IES-VE User Forum

Hi,

I have a room. It's too hot and too bright. I go into the constructions database to modify the glazing and I add a louvre and set it to shade the window (eg I set the transmission factors for all angles to say 0.3).

I run apache .... my room is now much cooler ... great!

I run Radiance ... my room is exactly as bright as it was before!??

Ah, I say, that must be the transmissivity ... so I go back into the construction database and check the CIBSE derived parameters of the window ... lo and behold, it is still the same ... it doesn't matter what I do with louvres and blinds, the "Total shading coefficient" remains unchanged.

So, how can I get Radiance to understand that I have blocked out the sun most of the time (my solar gains are halved!).

Why won't Radiance make my windows darker?

I know how to change the window's transmissivity in Radiance ... so if I change the reflectivity of my glazing (eg using SunCool or similar) I can both cool the room and also darken it ... I just cannot for the life of me do the same thing by using louvres or blinds.

Can you help?

thanks

Norman

Come on man...

you're not still buying this integrated idea are you?

Radiance knows nothing about the window you set up in the thermal section... even though there's nothing to tell you this.
You have to set everything up each time for each module.

There's not even a hint of integration... unless you go in flucs which now changes your glass when you don't want it to.

Do they have focus groups?

Zap.

Hah! That's funny (and odd too).

Anyway, I think I have a solution.

There is a linear relationship between the solar gain and sky and ground diffuse transmission factors. The closer the latter are to 1, the higher the room's solar gain.

Ergo, I think I can use the window's Shading Coefficient and multiply it by the sky/ground TFs and use that as the Tramsmittance of the window in Radiance?

Is good?

thanks
Norm

Hi Norm,

We seem to be 2 of the few that keep dialogue going... I of course acknowledge the mighty JosephG who is, or should be, a God to the nameless and silent masses who leech of his knowledge.

I know this might be going back a step or two but have you tried just modelling some shading on the exterior facade?

Just realized we hadn't even mentioned physical modelling yet.


Zap.

yup, I know I can add some external shades in ModelIT but if I have a bunch of windows I;ll have lots of physical modelling to do if I want to vary the amount of shading to each to optimizethe model. It's a lot easier to use th econstructions database through which I can vary an external shade (eg brise soleil) as well as an internal shade (eg a blind) and the windows reflectivity. It's a bit more elegant.

not to mention that using a phyiscl shade as well as brise soleil (via the constr db) is odd to say the least.

ta
Norm

PS I can understand why IES is reluctant to mesh the thermal and daylighting databases. They're used in different ways. Radiance generally uses the CIE standard sky and lo is less sensitive to direct irradiation and hence to shading.

Hi,

I find it annoying that the Radiance documentation does not explain how to convert glazing transmittances from the construction database into a form that is usable as a surface property in Radiance. This is a fundatmental part of any building design or investigation and while I can understand that keeping the Apache and Radiance databases separate is the expedient thing to do it really sucks that the documentation doesn't mention anywhere how this is done!

Here's my take on this. ....
1. glazing in the construction database has a property called transmittance - or to be clear, a single pane of glass has this property.
2. The Radiance programme has a surface property called external glazing transmittance (from which the glazing transmissivity is derived).
3. In the construction database the glazing unit has another 3 properties called the short-wave shading coefficient, long-wave shading coefficient and the total shading coefficient.
4. The short-wave shading coefficient = glass transmittance (from the constr db) / 0.87
5. The other 2 shading coefficients follow from this (don't ask me how!)
6. Daylighting (ie Radiance) is affected by the short-wave shading coefficient only (ie visible light not IR)
7. So, if you have a glazing unit of say 2 panes of clear float 6mm glass (with transmittance of 0.780) with a 16mm argon cavity then you can go right ahead and do your thermal simulations until the cows come home ... but how does your glazing unit affect the daylighting.
8. Simple ... just find the short-wave shading coefficient (ie 0.7028) and multiply it by 0.87 to give 0.61 then go into the Radiance programme and plug in 0.61 as the glazing transmittance and you get 0.65 as the transmissivity

I'd like to know if this is correct ... anyone?

thanks

Norman