IECC 2012 leakage test versus the ASHRAE MVR limit
user-972873
| Posted in Building Code Questions on
Comparing the two equations is confusing. If you compare the IECC 2012 requirement for 3ACH50 in zone 4 with the MVR equation for a 2000 sq ft home with 8 ft ceilings and 4 beds in zone 4 with the climate N value of 24.5 the IECC cfm is 32.6 while the MVR cfm is 57.5. The MVR is larger than the IECC leakage level. Am I missing something?
MVR=0.01(2000)+7.5(5)= 57.5cfm natural
3ACH50= 48000 cu ft/24.5=1959ACHn/60=32.6cfm natural
The leakage test is looking for less leakage than that required for MVR. This would require the builder to provide mechanical ventilation on all code homes.
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Replies
Paul,
I'm not an expert in this field, so I stand ready to be corrected.
My first reaction: The ASHRAE MVR calculation method is obsolete. More information here: "The BTL, Minimum Ventilation Rate (MVR), and some other existing dwelling CFM50 threshold values are all based on the older, and now obsolete, ASHRAE 62-1989 ventilation standard. As weatherization programs implement the current ASHRAE 62.2-2010 Standard, the use of the BTL, MVR, or other thresholds to limit the air sealing activities on a dwelling based on ventilation requirements will cease. There is no comparable threshold to use with the current ASHRAE 62.2-2010 Standard. Since there is no set limitation for building tightness, homes can be tightened as much as is cost effective through the Savings-to- Investment ratio (SIR) as determined by the audit. The minimum ventilation rate as used in the Standard defines the required CFM of ventilation, and is unrelated to the MVR or BTL used previously as a CFM50 threshold."
My second reaction is: even if you are interested in making the obsolete MVR calculation (for which, by the way, you can use this worksheet), what makes you think that the IECC 2012 code requirements for building envelope airtightness need to be correlated in any way with the obsolete MVR calculation?
ALL homes are better served with mechanical ventilation, even those that leak like sieves.
The derived ACH-natural numbers based on any ACH/50 number are completely bogus in the first place, since the location of the actual leakage makes an order-of-magnitude difference in how much air actually moves through the building. Even in very leaky buildings there's no reason to assume the leakage is at locations where the ventilation is actually needed or at the volumes necessary.
HI Martin,
Thanks for your reply. I did see the extract you show and the worksheet you reference appears to be older than the 62.2-1989 version. I used the 62.2.2007 version for my calculation. I wonder if anyone knows what the 62.2-2010 or better still the 62.2-2013 version uses for MVR?
Paul,
The most recent ASHRAE 62.2 formula is 3 cfm per 100 sq.ft. of occupiable floor area plus 7.5 cfm per occupant. The older formula was 1 cfm per 100 sq.ft. of occupiable floor area plus 7.5 cfm per occupant. In the 2010 version there was a built-in infiltration credit of 2 cfm per 100 sq. ft. to get the number. In the current version, however, that default infiltration credit is gone. You can, however, get an infiltration credit based on a blower-door measurement.
Martin,
Is the most recent you refer to 62.2-2013? If so they have increased to ventilation level from 57.5cfm in 2007 to 97.5cfm in 2013. I prefer to use the bedrooms over occupants as the MVR would be different if only two people lived there and then a family moved in. Either way the new calculation makes the situation worse as regards the maximum allowable leakage from the 2012 IECC and the 2013 MVR. It also doesn't make a lot of sense as many of the pollutant generators are going away with direct vent appliances and HVAC systems.
This will be a dilemma for authorities that adopt the IECC 2012 testing methods for leakage because there has to be a MVR for healthy living, but this spec is not in the IECC outline.
Paul,
As I understand the 2012 IECC, the code:
- Includes airtightness requirements for the building envelope, namely 5 ach50 for climate zones 1 and 2, and 3 ach50 for homes in all other zones.
-- Requires a mechanical ventilation system in all homes with a blower-door test result of less than 5.0 ach50. Since the new code requires homes in all zones except zones 1 and 2 to achieve 3 ach50, the code effectively mandates a whole-house mechanical ventilation system for homes in zones 3 through 8. If a home falls under this requirement, the ventilation system must comply with requirements listed in 2012 IRC section M1507.3 (shown in the table below).
The code does not reference ASHRAE 62.2. For more information, see An Overview of the 2012 Energy Code.
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Martin,
Looking at your table my example shows the ventilation at 75cfm which equates to 4500 cubic feet an hour. The IECC result is still 2000 cu ft hour. The bottom line is that the leakage test has a limit of (48000cu ft) 2000 cu ft natural and the ventilation required is 4500 cu ft natural. My original position. It seems the powers that be have a conflict that will confuse builders and building departments once they put two and two together.