Prescriptive Path or PRM Path to be followed for recommendation of envelope U value/SHGC values during tender design stage

Hi Anurag,

For the EAp2 prereq in v4.1 you can demonstrate compliance with 90.1-2016 via prescriptive, ECB in Section 11 or PRM in Appendix G. Which of these paths is the design team planning to use for LEED?

If you're going perscriptive, then you'll need to be at least as good as the glazing tables for your climate zone in the envelope tables in Section 5.5 (if we ignore Envelope Tradeoffs in 5.6). 

If you're using a performance path, then the glazing in the model's baseline is from Table 5.5x for ECB or Table G3.4-x for PRM.

You won't want to tell the design team the values in Appendix G as those are for the 2004 stable baseline and you'll have to beat those by a decent margin, so don't send the team those as it will misled them. 

Jamy

I assume this is the same project you have been asking about in other forums? For a LEED project you should be doing early stage energy modeling to evaluate your options and inform those selections (you do not need to know all about the design in order to model it as that misses the whole point of energy modeling). Looking at window performance values in isolation will not give you very well informed answers regarding energy performance. You should be doing early stage energy modeling to evaluate bundles of interrelated strategies to optimze the whole building performance. What is the project's energy goal?

I generally agree with your approach. You don't have exact criteria and data. You can do sensitivity studies and identify ECMs per ASHRAE Std 209-2024.

But there is zero value in giving the design team the 90.1-2004 Appendix G values ...unless you want to include the BPF applicable to your building type and climate zone along with your targeted EAc2 points/savings. What good is it to tell them a 20-yr old code minimum when you're targeting net zero?

I don't agree with your approach as it includes some assumptions that in my experience are not accurate and actually harmful when designing a net positive energy project.

I will once again challenge the notion that you have to have floor plans, elevations and sections to create a sueful energy model. We use eQUEST but this is not exclusive to that software. There is a "wizard" mode where I can go through about 30 screens of inputs and create a model that is representative of what is intended to be designed. This can usually be done in less than 2 hours. It is likely that in the very beginning you know the building type, there is a building program so you know rough areas, you likely know how many floors it is likely to be, etc. You can use what you know to create these early stage models and use them to help guide design decision-making including building massing, orientation and configuration. If you are waiting for a mostly completed architectural design then it is too late to use the energy model to guide design decisions and you are left with the ablity to make only small incremental improvements. 

A net positive project requires a different approach if you want to do it in the most cost effective manner possible. For these types of projects we establish an EUI goal. For most exteranl load dominant commercial buildings we often start with around 15 kBTU/sf-yr. We apply green building design strategies like proper orientation, exemplary daylighting, massing and shading strategies, etc., in other words passive design strategies. You must impact the building design, not just the systems design within it. Once you implement strategies to reduce the building's loads you then select and optimize the HVAC system. We create a miximum potential model in the very beginning that significantly exceeds code minimum values. We start from there and then iteratively work our way back as necessary. If you start with code values and try to work your way up you will never get to a fully optimized approach in my experience. If you don't account for the impact of the building design on the loads you won't get there either. 

I don't believe you've mentioned what type of building you're working on, its solar access, the number of floors and what ASHRAE 169 climate zone it's in. Here are 3 free prescriptive net zero guides which might be useful depending on your project:
https://www.ashrae.org/technical-resources/aedgs/zero-energy-aedg-free-d...

Also worth a read is Charles Eley's 2017 ASHRAE Journal article on net zero potential:
https://www.eley.com/sites/default/files/pdfs/ASHRAE_Journal_July_2017_[...

You mentioned the project is: targetting for a LEED Platinum & Net Zero by 2030.
I'm going to assume since you're asking a NC v4.1 question you're using LEED v4 or v4.1 BD+C for that Platinum target, but I have zero idea which net zero definition or certification you're using and if it's energy or carbon. LEED Zero includes options for energy or carbon for existing buildings--not new--and includes user transportation and operational emissions for a single year but omits embodied carbon. ILFI has different boundary conditions in its energy or carbon certifications. While I have luke warm support for many of these systems or ratings which wholly ignore entire GHG emissions portions, I would encourage your team to review ASHRAE Standard 240P--either the draft version or hopefully the final version when it's released later this year for a Whole Life Carbon approach to zero carbon.

Sounds like you understand. To design a net positive project cost-effectively it needs to be super energy efficient. Most energy efficiency strategies are still cheaper than solar panels. So you need to reduce energy use dramatically. Every kWh saved is one less that you have to produce, so the solar system gets smaller and cheaper. When evaluating the economics you have to take this into account.

There are sources for US based projects but I am not sure about availability in other parts of the world. In the US we access the CBECS data from the US Department of Energy and this data is accessible through the US EPA's Target Finder tool. This data does not really apply outside the US. You can also obtain this information from existing building projects, maybe others you have worked on and calculate the EUI to get an idea of where your projects stack up.