Calculating Infiltration Reliably
Hi All,
I'm an Architect who focuses on parametric design. I've been taking a lot of time learning the ins and outs of Energy Plus via Grasshopper/Honeybee. So far I feel like I have a good handle on things, but one thing is always sticking out as a problem for reliable EUI calcs.
How do y'all manage infiltration in your models? From my understanding, the infiltration really depends on the detailing and the skills of the contractor. In Washington State, the energy code requires a certain ACH target, which is fine, but the difference between, say 2 ACH/50pa and 3 ACH/50pa is like 10 to 20 EUI points in some models. I always find that tweaking the thermal performance of the building moves the EUI a few points up and down, but even small tweaks to the infiltration rate has a huge effect.
I am pushing for more blower door tests just to understand how our detailing and contractors work when it comes to infiltration, but we don't always have that option.
For what it is worth, this is only relevant as we are participating in the AIA DDX 2030 challenge and I'd like to feel good about the projected EUI's we are posting.
So how do you determine your infiltration rate in EnergyPlus? Is it with a baseline, with a code/certification target, or with historical data from past projects?
Sounds like 2 questions: (i) airtightness parameters (input) + (ii) E+ modelling. ACH@50Pa? I think of small-sized residential (vs cfm/ft2 @75Pa for larger buildings). Reliable characteristics are obtained from in situ testing. Otherwise, postulated (think 90.1). ACH@50Pa to E+? A nice overview here, or dive into RESSTOCK code.
Thanks, yeah, I'm more interested in how we determine what would be a good target for infiltration, especially since it makes such a huge impact on the final energy model. It sounds like you are recommending using 90.1 standards to create a target, which is great and I'm on board, and then back that up with testing during or post construction. It seems that any energy model where there is a target EUI really hinges on the infiltration control. Maybe I'm over-inflating its importance, but I'm getting the feeling it is the most important metric, but also the most difficult to predict.
I was simply pointing to 90.1 requirements as an example. I see more houses now < 1.0 ACH@50Pa. Achievable, yes. But there's a learning curve. As AIA DDX 2030 seems more geared towards (larger) commercial buildings, a more suitable metric would be cfm/ft2 (or L/s/m2) @75Pa. The following summaries may be helpful in picking a target: last page here, and introduction here.
And I don't think you're over-estimating the importance of airtightness, but its impact would depend on a number of building-specific conditions, e.g. mild vs cold climate, building height, single volume (houses) vs compartmentalized layout, internal- vs external-load dominant, mechanical ventilation de/pressurization. But yes, its impact can be quite significant. Neglecting airtightness in well-insulated, thermally-broken envelopes is like having the best Arc'teryx ski jacket/kit ... and zipping it halfway up.