I frequently find that if I include basement area and volume in a BPI calculation for Building Airflow Standard then relatively high flow rates will be considered too tight. 


As I have gone through these projects these past 10 years, I have been ignoring basement area and volume when I calculate BAS, no matter how much the basement is used.  I really think when they set up that calculation they did not anticipate anyone building with a basement.  It also is logical that the basement would be ignored because you will not get a lot of infiltration from below grade walls. 


The anecdotal answer whenever I ask this is ‘where is the insulation’? but that is a ridiculous proposition because insulation in a basement is not likely to influence infiltration in the house. 


Insulation in a rim joist may influence infiltration, but if you are retrofitting a house with an unconditioned basement, and rim joist insulation and airsealing is part of the plan, you do not necessarily include basement area or volume when you calculate your ventilation requirements.  Why would that change if the basement is conditioned? By airsealing the rim joist in the unconditioned space you influence the performance of the conditioned space but you have ignored the volume of the basmeent in your BAS calculation.  Why would that logic change if the basement is conditioned? You still have the same number of occupants, appliances, and location of those appliances. 


This determination is important because if you include basement volume, you may walk away from a job that really needs work because the calculations say the building is ‘too tight’.

These discussions will become much more murky when walkout basements are considered.  Half the volume anyone? 

Tags: Airflow, Building, Standard, Tight, Too, airflow

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Hi Pat, I hate bringing a basement into the house as well. For me it is because most basements are anything but pleasant. I do think including the basement in the BAS does give a little buffer to prevent us from tightening the house too much. I test airflow with basement open and basement closed just for my information. I'll either bring the basement in or leave it out based on use. I still include the area in the equation either way because that is the way the formula is written. Once ASHRAE 62.2-2013 comes out and is adopted by BPI we will be tightening to .5 ACH instead of .35. I'd rather waste energy on excessive airflow than create health hazards by tightening too much.

Most homes in this area have full conditioned basements.  They are finished and used for all types of living activity.

If the basement is intentionally conditioned, you have to include the volume.  If it is unconditioned, you really should provide an effective air barrier between the house and the unconditioned space.  You could define your thermal enclosure at the basement walls, provide no conditioning and think of it as semi conditioned. In that case the volume would still be considered as part of the home.

You have to do this because your blower door is effected by the volume.  When you calculate the shell area you have 6 sides.  You include all six sides. You don't leave one out because it leaks less than the rest.  If you build with ICFs, you still include the shell area.  A concrete basement wall is more porous than an ICF.

The issue is not .5  VS. .35.   ASHRAE 62.2.2010 says, 'All homes need mechanical ventilation. The question is how much.' The 2013 update will not change that.

I have yet to see the basement that was more connected to outdoors than the living area above, so all my basements are included in the volume calculation.

The only basement (in the mid-Atlantic) I would consider unconditioned is a basement in a baseboard electric heated house where the water heater was upstairs, say in the kitchen.  Only then would I seal and insulate the basement (or crawlspace) ceiling.  If there is a heater or a water heater in the basement, the entire basement is conditioned, whether intentionally or not, whether partially occupied or not.  Look at the example on page 4 of the BPI BA standard - it says nothing about occupied, conditioned, etc.

The 2009 and 2012 IECC state that a conditioned space is any space open to a conditioned space, or with un-insulated ducts running through it - note there is no requirement for a diffusor.

So every basement with a duct trunk is conditioned by the latest code.

AIr leakage in a basement or crawl is very important because it is the bottom of the stack during winter.  It must be sealed to reduce the stack, and when you do this, you need to pay attention to your CAZ.  If the basement is an icky place, well part of your job as a building scientist is to make it un-icky.

And making the house volume larger by including the basement gives you a larger BAS and means you can do more tightening before you install ventilation - not the other way around. Adding the basement makes the house less tight, not more tight.

Testing with the basement door open and then closed is informative and may help with understanding the big picture, but does not matter in the long run - the leaks at the basement (or crawl) walls must be sealed.  You could also close off the first floor from the second to help with  your diagnostics.

Price and need for insulation varies with above grade and below grade walls, so cost-effectiveness will vary and may mean that you won't be able to sell that insulation job - but you should still seal it, and insulate the bands.

ASHRAE 62.2-2010 does not say that ALL houses need ventilation.  For new houses, the starting point is 5 ACH50 (when coupled with the IBC), and for existing houses it uses a complicated mix of fans, windows and blower door numbers, and you can certainly have an existing house that does not need ventilation.

And is 62.2-2013 using .5 ACHnat or 5 ACH50??  I know 62.2-2010 uses 5 ACH50 (through the IBC) as a point to start ventilating, the 2009 IECC requires 7 ACH50, and the 2012 IECC requires 3 ACH50.  This means that every new 2012 house must meet 62.2-2010.

And a concrete wall is no more porous than an ICF, however a BLOCK wall is.

Do good work.

Ed Minch

Generally, the older the house, the nastier the basement.  If we do a good job of air-sealing a house, where is the remaining infiltration air coming from? If the path from outside to the living space is via the basement, how much is the air quality compromised?  Depends on the basement.

Stopping air-sealing short of needing to add mechanical ventilation meets 62-89 and current NY State code, but not the current thinking about IAQ and ventilation needs.  

Some folks set up a TV, Lazy Boy, and a kegerator in the basement and call it finished.  Should it be included in total volume? A new finished basement may be included, a dirt floor rubble stone foundation basement won't be included.  It's all the gray areas in-between those two extremes that we have to deal with.  Thinking about the air quality in a basement and whether you'd want to be breathing that air, may help decide whether to include the basement in the volume.

On a different note, the NYS-RCC-2010 code book states:

M1701.4  Prohibited sources.  Combustion air ducts and openings shall not connect appliance enclosures with space in which the operation of a fan may adversely affect the flow of combustion air. Combustion air shall not be obtained from an area in which flammable vapors present a hazard.  Fuel-fired appliances shall not obtain combustion air from any of the following rooms or spaces:

1. Sleeping rooms.

2. Bathrooms

3. Toilet rooms.

As I read it, after sealing holes in the pressure boundary, you may need to create a pressure barrier at the basement ceiling to keep house exhaust fans from communicating with atmospheric appliances via plumbing chases and the like.  The same pressure barrier limits make-up air from coming into the house via the basement.   You would also need to keep the basement door closed, which is the way most homes are used, anyways.

Someday, balanced pressure HRV/ERVs will be standard, but until then, think about IAQ, the pathway for outdoor air coming in, and how much. 

I understand all the theory and logic you all are putting forth here. Most of it a agree with. But, in the end the owner sets a budget and we need to stay within it. If that weren't the case I would always foam a basement.

What I am asking for is not what you are answering though.

Does BPI, or anyone else, have a rule about including or excluding basement volume for a minimum airflow calculation? We all have our own 'best practice', which is gleaned from our experiences. Personally I have seldom seen any house experience ill effects if we excluded basement volume in this calculation, but I have seen dozens of occasions where a house that has obvious flaws in thermal and pressure boundaries of above grade surfaces will test in at or below minimum airflow standards if I include basement volume. In those cases, if I tighten without venting we don't see problems with condensation or IAQ as long as we don't tighten beyond the minimum guideline for the volume of the above grade sections.

Of course, if you see obvious IAQ hazards, you have to vent or change behavior or circumstance to remove that hazard. Pa cannot continue to melt lead for his black powder club in the basement. Mom cannot continue to dry clothes on the line in the basement. There are hundreds of things like this to be aware of and act on. But these actions are actually rare.

Look at the example on page 4 of the BPI BA standard - it says nothing about occupied, conditioned, etc.  BPI says to use the basement as part of the calculation and has never said that this changes with climate. There will certainly be times, as has been noted, where extenuating circumstances dictate otherwise.

You say:

"I have seen dozens of occasions where a house that has obvious flaws in thermal and pressure boundaries of above grade surfaces will test in at or below minimum airflow standards if I include basement volume."

But making the house volume larger by including the basement gives you a larger BAS and means you can do more tightening before you install ventilation - not the other way around.  Adding the basement makes the house less tight, not more tight.  The blower door readings with the basement door closed and open will be close, and be outweighed by the added basement volume - this will allow more air sealing.

Inquiring minds?

Ed Minch

Here's how I figure the opportunity for air sealing: Measured CFM50 - BAS.  The smaller the CFM50 or the larger the BAS, then the less room for tightening.  

Another way to look at this is that the air flow requirement gets larger as the volume of the house gets larger.  So yes, adding the volume of the basement means a larger ventilation requirement, i.e. less air sealing.

Although I have yet to find an explicit requirement to include the basement volume in the minimum airflow calculation, the BPI BA standards example strongly implies that basement volume should be included always.  BPI instructors I've chatted with agree that we should always include basement volume.  

BTW, by the same logic, unvented crawl spaces should be included, but that may be another discussion.

I, too, run into similar problems where an unfinished basement that is only used for storage and mechanicals and has been sealed at framed-flooring above doesn't seem like it ought to be included in calculation of the home's airflow requirements.  

The way I've come to rationalize always including the basement is that the BAS is on the low side of requiring fresh air when compared to later revisions of ASHRAE 62.2 and so it seems that adding the basement volume helps the BAS to be more conservative and err on the side of more air flow.

We can make use of the 70% BAS range to allow additional air sealing.  Many of our customers are already tuned-in to indoor air quality and so have chosen to reduce the sources of pollutants in the home such as off-gasing floor coverings, furniture, and finishes.  That helps me feel comfortable about being more aggressive with the air sealing.  And, yet we have to keep in mind that the next person who owns the house could have totally different habits (or more children, dogs, etc!).  So my advice is make the house as tight as is affordable, provide mechanical ventilation in the right capacity according to BAS, show the customer how to control it, and help them set it at the appropriate level for the situation.

One size does not fit all, and Standards cannot be written to meet every type of issue presented in the field. BPI always refers to State or local codes. In the absents of clearly defined measures it still all comes down to a competent person to evaluate the whole system and apply best practices.

I've tested tight houses with really great iaq, and really loose houses with terrible iaq.  That got me thinking about ACH seasonality due to stack pressure.  Think a house with good IAQ on a 15f day has the same IAQ on a 55f day?  Certainly is NOT going to have the same air changes!  

This lead me to conclude BAS and leakage are not proxy for IAQ.  I think best practice is always recommend mechanical ventilation.  This gets the educational conversation going, and shifts responsibility for things we can not control to the homeowner, who can control them.  (Unfortunately in this era of "shortening the audit" and "approved/cost effective measures" I think a lot of the consumer consultation and education goes out the window in the rush for quick, prescriptive sales...)

Soon monitoring and logging will be so inexpensive that I suspect that will be the recommendation we will make for all homes - manage fresh air based upon need.  


What measure did you use for IAQ?  What pollutants did you look for? Do you do it in every house?  I recently had to refute a guy who measured "high cellulose" in the carpet of the bedroom with the pull down stair to the attic when there was no standard to interpret his readings by.

Ed Minch


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