Frame Cavity Slightly Ventilated NFRC 100 - "open to the exterior" clarification

[Zack Boshart]

Hello,

I have a question regarding the use of Frame Cavity Slightly Ventilated NFRC 100.

In the THERM simulation manual, it states that for cavities open to the exterior with an opening between 2mm and 10mm we are to fill that void with the "Frame Cavity Slightly Ventilated NFRC 100"

When the manual says exterior, is it referring specifically to the exterior boundary, or just exterior of the modeled item (in my case curtainwall)

The example from the manual only shows this being used for the exterior boundary areas such as under the sash of a casement window.
In my case, for a curtain wall section let's say there is a 4mm gap between an insulated spandrel infill, and my mullion framing on the interior side of the model.
Is this space left to the interior boundary condition or should it be Frame Cavity Slightly Ventilated NFRC 100.

Please see attached for snip from the manual as well as my specific case in question.

Thanks

[yal...@gmail.com]

Zack,

ISO 15099 6.7.1 which NFRC simulations are based on says sightly vented cavities are applicable to both interior and exterior cavities.

However the gap being narrow and deep and with the gray radiation calculation on the inside it is unlikely this gap will make much of a difference either way.

Yalin

[itsmeryguy]

6.7 Ventilated air cavities and grooves 

6.7.1 Slightly ventilated cavities and grooves with small cross section Exposed grooves with small cross sections (see Figure 14) or cavities connected to the external or internal environments by a slit greater than 2 mm but not exceeding 10 mm are to be considered as slightly ventilated air cavities. The equivalent conductivity is twice that of an unventilated air cavity of the same size in accordance with 6.6. For cases where the slit is less than or equal to 2 mm, treat the cavity as completely enclosed in accordance with 6.6. 

[Shaun Martin]

I was recently reading some information from ABTG research, that measured 2-3 ACH for an air space behind exterior cladding where bug screens are installed top and bottom (a common rainscreen construction in the Pacific northwest). Does anybody know if 'Frame Cavity Slightly Ventilated NFRC 100' is a good representation of this condition?  If the "equivalent conductivity is twice that of an unventilated air cavity", I would think this conductivity would be too low. 

Note, where screens are not installed the ACH rate is so high that the R-value probably approaches/or is zero.

Thanks

Shaun