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RE: [EnergyPlus_Support] Use of radiant barrier systems in walls/roofs
To answer your original questions about how EnergyPlus models things . . .
> > Can EnergyPlus correctly model the effect of the radiant barrier on
> > the roof heat transfer? I dug around the documentation quite a bit
> > and couldn't find the answer. For now, I have created a construction
> > containing: 1. Steel roof (Material:Regular) 2. Air gap
> > (Material:Air) 3. Radiant barrier (Material:Regular-R)
> >
> > Will radiation cross the air gap, or is the air gap just acting as a
> > thermal resistance (conductivity only)? If there is a better way to
> > do this, I would appreciate any pointers!
>
Material:Air is just a fixed thermal resistance, radiation does not cross this
gap.
The best way to model this would be to make the air gap in the roof a separate
thermal zone. Then radiant heat transfer from the roof to the radiant barrier
layer will be taken into account. The air temperature in the gap will be
uniform, but that is probably not a bad approximation for a thin gap. If the
radiant barrier material is not the same on both sides, you must make two
material layers so that the inside (facing the gap zone) thermal absorptance
can be different from the outside absorptance (facing the interior zone). The
detailed interior convection model will model the natural convection within the
gap but it will model each surface as an open flat plate with no interactions
due to the gap being narrow. If the gap is vented, then you would need to add
the COMIS airflow model to simulate natural convection through the vents.
The question remains, though - is this a good model? Maybe, maybe not. I
can't say for sure, because I have not seen any studies comparing this type of
model with actual data. So, scrutinize the results and do some sentivities to
assure yourself the results are reasonable.
Mike
On 16 Mar 2005, at 20:54, Scott and Roxanne Munns wrote:
>
> Agas,
>
> Thank you for your feedback. I believe we are also considering painting the
> roof white, in addition to a possible radiant barrier. The data I had suggested
> that a white painted roof would still have an absorptivity of 0.2, which I used
> in my model. The aluminum foil in the radiant barrier is 0.03-0.05.
>
> I will pass on your comments about how to best paint the roof to our project
> manager.
>
> Thanks again,
> Scott
> -----Original Message-----
> From: Agas [mailto:agas@xxxxxxx]
> Sent: Wednesday, March 16, 2005 9:49 AM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx; munns@xxxxxxxxxxxx
> Cc: carl.bauer@xxxxxxx
> Subject: RE: [EnergyPlus_Support] Use of radiant barrier systems in
> walls/roofs
>
>
> Modelling exercises backed up by practical experience in Botswana have
> shown that the greatest benefit for least cost option to reduce heat gain
> through galvanised steel roof sheets is to paint the outside surface white. A
> good quality PVA paint will last quite a few years. If you paint the sheets
> after they have been exposed to weather for about a year, very little
> preparation is needed. Just wash the roof with detergent to remove dust and oil.
> A new galvanised roof will need more expensive preparation to get the paint to
> adhere. I have heard that washing with a thin cement wash does the job quite
> well.
>
>
> The next best intervention is a ceiling with insulation (with a reflective
> upper surface if available), and ventilated roof space.
>
>
> Regards,
>
>
> Agas
>
>
> >
> >
> > ________________________________
> >
> > From: Scott and Roxanne Munns [mailto:munns@xxxxxxxxxxxx]
> > Sent: Tue 3/15/2005 11:36 PM
> > To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> > Subject: [EnergyPlus_Support] Use of radiant barrier systems in
> > walls/roofs
> >
> >
> > Hello,
> >
> > I am doing analysis on a steel-roofed building in Guatemala. Typical
> > construction there is just a simple corrugated steel roof, with no
> > other insulation, etc. Ventilation only, no air conditioning.
> >
> > I am investigating whether using a radiant barrier (aluminum foil
> > facing on a 4mm thick plastic bubble sheet) is appropriate. Sources
> > have recommended a 25mm gap between the roof and the radiant barrier.
> >
> > Can EnergyPlus correctly model the effect of the radiant barrier on
> > the roof heat transfer? I dug around the documentation quite a bit
> > and couldn't find the answer. For now, I have created a construction
> > containing: 1. Steel roof (Material:Regular) 2. Air gap
> > (Material:Air) 3. Radiant barrier (Material:Regular-R)
> >
> > Will radiation cross the air gap, or is the air gap just acting as a
> > thermal resistance (conductivity only)? If there is a better way to
> > do this, I would appreciate any pointers!
> >
> > Thanks,
> > Scott
>
>
> [Non-text portions of this message have been removed]
>
>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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