Hi
How do I model a naturally ventilated double skin facade in proposed and baseline building?
- proposed building: as designed, with a thermal zone for the intermediate area. The insulated exterior wall is located between the building interior and the intermediate area, the "double skin" is a lightweight uninsulated construction between intermediate area and outside.
- baseline: with or without the double skin?
For the baseline building, I have to limit the vertical fenestration area to 40 % of the gross above-grade wall area. If I model the baseline building with double skin, is this based on the outer shell or on the inner shell of the building? For me, the logical solution is the inner shell (this is the insulated shell), but our software support proposes to do it based on the outer shell.
I found an old post from 2013: "We faced this issue for a project in Italy which is now certified. Essentially we removed the outside of the double facade in both models and reported those results in the form. In this way we are not violating the modeling protocols for zoning or area. We then added the double facade to the proposed model and submitted the savings as an exceptional calculation (one reason to do an exceptional calculation is that in order to show savings you need to violate a modeling protocol)."
This sounds interesting, but it would be nice to have some confirmation of others who have done it like this.
Thanks!
Sarah
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 15, 2019 - 4:30 pm
I was the one who did that for the project in Italy. It was accepted. Using the outer shell makes no sense.
Sarah Leenknegt
Dr.Lemon Consult AG
4 thumbs up
March 25, 2019 - 9:18 am
It seems I misunderstood our software support. Their proposal:
- proposed building facade considered as 1 element (inner + outer shell)
- baseline building facade: inner + outer shell is replaced with the building element according to ASHRAE 90.1. The 40 % fenestration area limitation is based on the inner shell in the proposed building.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 25, 2019 - 2:01 pm
Here is the narrative we provided to the reviewer for the exceptional calculation we did for the double facade we modeled. Maybe your situation is different.
The double façade for this project is not typical. It is not connected vertically, but is isolated on each floor instead. The cavities formed by the double façade are unconditioned (using ASHRAE 90.1-2004 definition) and are automatically ventilated on each floor with exhaust fans which are activated based on temperature, with variable set points by season. The makeup air comes directly from outside. In addition, on the outside of the cavity interior glazing are slatted shades which automatically adjust the slat angle based on solar radiation sensors. The envelope thermal barrier to the occupied spaces is the interior wall of the cavity.
While the cavities are unconditioned, the exhaust fans necessitate modeling the double façade cavities as additional unconditioned spaces / thermal zones on the exterior of the building. These spaces are uninsulated, unoccupied, contain minimal lighting, and have no additional equipment loads beyond the exhaust fans. Therefore, we have modeled the double façade cavities using the Exceptional Calculation Method in order to demonstrate the savings attributed to the double façade since these spaces violate the thermal zone modeling protocols found in Table G3.1 Section 7.
In the Baseline building, the cavity spaces, shades, cavity lighting, and exhaust fans were eliminated, and the interior cavity wall (which is the actual envelope thermal barrier to the occupied spaces) was modeled as an exterior wall with the steel frame exterior wall construction in accordance with Table G3.1 Section 5(b) and Table 5.5-4. Since the Proposed building without the double façade has a window to wall ratio greater than 40%, the Baseline building glazing was reduced proportionally on each façade to the maximum 40%.
The Proposed building model used to fill out the Template also eliminated the cavity spaces, cavity lighting, and exhaust fans were eliminated. The remaining interior wall of the cavity was modeled as designed except using air films for exposure to the outside, and the glazing was modeled as it was actually designed on this wall. The shades were modeled as mechanically controlled shades on the exterior of the glazing.
For the Exceptional Calculation Method, the Proposed building was re-run with the double façade cavities included. The double façade cavities were modeled as unconditioned spaces with the exhaust fans, cavity lighting, and the mechanical shades were modeled on the outside of the interior glazing and located within the cavity spaces as designed. The exhaust fans were modeled as they were designed using a temperature control manager within EnergyPlus with activation based on temperature and variable temperature set points by season.
Sarah Leenknegt
Dr.Lemon Consult AG
4 thumbs up
March 26, 2019 - 3:21 am
Hi Marcus
Thanks for the clarification. Your building does have some significant differences. Our double facade is naturally ventilated and has no lighting, equipment or fans inside the cavities. According to the definition in ASHRAE 90.1, the cavities are not considered "enclosed spaces". In the proposed building model, the cavities are zoned, for the purpose of correctly modelling the thermal effects of the facade. In the baseline, we will omit the double facade, as proposed by our software support.