Decision on Infiltration Losses 1

[Drazen]

I would like to open discussion about current practice of everyone who calculates thermal loads these days.

Infiltration was allways hard to define, and the most honest approch is desribed in some ASHRAE papers, where all figures given by norms were declared as a guide only, and discretion of experienced designer is considered the best way to deal with that.

Most of my concerns come from new, tighter building envelope issues.

Before, I was able to make informed guess based on expected level of contractor's performance, luxury level of building, and climatic zone (some zones are sensibly more windy in my country).

New carpentry changed tightness values so much, and right now it is hardest for me to define how much of that will be actually achieved during construction, as more stringent requirements impose more considerations about contractors's performance and difference is sensible.

How to express is in simpler words? Right now, with such low values of infiltration, contractor's performance can decide whether the system will acheive its function or not. To reach adequate safety, we need to apply some allowances which automatically annulate savings imposed by "theoretical" values of new standards.

How do you approach to this problem?

 

[ChrisConley]

In our climate infiltration represents the one of the largest heating loads we have to design for.

We have a young engineer who loves modelling everything in the building down to the last Btu (to three decimal places) and when I pointed out that our infiltration numbers was a guess based on experience he was indignant.

We've been using a 0.1 cfm/ft2 of exposed wall on new construction fairly safely for the last number of years. I reduce this number for a wall with no penetrations, and increase for operable windows and fire doors.

I haven't seen a good infiltration test spec from an architect ever.

 

[mauricestoker]

The NIBS 3-2006 might be helpful. As for the architects, I have a bad enough time trying to get them to understand the relation between how well the envelope is sealed and how the HVAC and utilities bills will follow.

ASTM E799-97, the blower door test, could be used for verifying whether the guess made from the drawings and ASHRAE HOF are accurate. Debating on whether to foist that on the A-E, commissioning agent. NIBS 3-2006 discusses this issue, as well as water/moisture.

 

[Drazen]

Exactly, Chris, actually I come to 60:40 or even 50:50 ratio often, which only puts more emphasis of accuracy senselessness, not to mention waste of time with thermal bridges calculation, except in very particular cases.

I use much radiant heating and cooling recently and infiltration challenges all achievements gained through thicker wall insulation/better glazing.

My guess is mostly related to ach, and I never had chance to attend some useful testings...

Maurice, the main issue of using standards is that it will not help me once building is finished. If I used brave figures and my radiant heating will not suffice, I can only escape to middle Africa - I never saw building is not commissioned for insuffient evelope tightness, but enraged tennants are very sensible creatures. And if I add too much allowances, radiant heating sometimes does not suffice (on paper).

 

[mauricestoker]

Drazen,

The standards are needed becuase there needs to be a performance criterion for accountability. I beleive that is the reason for the NIBS guideline, and why commissioning is performed. It might not be applicable in general, but when infiltration or exfiltration is a critical issue, a performance and testing criterion needs to be included. I've had a job where soap bubble testing of enclosure at 4 IWG was required, and until met the contractor bled time and money reinforcing and repairing any deficiencies (for a biological weapons receiver lab). It was not inexpensive, but it met standard.

 

[ChrisConley]

We had a project were the tenants were cold (obviously not enough heating). Our young engineer checked his calculations a dozen times and couldn't figure out the problem.

We went out to site with a thermal camera and noticed the ceiling tiles were cold. Popped a ceiling tile and was able to see daylight through the wall (not a little, full connection to the outdoors). Now that's infiltration! The ceiling space was nearly outdoor temp.

Contractor came back to fix the walls, was mad at us because if we would have just specified more heat no one would have noticed...

Agree though, if you carry a huge load in infiltration (and the building is built well) the Owner ends up buying a lot of radiant heating and cooling panels that never come on.

We generally pressurize buildings (contrary to building science recommendations, but acceptable in very dry climates like ours) and this tends to further reduce infiltration.

 

[sspeare]

I have somrthing to contribute, but only for residential construction. Yes, today's dwelling units (apartments & single-family houses) are very tight. We design them to be 10% better than the requirements of the International Energy Conservation Code. We have a "blower door" test performed on each dwelling unit. The "blower door" test gives you an infiltration number. Typically the infiltration number from the blower door test is in the range of 0.40 to 0.50 Air Changes per Hour (ACH). Some people claim to build units that get as low as 0.28 ACH, but I have never seem that happen.

So, you take the ACH, figure out your CFM infiltration, and calculate the BTU/hour loss for winter & summer conditions from the fundamental psychrometric relationships.

 

[Drazen]

Thank you for many useful insights.

Actually if I would summarize it all and apply to my actual circumstances, I would say one crucial thing would be to have building tightness test as regular part of commissioning (which is not the case in my area), and that would give me more courage to apply values that befit new "energy efficient" standards.

Right now, I have to balance between rage of contractor (as in Chris' case), rage of tennant or allowances after which I cannot present my HVAC systems as modern and energy efficient.

Whole this story is about residential applications, all kinds of commercial applications have everything much better defined and applied...

 

[KiwiMace]

NIST, particularly a guy by the name of Andrew Persily, have been churning out and refreshing blower door data on manufactured/residential and commercial buildings for yonks, and finding that nothing much has changed.

Also over at LBNL, Sherman and Matson have been efficiently paralleling the work. You can read in the linked report that nothing much had changed.

http://eetd.lbl.gov/IED/pdf/LBNL-48671.pdf

So, if you were expecting housing or buildings on average to have clammed up over the last 10-15 years, you might be disappointed. Not to say that the buildings in your area haven't though.