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Best method for infiltration heat loss

jinmtvt | Posted in General Questions on

I would like to ask the math pros here on how to calculate heat loss from infiltration rates.

I have seen so many different formulas in the last weeks that my head is hurting.

So would like to have a “reliable” formula that could be used in my never-ending expanding excel spreadsheets !

Something with at least a little bit of proven precision, that could be used to effectively get additional heat loss using door blower results or planned target value.

Links or straight formula please 🙂

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  1. GBA Editor
    Martin Holladay | | #1

    The answer can be found in my article, How to Perform a Heat-Loss Calculation — Part 2.

    Here are the relevant paragraphs:

    If you want to do a pencil-and-paper calculations of the heat loss due to exfiltration and mechanical ventilation, you can also perform the calculations using the following formula:

    Ventilation heat loss (in BTU/h) = Ventilation or exfiltration rate (in cfm) ● ΔT (in F°) ● 1.08

    Where does the 1.08 come from, you might ask? It's the volumetric heat capacity of air (in BTU ● minute / hour ● cubic feet ● °F).

  2. Expert Member
    Dana Dorsett | | #2

    There is no reliable formula for infiltration volume rates based on blower door test numbers, since there is no way of predicting where the air leaks are. A single hole in the wall will have orders of magnitude lower heat loss than two equal-sized holes, one at the lowest elevation of the house, the other at the highest elevation, which maximizes stack effect infiltration drives.

    Treat any of those formulas with a large degree of skepticism, with at least 25% error bars- assume it is likely to come in between 25% higher and 25% lower than what the formula predicts. That is a large range, to be sure, but the lower your blower door numbers, the smaller the fraction of the total.

    Then there is the unresolved issue of the "heat exchanger effect". The air entering your house via leak points does not enter the house at the outdoor temperature, it is warmed by the materials it passes along it's path. Similarly the air leaving the house doesn't reach the outdoors at the full indoor air temperature. This is quite UNLIKE active ventilation through smooth walled vents. No matter how big the high end of your infiltration volume estimates are, the heat loss associated with that will be lower than a ventilation air heat loss calculation would indicate. So treat it as a truly worse-than-worst case.

  3. Airithol | | #3


    I've already read both the heat loss articles you wrote (and many others online).

    After reading this: I am still left wondering one thing. The article suggests that ACHnat is a rough estimate, and not to be taken that seriously.

    So what value should I use for the CFM in the above equation? The CFM value from a blower door test is at 50 pascals, surely that is of no use in calculating infiltration for heat loss under "normal" conditions?

    To summarize my current understanding;

    CFM = Absolute volume of air per minute, at "normal" conditions. (aka CFMnat)
    CFM50 = Absolute volume of air per minute, at 50 pascals.
    ACH50 = CFM50 * 60 / Volume (for air tightness comparisons).
    ACHnat = ACH50 / 20 (to approximate "normal" conditions).
    CFMnat = CFM / 20 (to approximate "normal" conditions).

    Dividing by 20 is the simplest method I found.. The Sherman and LBNL methods are a few others.

    CFM * ΔT * 1.08 can also be expressed as ACHnat * Volume * ΔT * 0.018.

    Maybe I am getting hung up on the math conversions between units and missed something?


    I dove into this a few weeks ago myself and found a fair bit of seemingly conflicting information online. This site had a good explanation.


  4. GBA Editor
    Martin Holladay | | #4

    There is no resolution to the fact that ACH(nat) is simply an estimate.

    ACH(nat) is higher in winter than in summer.

    ACH(nat) is higher on windy days than calm days.

    ACH(nat) is higher when envelope holes are evenly distributed between high holes and low holes than when envelope holes are clustered near the neutral pressure plane.

    In Minnesota, ACH(nat) equals approximately ACH50 divided by 17, while in Florida, ACH(nat) equals approximately ACH50 divided by 30. This are estimates only. Your mileage may vary.

  5. Airithol | | #5


    Thanks for clarifying. There is a lot of information out there on this topic, and some sources portray it as more of an exact science than educated guess.


    Thank you, and your point is well taken - especially about the lower the blower door number the lower the absolute error range. +/- 25% sounds like a a huge margin, but once we approach passive house air tightness, it's just not a huge amount of actual heat loss.


  6. jinmtvt | | #6

    JAson : i espeially like this sentence :

    There is some evidence that as air leaks out thru an insulated cavity, the cavity acts a bit like an HRV, but given that you don't really want air leaking thru an insulated cavity, especially not at a slow rate where condensation can happen, its best not to assume this happens.


    Martin : thanks for the re-link, but i was more looking for the non obvious part of the situation.
    i hate BTU and Farenheit :p

    Dana : +-25% of which ? is it all linear equations ?

    Something i am not sure i understand properly,
    isn't ACH50 take at 50 because it simulates almost worst possible conditions ?
    and thus would represent on a simple house type building, maximum possible leak or very close to it ?

    The Sherman and LBNL are the minimum i would ever consider to use in any serious energy calculations.

    Isn't there anything better ???

    Can we use HDD to calculate year long losses ?

  7. Expert Member
    Dana Dorsett | | #7

    The short answer is, no, there isn't anything better.

    The degree of precision will never be good enough to be worth calculating. Use one of the simple estimation calculations, assume that the air volumes could be 25% higher than that, as a worst-case but without knowing how much heat-exchange/recovery you're getting out of that you can't really convert that to Joules/hour let alone MJ/year.

    Of course, tighter is always better, but how much better can't really be known. A blower door test doesn't measure stack effect, it only measures the total cross sectional area. Even using tracer gases to follow the leakage paths from stack effect won't tell you the volumes of wind-driven infiltration, nor will it tell you how much heat is transferred between the air & building materials on its way into and out of the house. The models are really guesses, but they are educated guesses.

  8. jinmtvt | | #8

    Thanks for clearing that up sifu Dana !

    Personally, i do not intend to do any other than new buildings in the future,
    and i will not settle for anything lower than near perfection as far as air tightness.

    So if there is no precise way to measure it, it must be avoided.

    If holes and leaks cannot be avoided,
    which factor influences pressure differential the mosts other than Delta T ?

    It seems fairly easy to double up on door seals and backdraft dampers.

    When aiming for super high efficiency design, it becomes easier to simplify mechanicals and envelope openings.

    Where could i find information about detected leaks on PH or similarly tight existing projects ??
    Could be interesting to see where problems are occurring at high rates.

    Using a single HRV for all venting, mini splits for climate control and adding a motorize damper to kitchen hood operation should minimize possible leaks.

  9. fitchplate | | #9

    There is precision to be had but its not by using the blower door and smoke pencils.

    While still not easy, I think the easiest part of the mission is in the treatment of the inoperable, fixed features fo the building: the bulk air sealing, the infiltration/exfiltration pathways sealing, and selecting the right materials for establishing optimum permeability --- doing this considering all 6 planes of the envelope.

    I think Jin is probably capable of attention to the detail needed to air seal and remove/overcome the fugitive exfiltration/infiltration points. Precision comes with taking the time to measure each surface (plane), seam, joint and transition in the inside and exterior surface of the sheathing layer.

    Rigorous attention to detail can result in nearly 100% air sealing of the fixed and inoperable elements.

    Instead of a blower door to be your testing tool, live with an IR camera hanging from your belt. The blower door test is an after the fact measurement tool. Builders and installers need real-time measurement and quality checking. Do it right before you do the pressure testing.

    The IR camera will show you the D-T between inside and outside anywhere you point it, it will show air flow issues, cracks, quality of insulation install, drywall seam quality and very importantly it will show conduction (ie bridging) issues that cannot be shown by any other method.

    It stands to reason that the weak spots (risky areas) are the operable components: mechanical penetrations in the envelope such as the doors, windows, combustion venting and air supply, conditioned air exhaust, pressure balancing, etc. The quality and performance of these components available no matter what you are willing to pay is shameful.

    Find a vent louver, back draft damper or a through wall cap that can be considered respectable! They don't seal and have lousy R values. More money buys better quality – sometimes. But these operable envelope penetrations (where we are adding more and more by code every year) are systemically weak and represent many square inches of low to no insulation and many square inches of infiltration/exfiltration.

    The worst weather stripping on doors, windows, vent covers and hoods
    Absence of closure designs that self seal and inherently cam lock
    No insulation in fan and vent housings
    Back draft dampers that are responsive to any minor pressure change - not calibrated or adjustable to their function
    No conditioning air-exchange or pre-warming features in any mechanical penetration equipment except the H/ERV's

  10. wjrobinson | | #10

    Finding leaks;

    Nighttime use light
    blower door and use smoke and the back of your arm and infra read cameras
    Like we do now with plumbing stacks, you could fill your home with water to the peak and check for leaks

    Every joining of material can leak. stop leaks at those locations during the entire building process.

  11. Expert Member
    Dana Dorsett | | #11

    I've found the filling the house with water method of leak detection somewhat less than useful, since caulk & foam is hard to apply reliably while under water. It's better if you use a method that doesn't require diving gear to fix the leaks as you find them too. ;-)

  12. jinmtvt | | #12

    Building a REMOTE/PERSIST wall system using peel stick that covers the basic structure completely
    ( including the roof/attic ) makes it much easier to get low ach numbers.
    ICF is also another method that usually results in wall assemblies that are less prone to building deficiencies.

    AJ: on some larger buildings, the additional water taxes from your testing method would probably push the "sealing efforts" back for 100+years of ROI :p

    Fitch : anyone trying to build efficiently should either rent or buy a thermal camera.
    The prices are really falling on the base models from FIR and some others now ...

    Problems also arise when budget limits the type of wall system to be used because of the required insulation quantity, which in turns multiplies the number of possible leaks vs a more desirable assembly and then you are still left with labor cost limits to sealing hundreds of joints and cracks.

    AJ: you seem to have much hand on labor experience,
    how much labor is worth air sealing step on one of the conventional stick frame/sheathing systems in an average size house ? ( tapes and caulking included .. )

    Sometimes, the labor intensive details value could've been invested in a more expensive wall system that wouldn't require them to start with.

  13. fitchplate | | #13

    I am almost embarrassed to reveal the labor time on air sealing my place. And I am still looking for after-market and after- construction components to improve on the through-wall/operational penetrations' sealing features.

    And I had to do it myself as no one hired has the patience or motivation, commitment, or energy management beliefs to be forensic and diligent about it.

    It definitely needs doing while building, not after the framing envelope is done. And the cost of poly ether and PVC caulks and high end builders tape (don't be fooled by crappy latex and silicone caulks or expensive plastic and asphalt based window and sheathing tapes) will be a couple of thousand $'s.

    Also don't be fooled by Lesco and similar electrical boxes (some recommended on this website), they do not seal the airflow. Electrical boxes need to be hand covered and sealed with putty pads or back sealed with poly ether caulk.

    Also, also ... don't use spray foam crack fillers as sealants. They do not seal well to the boundary materials. If you fill door and window frame to rough buck spaces with backer rod or non-expanding foam, make sure you set it up so you can caulk the face of he entire opening with poly ether caulk.

    Also, also, also ... poly ether is the only way to go. It has indefinite life, it will not shrink with time or temperature, it has 150% elasticity, it works on oxidized materials, can be applied to damp and condensing surfaces (if you get the right grade), it adheres to metal, glass, wood and plastic, and its hot water and soap clean up. Latex fails within a year and silicone within two to three years. PVC and PU caulks and adhesives have a place, but not where you want ease of working and cleanup.

  14. jinmtvt | | #14

    Thanks Flitch for the advices, i neve worked with poly ether caulks i believe, i will look into it now.

    First quality silicone from GE is tough though ..i've used it extensivly to caulk windows to exterior finishes and it is still flawless 5-6 years after installation even on south east side of the building.

    But, you are pushing the same conclusion here , if speding thousands on material and labor to effectively seal the enveloppe, why not get rid of those problems in the first place.

    Electrical boxes wouldn't be a problem if all of the framing was sealed from the exterior.
    On my ICF house as example, the only things i need to worry about are doors and window opening,
    and a few ventilation openings.
    even the vents are hard to miss when you have 12-14" of EPS/concrete/EPS wall witdh to seal against.. spray foam can from interior and caulk from exterior and it make it pretty impossible to leak around the systems.

    Back to formulas, isn't there a location specific data that is used for altering infiltration rates ??

  15. exeric | | #15

    A big +1 for using engineered backdraft dampers. You want to be able to control where the air comes in. But you should treat it just like you treat windows in that they will always be areas of poor insulation. Don't overdue it. One in each bedroom and one near enough to the kitchen to serve the range hood. Implement them with a tubed wall hood, (like the wall exterior bathroom vents) but get them without built in backdraft dampers. The built in ones always seem to be inferior. Get a retrofit one like the kind American Aldes makes. They are very good quality. I plan to trim a little sheetmetal off of one of those so I can fit two dampers in one wall tube. I expect that will stop a substantial about amount of the inside and outside temperature exchanges at this weak point. You DO need a thicker than 2x4 wall framing to get away with this simple trick. I should add that having two in series in a tube should cut down on nuisance air exchanges caused by high winds.

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