Do the 2012 IMC and IECC preclude natural ventilation for dwelling units?

[mrev23]

Paragraph 401.2 of the 2012 IMC requires mechanical ventilation for dwelling units if the infiltration is less than 5 AC/hr (0.667 cfm/sf with an 8-foot ceiling) at a differential pressure of 0.2 iwg.

Paragraph C402.4.1.2.3 of the 2012 IECC doesn't allow the leakage to exceed 0.4 cfm/sf (3 AC/hr with an 8-foot ceiling) at a differential pressure of 0.3 iwg.

Converting the limit using the formula Q = c * (delta_P)^n , and with n = 0.65:

The IMC limit translates to 6.5 AC/hr at delta_P = 0.3 iwg.

So in a blower door test at 0.3 iwg:
1) IECC doesn't allow leakage to exceed 3 AC/hr
2) IMC doesn't allow natural ventilation unless the infiltration exceeds 6.5 AH/hr.

Are these conflicting limits code-speak for, "Don't use natural ventilation in dwelling units?"

If so, why didn't they just say that in Paragraph 401.2 of the 2012 IMC?

 

[cry22]

Pay attention to the wording - Section 402 does not requires Mechanical ventilation, it does not prohibit Natural ventilation.
the section states: "Shall be ventilated by Natural ventilation in accordance with section 402 [highlight #FCE94F]OR[/highlight] by Mechanical means".

"OR"[highlight #FCE94F][/highlight] means one or the other.

 

[mrev23]

Understood.

The 2012 version of IMC adds a caveat about the minimum infiltration rate that must exist in a dwelling unit before the use of natural ventilation is allowed:

http://publicecodes.cyberregs.com/i....htm?bu=IC-P-2012-000005&bu2=IC-P-2012-000019

"Where the air infiltration rate in a dwelling unit is less than 5 air changes per hour ..."

This minimum infiltration required in IMC is more than the maximum leakage allowed in IECC. Thus, it seems impossible to use natural ventilation that complies with IMC while still meeting the leakage requirement in IECC.

Doesn't an attempt to comply with both codes make it impossible to use natural ventilation in dwelling units?

 

[tys90]

I agree with your assessment, looks like you can't use natural ventilation if you are under 2012 IMC and 2012 IECC. Interesting to note that both requirements are new to 2012, so if you are under 2009 for either one, you would still be allowed to use natural ventilation.

 

[cry22]

Agree with the interpretation on the oddity, however, there is another twist - When we talk code, We are talking Design Phase, yet the IECC requires the 5 ACH be established through blower testing "when tested with a blower door at a pressure of 0.2-inch water column (50 Pa)". Building testing can be established only at building completion.

If one is designing a Ventilation system, one must have a clear set of defined data, since the 0.2" (50 Pa) cannot be established during the design phase, I would conclude that Natural ventilation is allowed until proven otherwise (at least in the design phase) - The designer must just specify a maximum leakage rate (architectural spec on the building envelope I guess) not to exceed that required by IECC to avoid code violation interpretation.

 

[mrev23]

Agreed, the post-construction blower door criterion in the IMC for plan review during the design phase is weird.

The "until proven otherwise" is problematic because it becomes an expensive fix if the building is tighter than allowed by the IMC.

However, one can anticipate that compliance with the leakage limit in IECC will ultimately "prove otherwise."

This portion of the IMC is not user-friendly because it does not say plainly what it means: Don't use natural ventilation for dwelling units.

And it provides no approved method for converting the leakage values from one differential pressure to another. There is no indication that using any leakage formula is acceptable; all the text (even for plan review in the design phase) is about post-construction test data. If the form of leakage equation used above is allowable, there is no mention of what value for the exponent is acceptable.

But the math for the conversions is all moot anyway because the intent of the code seems to be to prohibit the use of natural ventilation in dwelling units. If only they would have said so plainly ...

 

[cry22]

There are empirical data that can be used in the design phase for infiltration.

An "Average" construction will have a round 0.3 ACH infiltration, while a "tight" construction will have no more than 0.1 ACH of infiltration.
Any normal "loose" building construction will have no more than 1 to 3 ACH (more like 1 ACH than 3). You'd need a wide open large door at all times (such as loading dock) in order to get infiltration in excess of 5 ACH.

I'd say if you have a "regular building", your IECC tests will never pass, as 5 ACH of infiltration will not be there, especially under a pressure from a blower.

Do you have more than 5 ACH of infiltration? or you just want to use Natural Ventilation for whatever reason.

I would approach code officials for interpretation prior to proceeding with Natural Ventilation.

Good luck