air tightness. But evaluations of energy efficiency in terms of ROI, comfort, and building performance make the value of air tightness and energy efficiency dependent upon location, and this seems to miss an important point. The greatest value of energy efficiency is in terms of CO2 ouput. That, in my opinion, is what we need to be focused upon. The CO2 output of a building isn't reduced more in regions where energy costs are low, and buildings in temperate climates aren't less responsible for meeting global climate goals than buildings in more extreme environments. (unless the suggested solution is for all people to move to temperate climates, which may very well be a good solution but which those already living in temperate climates may not like). My point is that energy efficiency has a common currency, and that's CO2. I live and work in Washington where electricity comes (mostly) from hydro and is the cheapest in the country. ROI becomes less motivating as a reason to build an energy efficient home pretty quickly. Meeting the goals set forth in the Paris Agreement does not, so air tightness has value beyond $ saved. I'd love to see a study which relates air tightness to total CO2 output while also taking primary source energy into consideration.
Hah hah - yes it is B.S. !
We in Vermont have had seemingly an entire winter of alternating ice storms and single-digit cold snaps. With the ever-present threat of a power outage due to ice-burdened trees falling on power lines I think a super-tight house could be a real advantage for passive survivability.
An exceptionally well-sealed enclosure also gives you a lot more flexibility in how a home is heated. Even in my sprawling ranch home I can heat with a woodstove or single mini-split because heat loss out of the bedrooms doesn't much outpace heat gain from the hallway. Great air sealing makes distribution optional.
One ACH50 is free. Tighter than that is good for bragging rights.
I'm reading a pascal is the pressure of 1 newton per square meter. Or 50 pascals = .0073 psi..
I'm trying to get a feel of what 50 pascals, or .0073 psi compares to. Or what conditions might be required for the inside pressure to be .0073 psi higher?
Because even 1 air change per hour sounds high, so I assumed 50 pascals was a exaggerated test condition which a house would never experience.
Edit.. Sorry just found something.
"50 Pascals is about 5 times the pressure a low level building might experience on a cold winter day."
So in reality, what ever your air change per hour number is, divide it by 5 to get a idea of how much actual air leakage is in the worst conditions.. Probably more like 10 or 15.
The standard n factor ranges from about 11-30, depending on geographical location, shelter and number of stories. This will obvious vary a lot from day to day, depending on weather. I think that represents typical conditions rather than worst, but I could be mistaken.
Mike, 50 pascals is sometimes compared to a constant wind of about 20 mph blowing on a house.
Thanks. But isn't it a stretch to say the tightest 1 ach homes leak all their air 24 times a day with only a 20 mph wind Michael?
The statement I posted before sounds more plausible to me.
"50 Pascals is about 5 times the pressure a low level building might experience on a cold winter day."
So, five Times worst case weather.
Mike, it does sound plausible to me that a 20 mph wind would lead to one air change per hour--why does it not seem plausible to you? As Trevor noted, to find the natural air changes per hour you can divide the ACH50 number but anywhere from 11 to 30; I've heard 17 as a good average, but the variables make it impossible to be accurate. That will tell you roughly how many air changes you would have on an average day.
Why does it not sound plausible that a tight house leaks all of its air once an hour, 24 times a day? Seriously?
Purely unscientific.. But when I start my diesel in my leaky 70's split foyer attached garage, I still smell diesel inside 4 hours later. If I actually exchanged all my inside air with outside air every hour, the diesel smell would be gone long before. In the winter when I go to work I turn the stat to 55. It was 0 this morning.. Wouldn't my house be 0 degrees inside after one hour if my home actually had 1 ach? Yet the furnace didnt run all day and it was 59 nine hours later..
Ive never performed a blower door test on my home. But I'd guess I'd be lucky to achieve 10, maybe 8 ach at 50 Pascals.
Mike, have you ever been around for a blower door test or have you been in your house when there is a constant 20 mph wind? I have a drafty old house as well (I swear, it's getting tightened up soon!) and when it's that windy out, continuously, it's much harder to keep it warm inside than when there is no wind. That's why the natural air changes per hour are so much less than the ACH50 number--very roughly 1/17 as many air changes per hour. So one or two per 24 hour period for a tight house, not one per hour. For a drafty house like yours, maybe one air change every few hours, which matches your experience with diesel fumes.
As for the interior maintaining heat, you can likely thank solar energy for helping you out. Not just through windows, either. Plus all of the interior mass is at room temperature when you leave, and the ground under your home is likely between 45° and 65°, depending on the time of year and your home's construction details. Plus any lights or appliances left on, water sitting in a tank, etc..
Sorry, I must have misunderstood. But I still think your estimates are high.
I would guess my leaky house leaks all it's air... I don't know, that's why I'm asking.. lol Maybe 5 time a day.. But gotta confess, I did alot of air sealing over the years, and my bill is 95 a month on the "plan".
You're building a house. One. Or maybe a few over a period of years, and you ask, "Is it worth it to go that extra length for $200/year in electricity savings?" From that perspective, probably not. So then you add in home comfort, occupant health, mold reduction... and you look at the same house and you ask, "Is it worth it now?" Maybe, maybe not. But then you consider that millions and millions of homes are being built, and if they all went that extra mile, maybe we wouldn't be losing a billion animals in Australia right now, that California wouldn't be burning out of control every summer, that global sea level rise from melting ice caps might not be such a problem, that every town, village and major city built along a waterway everywhere in the world might not be under water in a few years, that the price of food might not skyrocket so much you won't be able to afford to buy it... is it worth it now?
There's a story:
“One day a sparrow was flying through the forest when it started to snow. It wasn’t a real storm or anything, just big flakes drifting down quietly. The sparrow spotted a bluebird sitting on a branch, and since the bluebird was larger and more brightly coloured than the sparrow, the latter therefore thought the bluebird must be wiser as well. After alighting beside the bluebird the sparrow turned and asked, “How much does a snowflake weigh?” “Why, nothing more than nothing” came the reply. “Why do you ask?”
“Well,” continued the sparrow, “the other day I was flying through the forest and it began to snow – just like it is today. I landed on a branch in a fir tree and sat there a while, watching the snowflakes fall and accumulate on a branch on the tree opposite from me. One by one they landed and stayed, and one by one I watched them, until all of a sudden one more snowflake – which you have just said weighs nothing more than nothing – landed on that branch, and just like that the branch broke off!”
Be a snowflake.
A Building Green article from about ten years ago: Net-Zero Does Not Live by Design Alone: The Human Factor - https://www.buildinggreen.com/blog/net-zero-does-not-live-design-alone-human-factor
P.S. From: https://www.greenbuildingadvisor.com/article/a-better-path-to-a-low-carbon-future
"Houses that were heated with natural gas were responsible for significantly more emissions than those heated with air-source heat pumps running on the Ontario grid—depending on the house, CO2 emissions could be 20 times higher with natural-gas heating. In itself, that’s not a surprise, but the report also found big differences in carbon emissions between code-compliant houses and those that were net-zero ready.
For example, a code-compliant single-family house would produce 160 kgCO2e/yr, while a single-family that was net-zero ready contributed less than a third of that, 50 kgCO2e/y. A code-compliant house that was airtight would produce 100 kgCO2e/y. In other words, air-sealing a house cut carbon emissions in half."
The results were similar for the modeled multifamily house: a code-compliant building produced 750 kgCO2e/y while the net-zero-ready multifamily produced 190 kgCO2e/y. An airtight, code-compliant house emitted 330 kgCO2e/y, less than half that of the code-compliant building.
By contrast, the single-family, code-compliant house heated with natural gas emitted 3000 kgCO2e/y; the multifamily 15,000 kgCO2e/y.
The airtight houses were modeled at 1 ACH50 with no upgrades to insulation or window quality, demonstrating the significant impact that air-sealing alone has on energy consumption and carbon emissions."
Very well said, Mike!
having just been in a fir forest in a snow storm, i find your analogy imperfect. There was little relief from the snow under the firs, snow just went thru/around/off them. You rarely see broken firs from snow but plenty of broad leaved trees get broken. Be a fir not a snow flake!
As long as we don't over-ventilate right?
I've been trying to figure what sort of comparative leakage rates the currently recommended ventilation rates would compare to. Its hard because converting from ACH50 to natural ACH doesn't seem that reliable. A good reason not to rely on 'natural' ventilation. But it still begs the question: does going from say 2 to 1 ach 50 get thrown out in the wash once we consider active ventilation?
In other words, is there a rough ACH50 equivalent we could assign to a ventilation rate?
Lets take a 2000 sq ft. 2 story house with 8' ceilings.
4 people. So 7.5(4)+(3)20=90 cfm
If volume is assumed to be 16,000: 90cmf(60) = 5400cfh : 5400 cfh/16000 cu.ft. = 0.338 ACH (nat).
If we allow ourselves to convert natural to 50pa numbers, using say an LBL factor of 15, that's (0.338)15 = about 5.
I'm not sure this number is particularly useful (and check my math). The conversion to natural is fraught with uncertainty. It is interesting though.
Of course heat exchangers invoke savings here.
And the assembly will likely thank us that air is flowing through ducts and not cracks between the sill and the concrete.
And if we're going to vent this much anyways (and perhaps we're not actually), then we should arguably get the 'uncertain' ventilation rate as low as possible.
But maybe not at extreme costs.
Great article, one thing that seems to be missed is the embodied carbon of the air sealing work which is much lower than other energy saving measures depending on what materials you use. Looking at if energy upgrades are worth it from a pure ROI or even comfort perspective seems to miss the point of why many people are interested in green building. I suspect that many readers are interested in doing the best thing for the planet which right now is decreasing the carbon we are pumping into the atmosphere due to the materials we use and energy we consume in houses. We still don't seem to have a metric to use consistently that we can measure design decisions against that shows the impact of the decision on CO2 emissions. Maybe we should be talking about embodied CO2 ROI or $/(Operational TCO2e saved). Maybe there is already a metric out there that can be used to compare the differences in overall climate impact for choosing something like air sealing over adding exterior insulation? This is a very tricky thing to calculate due to differences in fuel sources, climate, embodied energy of materials etc. but if we can't measure what we are trying to achieve how do we have any chance of affecting it?
and.. brent's comment is absolutely on point and is possibly be taken into consideration in some of the scoring systems?... if you are going to additionally consider the global warming potential prevention characteristics of a build, what is the embodied co2 of the prevention methods . I don't know, but it just gets more complex. cheers
Often times when discussing passive house air tightness I've seen the parallel of a fish tank. You build the house as airtight as possible and then control it. "Build it tight and ventilate right." Well, what about a different parallel? How about that of a cave where temperatures inside the cave are pretty much the same summer and winter. It's not about air tightness its about insulation. A cave is well insulated from below, above, and on the sides. But....caves have humidity problems. HVAC comes in and solves the humidity problem. Caves are open all the time. They aren't sealed well at all. But what if you could control bulk water and vapor in a cave? Don't we control bulk water and vapor pretty well in a Pretty Good House?
Think of another parallel...a coat. The dang thing is all open at the bottom...lots of ventilation.
Some coats also have extra vents for vapor build up. But coats still work well in the winter because of the insulation.
I throw all that out to say that sometimes we do fixate on tight, but if we are controlling bulk water, water vapor, and are insulated well it's not that big a deal. My two cents.
Which would you rather spend a cold winter night in: A very small cabin made from R60 foam blocks with only three sides, or an uninsulated, but fairly air-tight tent?
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