[Bldg-sim] Unusual Result

Keith Swartz kswartz at ecw.org
Fri Sep 24 15:03:41 PDT 2010


Jaigath,

Karen is right on. Lower window U-values and additional insulation in walls and roofs can cause a net energy increase in buildings with high internal heat gains. If you look at the month-by-month consumption, you may see that the "better" envelope helps out in winter and summer, but hurts in spring and fall. The overall net effect over the year is sometimes negative.

When the air outside is cooler than inside, then some of the internal heat can get out of the building through the envelope. A more insulated envelope slows the heat escaping through the envelope, so more of the heat has to get out of the building through the HVAC system, resulting in more cooling energy. If there is an economizer ("free cooling"), the effect is not as severe the net effect of more insulation could be positive.

________________________________
Keith Swartz, P.E., LEED AP

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From: Karen Walkerman [mailto:kwalkerman at gmail.com]
Sent: Friday, September 24, 2010 6:15 AM
To: Jaigath Chandraprakash
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Unusual Result

If most of your cooling loads come from internal loads as you said, then having windows with a higher U-value allows the building to lose heat to the outside any time it is cooler outside than inside.  If you look at ASHRAE 90.1, the requirements for windows changes dramatically as you move from colder to warmer zones.  In warmer zones, the allowed U-value is higher, but the SHGC is lower.

Also, it is usually good to verify that this is actually the case, often installed equipment runs only a fraction of the day, or at a fraction of the peak energy use.  If your internal loads are not actually as high as you are modeling, this could change things.

Lastly, does the building have an economizer?  This may make sense for this particular project.

--
Karen
On Fri, Sep 24, 2010 at 5:59 AM, Jaigath Chandraprakash <cjaigath at yahoo.com<mailto:cjaigath at yahoo.com>> wrote:
The project I am working on is an office building that operates 20 hrs per day with most of the loads occuring at night and low occupancy on daytime. It is a cooled by a DX roof unit and has a very high internal load. No heating equipment is required because the building is in Climate 1. I was surprised that when I changed my window U-value from 0.3 to 0.7 I got less cooling consumption. I always thought that getting low u-value will give me less coling consumption. Can somebody tell me why I am getting less cooling when I use a higher window U-value?

Thanks,

Jaigath


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