Hello all,
I am interested in modeling the energy savings that occurs when windows are replaced with lower-infiltration alternatives. In the wizard mode, the only place that I see infiltration is under the "Building Envelope Constructions" Screen.
I'm hoping someone can confirm my methodology:
Can I input a weighted average of window and wall air infiltration here? If I'm looking at a building with 20% Window to Wall Ratio, and the windows have 0.1 CFM/ft? while the walls have 0.04 CFM/ft?can I apply an area weighted average into this box? I would take (20%*0.1+80%*0.04) as the fa?ade infiltration value.
That way I could use the same formula to determine the area weighted infiltration of the fa?ade if better windows were installed.
How do other people model this?
Thanks in advance!
Alex Krickx
I've done exactly that in the past for the exactly the same purpose!
The only difference is I believe I always punch in the differences in the metric of ACH. ASHRAE fundamentals provides some guidance regarding air change rates to expect for various classes of construction as a function of outdoor design temperature, and that was the best I could base my existing construction off of from what info I had available.
Come up with a "leakiness" for both you construction, old and new windows, and do the weighted average jig =).
Kudos on your fancy facade spelling also ;)!
~Nick
NICK CATON, E.I.T.
Hi Nick,
Do you have a reference for the ASHRAE ACH estiamtes you mentioned?
thanks!
Steve
Sure!
Table 7, Chapter 28, 2001 ASHRAE Handbook-Fundamentals.
NICK CATON, E.I.T.
A good rule-of-thumb assumption for any new construction is ACH=0.20. I
believe this is based on an ASHRAE reference. For existing building
construction it can be slightly higher than that up to significantly higher
depending on the construction type and age of the building.
Generally for new construction 0.20 is a good solid assumption.
pk
I tried to fire back a reference for you yesterday but it seems to have been held up a bit...
Here's where you can find some ACH as a function of airtightness of construction/temperature:
Table 7, Chapter 28, 2001 ASHRAE Handbook-Fundamentals
~Nick
NICK CATON, E.I.T.
Hi all,
Thanks to Pasha and Nick for their input. It's always good to know that someone else is doing the same thing! Most of the data I have on windows leakage rates is in CFM/ft? and as I don't have information on most of the wall infiltration values on projects I work on (I usually do feasibility studies on existing buildings) I just leave it at the eQUEST default (usually 0.038). Does that sound like an acceptable way of doing it?
Thanks,
Alex
Whether deciding to stipulate/leave-at-default infiltration/exfiltration rates in your energy models is "acceptable" is a subjective question - I'd venture if you're specifically trying to model the airtightness of the new construction/windows against the existing envelope, it follows you should come up with a figure that somehow accounts for what's really out there, even if it's only subjective and without complicated measurements.
To be more descriptive:
Chapter 28, Table 7 and Table 8 of ASHRAE Fundamentals 2001 give you air exchange rates (ACH) as a function of tightness of construction for a range of Summer and Winter outdoor design temperatures. Each table assumes a certain indoor temperature and outdoor wind speed. The three categories of tightness are "Loose," "Medium" and "Tight." Those categories are described further in the preceding text. The ACH rates vary quite a bit as you go from warm to cold design temperatures and from loose to tight constructions.
It's worth noting that (1) these tables are in the "Residential cooling and heating load calculations" chapter, and (2) this is the 2001 Fundamentals. These tables are compiled using raw data from single/multifamily housing structure constructions. The descriptions of each class of airtightness will help to assign to an existing building depending on just how leaky it is. I checked out the corresponding sections in the 2009 Fundamentals, and the Residential chapter currently recommends going through a rather involved string of equations to arrive at your ACH rate. The following Nonresidential chapter doesn't give much more guidance than stating ACH rates aren't easy to measure as they depend on a bunch of variables, so not too much help there...
Unless there's something I'm missing that's more current, I think these 2001 tables are the easiest way I've found to subjectively assign an ACH rate appropriate to old existing buildings that accounts for winter/summer climate temperatures and just how subjectively leaky the construction appears to be.
~Nick
NICK CATON, E.I.T.