Knowledgebase : Materials and Constructions
1. For radiant barriers which are exposed to a thermal zone, such as an attic space, specify a reduced thermal absorptance for the innermost material layer.

For example, an attic roof construction might be (outer to inner)

Asphalt shingles,
R-30 insulation
Radiant barrier;

The radiant barrier material would be a thin layer with some small resistance with a low thermal absorptance value. This will reduce the radiant heat transfer from the roof surface to other surfaces in the attic zone.


2. If the radiant barrier is within a cavity which is not modeled as a separate thermal zone, then there is not an easy way to model its impact. For example, a wall construction:

Brick,
R-12 insulation,
Radiant barrier,
Air gap,
Gypsum board;

Here, the radiant barrier would reduce the radiant transfer across the air gap. But EnergyPlus air gaps are a fixed thermal resistance, specified in the Material:Airgap object. The user would need to compute an average effective resistance which represents the reduced radiant heat transfer across the air gap due to the radiant barrier. This resistance could then be assigned to the radiant barrier material layer.
Emissivity is not the same as absorptance/absorptivity. Not even close. Emissivity is measured in Far IR range (thermal radiation), while asbortptivity is measured in Solar (UV, VIS, Near IR) range. Big difference, since solar radiation is at approximately 5600K, while thermal radiation is at approximately room temperature (plus/minus 40K).

BTW, this is a common misconception and very often these quantities are confused, leading some to believe that white painted surface has low emissivity, which is not true. Unless the paint is specifically designed to be low-emissivity (not easy), all paints have emissivity of 0.9 regardless of color or appearance. However, shiny white paint will have very low absorptivity, while dull black paint will have very high absorptivity. This behavior is utilized in the design of cool roofs, where low solar absorptivity is sought.

Since all surfaces in buildings are painted, it is safe to assume that emissivity is always 0.9, which is probably the reason that there is no field to enter it, however this should change as we see more and more specially designed low-e paints, even in window frames.

D. Charlie Curcija
DesignBuilder Software
16 Bridge St.
Millers Falls, MA 01349
In the Engineering Reference, there is a section named "Outside Surface Heat Balance"... which contains a subsection named "External Longwave Radiation":

External Longwave Radiation
q"LWR is a standard radiation exchange formulation between the surface, the sky, and the
ground. The radiation heat flux is calculated from the surface absorptivity, surface
temperature, sky and ground temperatures, and sky and ground view factors.
The longwave radiation heat exchange between surfaces is dependent on surface
temperatures, spatial relationships between surfaces and surroundings, and material
properties of the surfaces. The relevant material properties of the surface, emissivity  and
absorptivity , are complex functions of temperature, angle, and wavelength for each
participating surface. However, it is generally agreed that reasonable assumptions for
building loads calculations are (Chapman 1984; Lienhard 1981):

* each surface emits or reflects diffusely and is gray and opaque (absorptivity = emissivity, transmittance = 0, reflectivity = 1- emissivity)
* each surface is at a uniform temperature
* energy flux leaving a surface is evenly distributed across the surface,
* the medium within the enclosure is non-participating.

These assumptions are frequently used in all but the most critical engineering applications.
Q: I would like to know how E+ is calculating the emissivity of materials (not the glazing ones). I am guessing it is an indirect proceedure, as there is no specific input field for this subject.

A: In EnergyPlus input there are three different wavelength bands for absorptance – thermal, solar, and visible. Assuming that by "emissivity" you are referring to an average for longwave thermal radiation, then in EnergyPlus, emissivity is “calculated” to be the same as the value you enter in the field called Thermal Absorptance in the Material object. The material located at the outer or inner layer in the Construction provide the surface property data for all surfaces that use that Construction.

Question: would you tell me please in which document or book or website i can find material's characteristic such as u value, density, and conductivity

Answer:

There's an entire database of materials and constructions (from ASHRAE and other sources) in C:\EnergyPlusVX-0-0\DataSets (if you install in the default location for VX).

You can use IDF Editor to open and copy materials and constructions to your IDF. (Hint, the datasets should come up with only the input fields that are in the dataset. If not, hit the Ctrl and L buttons. It will toggle between all objects and only those with data.)

If other materials are needed, these references list many material properties.

- 2009 ASHRAE Handbook of Fundamentals
- DOE-2 software has a library of materials with characteristics
- Sustainable Home Refurbishment, David Thorpe, Earthscan expert issues (p 23) (mostly residential).
Question:

I am working with energy plus in sketchup on the quest for one model.

What I am trying to do is to trace the external geometry from a 3d daylighting model and to build my energy plus model and then apply thicknesses internally afterwards to account for the volume accurately.

I know I can apply the floor to ceiling height in energy plus (which will hopefully overule this issue in the Z plane), but I am struggling to find where I can apply the wall thickness without individually changing each co-ordinate.

Does anyone know if it is possible to assign a wall thickness specifically to go internally to ensure the internal volume will be accurate if I trace external walls?

Or do I have to trace internal walls. The issue with having to trace internal walls is that you are then left with gaps inbetween rooms.

Generally, how does energy plus account for wall thickness in the wall materials, does this just impact the thermal properties of the material, or does this also impact volume? If so, which direction will it impact the volume? Inside, outside, or half-half?

Thanks in advance.



Answer:

In EnergyPlus wall thickness only applies to heat transfer/thermal mass.

Geometrically, all walls in EnergyPlus are infinitesimally thin - i.e. they have no thickness. So, once each surface has been placed changing the material thickness will have no impact on zone volume, ceiling height, floor area, shading, or daylighting.

For most modern buildings the choice of inside vs outside vs centerline should have little impact on results so many modelers just pick one a let the volumes be off a little. Using centerlines throughout the model splits the difference. Or some modelers use outer edges for exterior walls and then use centerlines for interior walls.

If you are modeling a very thick wall, such as an old stone building, then you also have thermal mass considerations. If you use the outside edges there will be too much mass, inside will be too little. Again, centerline will split the difference and will be very close to the correct amount of thermal mass (possibly losing some corner mass).



Alternate Answer from Bldg-Sim group:

So if I understand this correctly you are starting with a thick wall model used for 3d daylighting. Tracing the exterior geometry is the correct approach for EnergyPlus modeling. The heat transfer surfaces should be at the exterior face of the wall vs. the interior. You are correct that the volume EnergyPlus calculates will change based on where you draw that boundary. Drawing the heat transfer surface at the interior face of wall, while creating more accurate volumes, introduces bigger issues such as changing the surface area of the heat transfer surfaces exposed to the outside air and ground.

Just as you can set the ceiling height in EnergyPlus, you can also set the zone volume. So if you use tools in SketchUp to help get this volume, you can manually enter it in your IDF. You will have to use an IDF or text editor for this vs. the SketchUp Plugin. Note that if you do this the volume is not dynamic. If you change your zone geometry EnergyPlus will still use the volume you set earlier. There is a warning when you run the simulation if the volumes differ drastically, but I’m not sure what that threshold is. Search for “field: volume” in the link below to learn more about the volume field.

http://apps1.eere.energy.gov/buildings/energyplus/pdfs/inputoutputreference.pdf

So, now on to the tools to use in SketchUp to determine accurate interior volume. If you are using SketchUp 7 one option you have is to make a copy of your zone floors outside of the EnergyPlus group and then use the offset too. The offset tool icon looks like two arcs with a red arrow. Click once near the edge of the tool, and then click again where you want your offset path to be. You can also type a distance after the initial click. If you select your new face you can right click and choose “entity info” to view the area of your new face. Multiply that times the height to get your volume. If you plan to do this on a model that is still changing I would locate these floor copies geometrically inside of the zone, but not physically in that group. You can also put the copies on a unique layer so that you can turn their visibility on and off. In SketchUp 8 you can use that same approach, or you can extrude the floor to a volume, and make it a group. Now you can right click to choose “entity info” and it will give you the volume of the group, assuming it is a clean manifold solid, airtight shape without extra surfaces or lines. Another option is to use a plugin called the “Joint Push Pull” plugin that among other things will perform a 3d offset. So if you have a 10 foot cube. You can offset all walls by 15 inches, and then get the volume of your new smaller cube (in SketchUp 8). Here is a link to the plugin, but you may have to register with the site to download the plugin and see the documentation.

http://forums.sketchucation.com/viewtopic.php?f=180&t=6708

Having shown you how you can use SketchUp to calculate the volume, and manually enter them into the IDF, I want to point out there are probably better places in the energy model for you to spend your effort rather than adjusting volumes. For example the effects of the internal mass in a zone would likely have a great impact on your energy analysis than minor adjustments to zone volumes.

I hope this helps, let me know if you have any additional questions.