Infiltration

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What 90.1 PRM Section(s) are relevant to this issue?
3.3.5 Infiltration

Explanation of issue:
Modify existing methodology for modeling infiltration.
Please review the new methodology proposed below. This has been added for 90.1 
2013 Appendix G. The value for I75 comes from 90.1 2013 Appendix C.
If possible, we'd like to move to this approach.

Proposed resolution:
The new methodology-

The air leakage rate of the building envelope (I75Pa) at a pressure 
differential of 0.3 in. H2O shall be converted to appropriate units for the 
simulation program using the following formula describing infiltration as a 
function of exterior wall area,
IEW = 0.112 × I75Pa × S/AEW
where
I75Pa = air leakage rate of the building envelope expressed in cfm/ft2 at a 
fixed building pressure differential of 0.3 in. H2O, or 1.57 psf
S = total area of the envelope air pressure boundary (expressed in ft2), 
including the lowest floor, any below- or above-grade walls, and roof (or 
ceiling) (including windows and skylights), separating the interior conditioned 
space from the unconditioned environment measured
IEW = adjusted air leakage rate (expressed in cfm/ft2) of the building envelope 
at a reference wind speed of 10 mph and the above ground exterior wall area
AEW = total above-grade exterior wall area, ft2
The peak infiltration rate of the building envelope (I75Pa) at a fixed building 
pressure differential of 0.3 in. H2O shall be 0.4 cfm/ft2 exterior building 
enclosure area.

The calculated value for Iew will be used in the 
zoneinfiltration:designairflowrate object, as a funtion of the exterior wall 
surface area.
The coefficients for infiltration would be 0,0,0.224,0 (DOE-2 methodology)

Original issue reported on code.google.com by supriya....@pnnl.gov on 4 Sep 2014 at 7:35

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Supriya, we are confused.  Is teh design infiltration rate to be included in 
the ZoneInfiltration:DesignFlowRate object 0.4 cfm/ft2, or are you expecting 
this to be adjusted in some way.  We don't understand how the ratio S/AEW is 
supposed to be used.  

Original comment by rhedr...@archenergy.com on 22 Sep 2014 at 5:20

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We read through Section C3.5.5.3 and the formula is clear now.  It seems odd 
that the value of "S" is dependent on below grade surfaces, but the ruleset has 
been updated to reflect this section.

Original comment by ncz...@archenergy.com on 22 Sep 2014 at 6:33

• Changed title: Infiltration

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ok, good to know that it's clear now.
I agree that it is odd, both 90.1 2013 App C and App G use this formula. I will 
ask Mike for any analysis that substantiated this change and get back to you if 
I can find anything

Original comment by supriya....@pnnl.gov on 23 Sep 2014 at 1:17

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It seems wrong to include the below ground surface area in the conversion from 
area of the envelope to exterior wall area.  Two otherwise identical buildings 
with disparate underground surface areas will have different infiltration 
rates.  

Original comment by rhedr...@archenergy.com on 23 Sep 2014 at 1:26

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Roger-
Mike clarified the approach-
The 0.4 cfm/sqft is based on measured results from buildings tested according 
to the ASTM Standard and that standard reports the leakage as a function of the 
entire pressure boundary of the building including below grade walls and slabs. 
That is why the conversions are necessary, since the simulation software 
reports it in terms of exterior surfaces. 

Yes, I agree that 2 identical buildings with different below grade wall areas 
will ahve different infiltration rates.
If you see 90.1 2013 App G section G3.1.1.4, I75 is supposed to be based on 
user input- i.e. it requires measures infiltration airflow.
We followed a simplified approach of fixing this to 0.4 CFM/sq.ft. The right 
interpretation of 90.1 would be to allow user input else use C3.5.5.3 and 
default to 0.4 cfm/sq.ft.

Does this approach sound right? 

Original comment by supriya....@pnnl.gov on 23 Sep 2014 at 2:28

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The adjustment of S/AEW makes sense when you are starting with an actual 
measurement for I75.  When you fix I75 at 0.4, though, I think it would make 
more sense to use the total above ground envelope area instead of S.  However, 
for 99.9% of buildings the impact will be small, so the current formula is fine 
to retain.  

Original comment by rhedr...@archenergy.com on 23 Sep 2014 at 3:42

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Original comment by rhedr...@archenergy.com on 23 Sep 2014 at 3:43

• Changed state: Resolved