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Building Tightness Limits
Last Updated: 08/18/2010
 
Building Tightness Limits (BTL) can provide a guideline for tightening up a house.
(Parts of this are excerpted from The Residential Ventilation Handbook by Paul H. Raymer published by McGraw-Hill in November, 2009.)

           

It is pretty obvious if you can’t run your range hood and keep a fire going in the fireplace, that the house is too tight.  The reaction is to open a window.  It is not always obvious that carbon monoxide (CO) is flowing down the water heater chimney into the house when the bath fan or range hood is on.  Ventilation devices are systems.  They mechanically move air into or out of the house, pressurizing or depressurizing the space, and relying on building leakage to make-up the difference.  Some systems, like Heat or Energy Recovery ventilators (HRVs or ERVs) are balanced.  They don’t provide extra for other appliances like water heaters or clothes dryers.  Those systems need their own make-up air.

 
Building tightness limit (BTL) (or Building Airflow Standard (BAS) or Minimum Ventilation Level (MVL) or Minimum Ventilation Rate (MVR)) is a general term for a house-tightening limit used for ensuring adequate air quality for the occupants of the house.  Weatherizing or reducing the energy consumption of a house requires tightening it up, closing up the air leaks and holes where pressure forces the air in and out of the building.  Many homes, particularly manufactured homes get a large portion of their air from the air leaks and once those have been carefully sealed, the requirement for mechanical ventilation increases because the natural air changes have decreased.  The Building Tightness Limit was developed as a benchmark, a limit beyond which the health of the occupants may be jeopardized.  Most homes can use a primary ventilation system.  The BTL benchmark should be considered a way to confirm the ventilation needs.

 

The BTL can be calculated using three formulas.  (See the “n” factor table below.):

Formula #1:   15 cfm x the number of occupants x n = CFM50 BTL     This formula estimates a tightness level based on the number of occupants.

Formula #2:   15 cfm x number of bedrooms + 15 x n = CFM50 BTL   This formula calculates the tightness level based on the number of occupants as estimated by the number of bedrooms.

Formula #3:   Volume of the conditioned space x .35 x n/60 = CFM50 BTL  This formula calculates the tightness level based on .35 air changes per hour of the volume of the building.

The BTL equals the highest number calculated using formulas 1, 2, or 3.  The number recorded would be the minimum allowable CFM50 of the conditioned living space.  For example a 1 story, 1200 square foot, 2 bedroom house in Massachusetts with 3 occupants is tested with a blower door to 960 CFM50.  (For this house in this location n = 14.8 from the LBL tables.)

Formula #1:   15 cfm x 3 x 14.8 = 666 CFM50 BTL

Formula #2:   15 cfm x 2 + 15 x 14.8 = 666 CFM50 BTL

Formula #3:   1200 x 8 (ceiling height) x .35 x 14.8/60 = 828 CFM50 BTL

All of these calculations are less than 960 indicating that this house would be loose enough not to require mechanical ventilation.  A better alternative would be to tighten up the house further, dropping it below 650 CFM50 and then adding mechanical ventilation.

           

The tightness of the house can be measured using instrumentation such as a “blower door”.   Many green building and weatherization programs have criteria for how tight a home should be before mechanical ventilation must be added.  This may be a reference to a BTL or to a Minimum Ventilation Rate (MVR).  Indiana, for example, sets their MVR at 1200CFM@50 Pa.  (This is the “CFM50” level.)  That means that a blower door must be used to exhaust air from the house until the house reaches a negative pressure relative to the outside (with respect to or WRT) of 50 Pascals.  If the amount of air flowing through the test fan to accomplish this exceeds 1200 cfm, then the house is leaky enough not to need additional mechanical ventilation (in their program).  Luckily homes are not being continuously exhausted to this artificially high level, which is used to exaggerate the conditions and to reach a standardized, comparative level.  The CFM50 level can be converted to a more realistic CFMNatural or CFMNat level by dividing it by a factor that takes into account the location or wind conditions and the height of the building known as the “N” or “n” factor.  Lawrence Berkeley Laboratory came up with variations on the “n” factor which used to be just averaged at 20.  They vary from a low of 9.8 for an exposed three-story house in a cold climate like North Dakota to 29.4 for a well shielded, one story house in a warm climate like Miami.  Dividing the CFM50 number by the “n” factor provides CFMNat or approximately how the air will move through the home under average natural conditions.

 

For example a two story 1200 CFM50 house located in Massachusetts that would be considered “normal” in terms of wind exposure would use an “n” factor of 14.8.  Its CFMNat would be 1200/14.8 or 81 meaning that under average conditions this house would be leaking 81 cubic feet of air per minute.  If the house were tightened up to 900 CFM50, its CFMNat would be reduced to 900/14.8 or 60 cubic feet per minute.  This is under “average” conditions.  Much of the time it would be leaking less than this.

 

Because it takes such a small amount of pressure to potentially backdraft a combustion appliance, caution should be taken in aggressively tightening up the house and employing mechanical ventilation.  It is highly recommended that combustion safety testing be done before, during and after the air sealing/tightening process.  The calculated CFMNat number is an approximation under “average” conditions meaning that half the time the natural ventilation will be greater than this and half the time it will be less than this.  Using a continuous, primary mechanical ventilation system assures that some ventilation will be happening all the time and is an advisable approach unless the house is obviously too leaky to warrant it.

 

 BTL (LBNL) Map

 

 

 

Zone

# of Stories

1

1.5

2

3

1

Well-Shielded

18.6

16.7

14.9

13.0

Normal

15.5

14.0

12.4

10.9

Exposed

14.0

12.6

11.2

9.8

2

Well-Shielded

22.2

20.0

17.8

15.5

Normal

18.5

16.7

14.8

13.0

Exposed

16.7

15.0

13.3

11.7

3

Well-Shielded

25.8

23.2

20.6

18.1

Normal

21.5

19.4

17.2

15.1

Exposed

19.4

17.4

15.5

13.5

4

Well-Shielded

29.4

26.5

23.5

20.6

Normal

24.5

22.1

19.6

17.2

Exposed

22.1

19.8

17.6

15.4

 

(See “Survey of Tightness Limits for Residential Buildings” developed by Rick Karg in July, 2001 that is available at http://www.karg.com/pdf/Survey_of_BTL.pdf)


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