More Building Science
One of the best things you can do to improve your home is to make it more airtight. Yeah, it’s good for energy efficiency, but that shouldn’t be your main motivation. It’s good for indoor air quality. A good air barrier keeps those pollutants from your garage, crawlspace, basement, or attic out of the air you breathe. It’s good for comfort. It eliminates the drafts and makes it easier for your heating and cooling system to do its job. And, it’s good for noise reduction.
But how airtight can you make an older house? I’ve got a sampling of results from homes I worked on earlier in my building science career, plus a history of the airtightness in my current home.
The difficulties of air sealing an old house
In my home performance contracting days, I did a lot of blower door tests both before and after air-sealing. For a couple of those years, I also did the air-sealing work. The good news is that you can make significant improvement in older houses. Here are a few of the results I got from making homes more airtight:
That fourth one had the highest air leakage I’ve ever measured: ~13,000 cubic feet per minute at 50 Pascals (cfm50). I measured that with a single blower door, so I couldn’t get the full 50 Pascals of pressure difference. I was able to get only a measly 15 Pascals, so the 13,000 cfm50 is extrapolated using what’s called a Can’t Reach Fifty (CRF) factor.
One of the easiest houses to reduce the air leakage in was the last one I worked on. That’s it in the photo above. Those two crawlspace vents, though, aren’t there to ventilate a crawlspace. They’re actually two big air leaks. On the other side of that wall is a finished basement. But no one knew they were air leaks because they were above the dropped ceiling. This being a relatively small house, sealing those two leaks alone probably got at least 15 of the 40 percent improvement.
Another fun one that made a big improvement was the refrigerator vent I discovered while crawling through an attic. The photo above shows only about half of that hole. The other half was on the other side of the joist. Sealing that one hole up reduced the air leakage by about 10 percent.
So, yeah, it’s certainly possible to make old homes more airtight. But going beyond about 25 percent takes luck, persistence, or making some big changes to the building enclosure. (Think: gutting a house to the studs.) It also takes skill because you have to understand air leakage and the air barrier.
Air barrier improvements in my home
I’ve lived at my house since 2019. Shortly, I’ll show you the results of the blower door tests done on the house since 2012, but first let’s talk about what has happened to the house to improve airtightness. Here’s a table showing how things have changed from 2012 to present:
The previous owner (my father-in-law) had the attic encapsulated with spray foam in 2012. It was a pretty bad job, though, as you can see in the photo below. They didn’t use enough spray foam so it didn’t have nearly the insulation it needed. But it also was quite leaky, as you’ll see in the test results.
Getting the crawlspace encapsulated was the second improvement made. That must have helped reduce air leakage some, although I don’t have any blower door test results to show the change.
After we bought the house, I had Woodman Insulation come in to install a lot more insulation. They did a great job adding thermal resistance and improving airtightness.
Shortly afterward, we had our soffits and fascias replaced, which led to an important discovery. A lot of the rafter cavities at the eaves didn’t have much (or any) foam in them, as you can see in the photo below. This meant a poor job of insulating and air sealing at the eaves. Read the full article I wrote about that and the followup article on how I fixed it.
And finally, we started remodeling our basement in February 2024. We’ve done several things that have improved the airtightness, but one of the biggest holes I sealed was just like one I mentioned above. I knew we had three crawlspace vents in the basement, but one of them was hidden until after demolition. Here’s what it looked like and how I fixed it:
All these things have made a huge difference in our home’s airtightness.
How much did it help?
The graph below shows how our blower door tests have improved. The columns show the test results before factoring in the size of the house. They’re measured in cfm50, shown on the left vertical axis. The line shows the results normalized to the volume of the house, measured in air changes per hour at 50 Pascals (ACH50) and shown on the right vertical axis.
From the first test done before any airtightness improvements to the latest one with the basement partially done, we’ve improved 48 percent in the cfm50 result. The normalized air changes per hour result improved more. Do you know why?
The reason the ACH50 improved more is that encapsulating the attic added volume to the house inside the building enclosure. Encapsulating the crawlspace also added more volume. The ACH50 is basically just a unit change. You multiply by 60 to change minutes to hours and then divide by the volume to change cubic feet to air changes. When you divide by a bigger volume, you get a smaller ACH50 and hence greater airtightness by that metric.
Using that metric, our airtightness improved 59 percent from 2012 to today. And we’re now down to 7.1 ACH50. That would have almost passed the previous Georgia energy code requirement of less than 7.0 ACH50. The state now requires less than 5.0 ACH50, which I plan to get down to by the time I’m done with this house. That would be a 71 percent improvement overall. I’ve made this house more airtight than the condo I used to live in.
If you work in the field of airtightness, what have you found? If you’ve worked on your own home—or had others do it for you—what results have you achieved?
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Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel. Images courtesy of the author.
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16 Comments
Wow, thanks for providing all this data, Allison! I just had my 1880s house blower door tested, and it came in at 17.4 ACH50... Room for improvement, needless to say. It feels like a Sisyphean task at times, but it's helpful looking at your improvements to see that there is, indeed, a light at the end of the tunnel.
Yep. It takes persistence, work, and money, but you can get there.
The good news for us old house owners is that when things are that bad, it is easy to make big early gains! It is a bit like new runners - they tend to break their personal records a lot in the first few months of training when they are making progress very quickly. It is a few years in when additional gains become much harder.
I'm in a 1922 house w/ finished basement, we started over 12 ACH50, got it down to 11.2 with just walking around sealing little gaps with a can of foam, now to 8.2 with airsealing our lower attics and knee walls, and restoring the double hung windows with metal interlocking weatherstripping. Up next is air sealing the main attic (wiring has to be done first), and adding storm windows. My hope is that I can get down to 6 or so, which I think would be great for an old home.
At 17 ACH50 you probably have at least a few giant holes (like the crawlspace "vents" in Allison's post) that will earn you a big improvement for very little work.
Great reference data, makes me feel better, thanks ! I've been working on my 1986 ranch triggered by I thought a horrible initial test result of ~11 ACH50. After about 3 months, we're down to ~ 5.7. The low hanging fruit are gone, but I target < 5, feasible without invasively pulling off cedar siding to externally seal / insulate (expensive, not justifiable until windows are needed)
Would have been nice to track if the air sealing actually made a difference in air quality
I don't know where I started, but I was able to get my 1993 spec built home down to 2.5ACH50. After reading about it for a while, air sealing and improving IAQ became my hobby during COVID.
Thanks for the article.
Allison (and others) who've had big airtightness gains, how noticeable is the change in the home in terms of comfort level and heating/cooling costs?
Will come back to you shortly with final heat load numbers: last year's baseline / after air sealing / after attic insulation. I'm in Rochester NY, and it's winter (duh!). Preliminarily, the difference with only air sealing and not yet insulation is quite remarkable. I had the rim joists sprayed with closed cell, and after removing some cabinet-top soffits and capping others, and capping all the recessed lights with Insulite boxes, we sprayed roughly 1.5" of closed cell across the entire ceiling plane sheetrock to air seal. not 100%, maybe 95% effective at that plane. We've been in that state for two months, ie. still without insulation on top yet completely comfortable, while I gathered data on the indicated heat load via detailed monitoring of the 25 year old 2 stage 92% furnace. 12" of Timberfill wood fibre (R40) will be blown in tomorrow ("flash and batt" approach). The house as is is far less drafty and cold, temperatures are more even throughout the home, two stage furnace is running only on low, and averaging ~ 50% run-time. Next sealing steps will address the two chimneys, each with a 6" fresh air intake, hope to get to 3-4 ACH50, tbd, prior to sizing heat pump
Thanks so much for the reply. Nice to hear a real world "before and after" account from someone living in the home.
I live in an urban 1920 three wythe brick two flat with a flat roof, zone 5. I long to get to air sealing the swiss cheese ceiling (existing recessed lights). The low "attic" under the flat roof makes access from above impossible. There's roughly 30" of space above the ceiling at the front of the building and it tapers to 6" at the rear. Tearing the bitmus roof off and installing insulation over the roof deck seems the best approach. Removing the 2" layer of old cellulose insulation over the ceiling which was squirrel bedding for years is another part of the job. I don't want that coming inside the conditioned space :/
You've given me some much needed energy to finally tackle the job.
Dang Dr B I am exhausted just reading about all that work. 😉 It’s war not a battle but hey I still live in my log cabin. If you ever want to do a series on air sealing a log cabin we should talk. Take care buddy.
As all I do is Deep Energy Retrofits on 1880-1940 homes, I can verify it's a lot of work. Every job of mine makes or breaks PHIUS 0.6ACH. Unfortunately with the new HOMES Fed money out there, I am the only one that does this work in WI and can't find any contractors to help. I want to retire!! I really need some of the east coaster folks who understand building science. Nobody here does:-(
Great Article and congratulations on your efforts. We are a Home Performance company in Kalamazoo, Michigan (ZONE 5) called Better World Builders.
We have been doing Energy Audits and Energy Retrofit for over a decade and a half.
We often achieve 30-60% blower door reductions and have learned ways to have our old Michigan housing stock be in line with today's code of 4.0 ACH 50 or less.
Our most successful retrofits happen when we remove old insulation. It reveals all the top plates and by-passes. We typically do the perimeter top plate before we install baffles, then all the other interior wall top plates, plumbing penetrations, chimney by-passes. Every house is different, but in attics alone, we have achieved 18-30% Blower Door reductions. Next is Rim Joists. In Michigan we have basements with cinderblock, poured concrete or in historic houses a rock/rubble foundation wall. Rim Joists can count for 5-10% Blower Door reductions, If a crawlspace is present - as the article states, almost always has a huge impact. We do less window replacements, but caulking window/door trim alone can count for another 4-8%. Retrofitting Wall insulation is not for the faint at heart. However, we have found this can account for 20% air losses. We typically remove old insulation from the walls then dense pack them with cellulose. Pitfalls include balloon framing cavities on 2 story historic houses, open substrate above the soffits, and failed insulation attempts left in place. This next statement will upset some, however, when it comes to water based injection foam insulation stay away. In my experience, we have always found that it shrinks and fails. Not a long term or reliable air sealing method.
For your cinderblock walls, if you were going to be encapsulating the area, do you do the vapor barrier on the cinder block and then the rigid insulation? I"m trying to see if my foam board adhesive will stick to the vapor barrier or not. I'm trying to avoid having to do mechanical fasteners as well just for labor reduction. But I was feeling like I probably didn't want my rigid insulation against the walls and then vapor barrier over the top of that as I wasn't sure if it would trap moisture.
I'm planning on only going about a foot up toe 2 foot crawlspace wall with the vapor barrier, so I suppose if the insulation still has some exposure it could dry even if the vapor barrier was on the inside face of the rigid insulation.
Thanks for the encouraging article. I recently concluded a several year effort at an energy retrofit on my small 500-ft2 condo from 1952. I don't have the exact original blower door ACH50, but I believe it was in the range of 15ACH50. I now have it down to 4.6ACH50 (negative pressure). My last go-round included a Quixotic but meticulous sealing of my kitchen cabinets, behind which lurked unknown numbers of penetrations. I'm sure it's not in any textbook to make your cabinets part of the air control layer. ;)
proposterosa: Yeah, kitchen and bathroom cabinets can hide a lot of leaks. That's another reason to put the air barrier on the exterior of the structure.
did you just caulk cabinet to countertop and all interior corners or the cabinets and cabinets to the floor? I have a toe kick and quarter round, I should probably try all inside corners/edges of the cabinets and the toekick to quarter round and quarter round to floor.
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