CO2 sensors and outside air reduction. LEED forschools & ASHRAE 90.1-2004

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Previous posts have confirmed my suspicions that increasing the R-value of
walls and roofs has little to no effect on energy performance, due to the
outside air requirements.

This raises a few questions:

1. What is the most effective way to use CO2 sensors to dramatically
reduce the outside air requirements?

2. Is it permissible to vary the CFM between the base case and design
case in the ASHRAE 90.1-2004 Appendix G model, and consequently in the LEED

3. What is the most effective way to use CO2 monitoring in a building
with 40 classrooms and other large spaces. If one sensor is triggered,
won't the entire zone get fresh air and defeat the energy savings?

4. If every classroom has a sensor, does any one have experience with
the replaceable / Mail in sensors to assure their accuracy.

5. If I model this in Equest, do I need a zone for every classroom.

Thank you for any feed back.

James F. Geers

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Joined: 2011-09-30
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I disagree with the claim that R-value has little effect. Look at the number of all-glass buildings in northern climate, and I'll bet they barely meet LEED or ASHRAE energy pre-reqs.

However, there is definitely a law of diminishing return at work. That is, after a certain R-value, additional insulation has very little effect. The curve of R-value vs. energy savings flatlines. In our case in Toronto, this happens beyond R-20 walls and R-30 roofs typically.

The other thing to keep in mind is the type of building. Offices are very much dependant on skin losses, but hospitals for example are dominated by ventilation loads and envelope losses are almost insignificant by comparison.

As for your question on CO2 control, our experince has shown that you need a very fine resulution to get any benefit. That is for a mixed-air system, you typically need one sensor per box/damper, so that you're not over-ventilating the rest as you pointed out. But mixed-air system are bad anyway so CO2 control is a marginal improvement. You get the most bang if you de-couple the ventilation from the heating/cooling by using compartmental system such as fan-coils or distributed heat-pumps. Why these systems aren't more popular in our market is beyond me...

Luka Matutinovic, B.A.Sc., LEED AP

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Joined: 2011-09-30
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Dear Bldg-Sim folk,

The insulation issue is one where more is NOT necessarily better, as is
indicated in Mr. Spielvogel's attachment. For a "standard" building type
such as an office, it won't be too hard to assign an average density of
lighting and plug loads, use the favored HVAC system, and then tinker with
the "R" value to see where "optimum" is found. Climate does make a
difference, as is also indicated in the article.

LEED, for the record, seems to give this interaction between envelope and
internal loads its due because the required energy analysis will account
for these factors. To be rigorous, though, you need to use a realistic
level of lighting and plug loads, NOT the connected load!

Another factor which I think ASHRAE 90.1 includes is the bridging effect of
many wall constructions. Wood studs in residential construction, for
example, consume 15%-20% or more of the wall area and have very little
insulation value, degrading the overall average insulation value. Steel
studs do the same thing in commercial construction.

James V. Dirkes II, P.E., LEED AP

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Joined: 2011-10-02
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