Existing Buildings

Yes assuming the space was previously conditioned.

How do you suggest accounting for poor infiltration in the existing/baseline building, when a building envelope project was a part of the renovation which improved/decreased infiltration in the proposed building? We are renovating a historic building which included window replacements, furring out the walls and insulating. We recieved the review comment, "...the infiltration rates must be modeled identically between the baseline and proposed cases. Revise the model, as required, such that the infiltration rates are modeled identically."

The building was leaking like a sieve before the renovation and we modeled it with "poor" tightness (in TRACE) for the baseline. This resulted in the entered values in the TRACE model being 1 ACH in the baseline and 0 ACH in the proposed. How should we alternatively model this to account for the improvement in infiltration between the old/existing building and the new/proposed building? I appreciate any advice!

Under the current rules you cannot claim any credit for reduced infiltration. The problem is that there is not a defined baseline of performance.

Your comparison assumptions are a case in point. What was your basis for 1 ACH? Was it measured? The proposed 0 ACH is frankly not possible. If you want to try and claim savings for infiltration you will need to present an ironclad case, as an exceptional calculation, for it based on actual pre and post measurements in my opinion. That may work for an existing building but still potentially raises many baseline questions regarding the existing conditions.

I don't believe there is a set approach as to how the proposed building average infiltration should be calculated? In my experience in heating led climates I have always used conversions from the air pressure test (m3/hr/m2 at 50Pa) to average ACH based on the building type and stories. Typically for a value of 10m3/hr/m2 at50 Pa this may be 0.15-0.25ACH

However I have read that when the building is positively pressurized the infiltration is likely to reduce but not to zero but there doesn't seem to be a standard to what. I have read 10% (which basically is almost zero at low air tightness).

I was wondering what others approach was?

I have read some research that indicated that infiltration is a much larger component of energy use than previously thought. The lack of infiltration in positively pressurized buildings is basically a myth. I think NREL did some of this research and I know there have been some ASHRAE Journal articles.

Also the UK has specific standards for air tightness and requires testing in certain buildings I think. Good links and information on this web site - http://www.infiltec.com/inf-larg.htm

We are currently working on a school project targeting <0.10 cfm/square foot @ 50 pa.

It is a few years after this discussion but our team has a similar concern. We will propose to the Owner that they test the air infiltration rate on the existing historic windows as is, and test a mock-up of a matching window with new improvements - better sealants, packing wood jambs and trim more securely, paint, re-installed storm windows, etc. Assuming the performance is better and measurable, the engineer would use the two rates in the model, base and design. Do you think that type of evidence and research testing would be an acceptable approach?

Comparing just the windows presents a potential problem in that this is not the entire envelop assembly. If your modeling software can account for the window infiltration separately and you have a valid testing methodology you might have a shot at an exceptional calculation but it will receive so pretty thorough scrutiny.

FYI since these previous posting ASHRAE 90.1 has established a baseline infiltration rate of 0.4 cfm/sf so projects can now use this as the baseline to demonstrate infiltration savings. The proposed should be based on post-occupancy testing.