I'm trying to interpret the blower door test / attic air sealing I had done. The two measurements listed on the report are:

Pre-sealing: 6758 @ 40CFM
Post-sealing: 6280 @ 40CFM

I assume these are pretty bad numbers. My home is about 4400 sqft of finished space (walk out basement, and 2 above grade floors. 2nd floor is "cape style" with lots of knee walls.)
The only sealing that was done were some strips on a couple of exterior doors as well as sealing of the attic air leaks.

The air sealing/test was all part of the free Mass Save program, so I suspect they did that absolutely minimum. If these numbers represent a very leaky house, I want to estimate my savings from additional sealing and figure out the ROI / what I should spend.

The measurements are likely 6758 CFM @ 40 Pa; they must have used 40 vs standard 50 Pa cause they didnt have the fan power to get to 50 Pa pressure or a typo. They should have provided an ACH number, if not figure it out, it is a comparative basis and when comparing figures remember your results were at 40 Pa, if they would have used standard 50 Pa they would have been worse.

Your reduction is pretty small, Im sure there is a lot of opportunity to easily get that number down but couldnt say looking from the internet. Savings would be based on modeling, and unsubsidized retrofits typically have long ROI, thats why you hear about doing retrofits for comfort because there isnt much else for sales tactics.

There are a lot of variables into energy savings other than cfm, so starting with a quality energy audit is really your only choice, unfortunately it lengthens your ROI. I can tell you your house is terribly leaky, I hope its old.

greentree - thank you. Yes, I think my next plan of attack is having a real energy audit by a company I actually pay.

I assume I can convert the CFM reading linearly between 40 and 50 Pa? I.e, I'd estimate a 20% increase in CFM leakage if I increased the Pa by 20%?

Greentree is correct. The building was so leaky that they could only get to 40 Pa with their blower door equipment. One rule of thumb is that you divide the measured CFM 50 Pa number by the building square footage and want to see something less than 1. Flow rate through an orifice is the area times the square root of the pressure difference, so more like 12% difference between 40 and 50 Pa.

Thanks guys. So assuming a 12% difference between CFM40 and CFM50, I get an ACH50 of 8.8.

Calculating volume for my home is very difficult. My 2nd floor is technically a "half floor" cape style, but it also has very high ceilings in some parts. I'll do more careful measurement, but the above ACH50 is if I assume roughly 60% volume for the 2nd floor versus what it would be if all the rooms had non-cathedral ceilings.

I was also unclear as to whether i should include unfinished areas of the basement. Some parts of the basement are nearly air-tight sealed by their doors (like the mechanical room), where as others have a normal interior door that could easily leak air to any exterior surfaces within.

You understand that current code maximum is 3ACH/50, or about 1/3 your current leakage is, right? That's more than a stripe on the floor, but it's still not a very high hurdle for current construction materials & techniques.

With mid 20th century and later houses sheathed in plywood with poured concrete or CMU foundations it's usually possible to retrofit-seal the house to under 3ACH/50. For 19th century plank sheathed walls (or clapboards nailed directly to studs) and uneven fieldstone foundations with no basement slab it can be more of an ordeal. But 7500+ ACH/50 is a very leaky house for this climate, and a double-digit percentage of your heating energy use.

One more thing about blower door tests... Blower door tests only give you the air flow leakage rate at CFM50 or whatever the maximum pressure differential than can be obtained. In any event, this blower door test air flow leakage rate is much larger than actual natural infiltration rates that are used for building heat loss analysis. The CFM50 rate is about the natural infiltration rate you might get if the wind was blowing at your house at 25 mph from all directions. Estimating the natural infiltration rate is an inexact science that is subject to a lot of variables. Perhaps the simplest rule of thumb is to divide the CFM50 rate by 20 to get the estimated natural infiltration rate. A more accurate method was developed at Lawerence Berkley Laboratory called the LBL Infiltration Estimation Model (Max Sherman). A tracer gas test is really the only way to get the natural infiltration rate and it would only be valid for the outdoor conditions that existed at the time of the test. So while blower door tests can provide useful data, one needs to fully understand and appreciate the limited relationship this data has to reality.

That's right- a blower door test only tells you the aggregate size of the total hole area, not where the holes are. The location of the holes matter- leaks at the foundation and those at the attic floor matter a whole lot more than leaks at the mid-level, since that defines the stack effect infiltration that goes on 24/365. That's why band joists/foundation sills, and chases that go from the basement up to a vented attic are high on the list.

What I find most interesting is the myth that blower door tests tend to promote. That myth being that old houses are leaky. Yes, if you do a blower door test on an old house you will certainly get a very high blower door test number. However, if you do a tracer gas test on this same old house, you will likely find that it has a lower natural infiltration rate than some of the most energy efficient houses built today. The reason being that these old houses are often heated with an oil boiler and radiators and don't have mechanical ventilation. Unlike modern air furnaces and even mini splits that use fans to move air which creates pressure differentials, the old style radiators do not create pressure differentials and the associated higher natural infiltration rates. So your actual natural infiltration rate is as much about how you heat, cool and ventilate your house as it is about how large a blower door test number you have.

Hey, that is really great information.

Great. Useful for many.

Also, this energy audit company likely did as little as they could possibly get away with, and likely fudged numbers on both ends.

I've seen companies caulk seams that only lead into unventilated dead spaces under stairwells, spray foam visible cracks, and not much more.

So, for the price of a tube of caulk and a can of foam, they make a few hundred bucks.


Betting big (relative to what little has been done) gains for you will be had by:

Popping molding around windows/doors and minimally expanding spray foaming into the gaps.
Spray foaming wall penetrations.
Recaulking your windows and flashing on the outside.
If you can get at your sill plate and rim joists, caulk them.
You can put in pieces of foam board and then foam them into place with expanding foam.
If you have an attic, get up there and pull the insulation back. Seal any penetrations as you find them.

I've done some blower door testing for local customer complaining of high energy bills and drafty homes and such. The blower door will measure the air changes per hour in a home. Now air changes aren't bad but too many of them mean the home is leaky and each time the air changes it needs to be reheated or cooled. Once we depressurize the home with the blower door we walk around with a thermal imager and look for the changes in temeperture to find the area of air infiltration. Here in Philadelphia were in climate zone 4 and IECC recommends 3 air changes per hour. It's been a law since 2018 for all new residential units to get a mandatory blower door test but it's not very enforced as of yet.

I've done some blower door testing for local customer complaining of high energy bills and drafty homes and such. The blower door will measure the air changes per hour in a home. Now air changes aren't bad but too many of them mean the home is leaky and each time the air changes it needs to be reheated or cooled. Once we depressurize the home with the blower door we walk around with a thermal imager and look for the changes in temeperture to find the area of air infiltration. Here in Philadelphia were in climate zone 4 and IECC recommends 3 air changes per hour. It's been a law since 2018 for all new residential units to get a mandatory blower door test but it's not very enforced as of yet. Feel free to email me or check out my site: localenergyaudits.com

You can also just use a $200 thermal camera first by itself to find cold/warm spots. Air is always moving through a house so it will show up.