Re: [EnergyPlus_Support] Green Roof

[Chris Jones <cj@xxxxxxxxx>]

You can estimate the effect by first simulating the roof with a standard 
insulation/roof finish.  Then increase the insulation by a large multiplier 
to see how much the total energy in the building changes.  The green roof 
will save some energy but not as much as a very high insulation amount.


At 12:45 21/05/2004, you wrote:
>I'm not sure that using either model makes a big difference.  As I 
>understand it, an EMPD model is only going to tell you how your interior 
>surfaces buffer interior air RH conditions.  An MTF model is attempting to 
>predict the effects of moisture transfer through an assembly.  MTF should 
>be better if the thermal changes in the system also stimulate 
>moisture/energy changes.  However, E+ has no reliable moisture quantities 
>and MTF assumes a wet absorbing exterior when it rains.  The calculations 
>could fail if the absorption is too great (see the reference about MTF 
>instability).  If you somehow adjust material properties to keep the model 
>calculating, you have no real basis for believing that you are accurately 
>simulating the moisture effects anyway.  MTF is also simulating moisture 
>flow to the inside air, and some portion of your roof had better be highly 
>resistant to moisture flow, or your roof could not really call it a roof....
>
>You generally may be better off to simply simulate the mass of the system 
>and use some shading to limit the solar exposure of the roof (that is the 
>resultant effect of the vegetation anyway).  You may be able to "schedule" 
>shading in a manor that more closely resembles the seasonal variations in 
>roof moisture and plant growth.  The problem still remains that you will 
>have to have a better understanding of the green roof thermodynamic 
>performance from other sources because you will not get it directly from 
>E+.  E+ can only tell you the impact of your roof shade assumption on the 
>building's energy use.
>
>Ned Lyon
>
>Senior Project Manager
>Simpson Gumpertz & Heger Inc.
>
>41 Seyon Street
>Building #1, Suite 500
>Waltham, MA  02453
>
>Phone:  781-907-9000
>Fax:  781-907-9009
>
>
> >>> fluizp@xxxxxxxxx 5/21/2004 11:45:48 AM >>>
>    About green roof simulation, we would like to make another two questions:
>1) If we use CTF or EMPD model, what aproximation would we achieve?
>2) What would be the best model to use?
>
>=====
>___________________________________________
>Engº Fábyo Luiz Pereira
>UFRGS - Laboratório de Vapor e Refrigeração
>www.mecanica.ufrgs.br/lafrig
>Rua Sarmento Leite, 425 - Sala 203 - Centro - 90050-170
>Fone: +55 51 3316-3167 - Fax: +55 51 3316-3355
>Porto Alegre - RS - BRASIL
>
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Chris Jones, P.Eng.
14 Oneida Avenue
Toronto, ON M5J2E3
Tel. 416 203-7465
Fax. 416 946-1005 



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