I used the Federal Department of Energy’s REScheck software for demonstrating compliance with the building envelope requirements of our Wisconsin Uniform Dwelling Code.
Now I’m experimenting with Sefaira to evaluate energy usage vs REScheck’s numbers.
Sefaira seems focused on commercial construction with a flat roof.
Our home design has a simple gable roof. The attic space is unheated.
It’s not clear which attributes to assign to an uninsulated gable roof and uninsulated gable walls.
The 5:12 pitch gable roof has 2’ overhands all around.
It’s clad with standing seam metal over plywood sheathing on conventional wood trusses.
I understand the ceiling is the thermal boundary.
What’s not clear is which attribute to assign to the gable roof and gable walls.
I presume they should be considered ‘Shading’ elements. ??
Are you putting any mechanical equipment in the attic space? I did in my house and now regret that decision. These days I’m wishing I had insulated the roof rafters instead of the attic floor. The attic storage would be cleaner, warmer, and the mechanical equipment there would be inside the thermal envelope. Since you’re using trusses, I would guess you’re not counting on using that space for much.
With the ice dams on the roofs we have here in the Midwest, it makes no sense to insulate the rafters. Instead insulating the attic floor with 2 inches of spray foam and a foot or so of blown in insulation along with a ridge vent is the way to go. This keeps the roof deck nice and cool in the winter and helps with the AC in the summer.
I had a look at the model and it looks great. You are absolutely right that Sefaira does not model pitched roofs when it comes to Energy Analysis (but does translate the model as is for the daylight analysis). In essence, what happens in the translation process (the translation of the SketchUp model to an EnergyPlus model) the pitched roof gets translated as a flat roof with the roof at the top of the wall level. So what you have done is good.
However, what you want to keep in mind is that any surface that is tagged as Shading will be an “Adiabatic” surface, in that there will be no heat transfer through that surface. So the heat that would otherwise enter through the roof (in this case tagged as shading), will not be captured. In order to capture the complete essence of the Energy model, my recommendation would be to do the following.
I believe your comparison with the ResCheck model would benefit from uploading the SketchUp model to the Sefaira web application via the “upload to Sefaira” button.
If you have some time tomorrow (6/26), we can hop on a call together and discuss?
The attic is essentially a plenum for natural convection, eave vents to ridge vent.
There’s a 30" 2-Speed Whole House Fan in the hall ceiling that utilizes those vents as well.
Except for wiring and plumbing vents, the 30” fan is the only mechanical thing ‘associated’ with the attic.
Heating & Cooling is cast-in-slab hydronic, some 2,300 feet of ½” PEX-a
The heat source is a dual use Polaris condensing water heater, claimed to be 96% efficient.
Cooling is passive. It’s an ‘open’ hydronic system.
In cooling mode, the incoming domestic water supply flows through the slab first.
In summer, our city water comes in at ~60°F
An ERV provides ventilation when the house is buttoned up.
By design, the ERV and its ductwork are within the thermal envelope.
Building Science Presentation with Dr. Joseph Lstiburek
I have no love for pink fluffy stuff.
As a kid, my dad ‘enlisted’ my help in insulating the attic of our old farm house(s)
There were three old farm houses over the years.
Horsehair plaster dust isn’t a fond memory either.
The ceiling of this house is 12” SIPs… R-45 EPS core with OSB skin both sides.
The perimeter wall SIPs are set first. They’re 4-5/8" R-27 polyurethane core.
And then, the 8’ x 24’ SIP ceiling panels are set, supported by the exterior walls and temp. shoring.
The trusses are set on top of the ceiling panels, then thru-fastened to the trusses.
Thus, the trusses carry the attic floor live load and the SIP ceiling dead load.
The interior partitions are built last. By design, they’re considered non-loadbearing.
We’re using Fink trusses, which give a clear alley down the center.
There’s standing headroom … almost.
That’ll help with plumbing and wiring in the attic space during construction.
As for storage, it does have a nice flat OSB floor.
But if I deem it storage space and create a larger attic access door, I’d have to go with heavier trusses.
WI Code calls for 5-pound live ceiling load in non-storage attic space.
It calls for a 20-pound live ceiling load in “storage” attic space.
Wisconsin Code defines “attic storage space” as any attic space with a floor.
It’s a bit of a Building Code Catch-22.
By definition, the Code deems it attic storage space whereas by design, it’s a place to store air.
That the attic has a ‘floor’ is incidental.
Our city Building Inspector is OK with trusses designed for 5-pound live ceiling load, provided the attic access door is kept to the minimum size allowed by the Code.
Yes, I did that straight away.
With this being my first go all the way through with Sefaira, I was keen to see what came out.
You can blame Hari’s demo at Basecamp for getting me started with this.
I gather the SU-Sefaira plugin is where one sets the Entity Types … floor, wall, roof and so on.
Then, the Sefaira Web App is where one fine tunes the properties of those Entities.
If you’ll PM an email address, I’ll hit the Share button in the Sefaira Web App
Here’s a screenshot of what I have so far.
Lol. That’s great news @Geo. Hari is very well known for inspiring people to perform building performance analysis in Sefaira. I am therefore not surprised that you are now diving deeper into the analysis…
I will PM you my email and I look forward seeing what you have so far.
OMG what have I done!!! Hello Geo! It’s good to hear from you. And thanks Niraj for taking it this far. You should absolutely model the roof the way he says for the following reasons -
For insulation purposes, OSB is practically paper, so it’s almost as if there’s no thermal barrier between the house and the pitched roof.
SIPs hold their rated insulation values across the assembly, unlike the pink stuff.
I´ve come across this topic while modelling a school and havig the same doubt about how to handle the attic, as well as the gabled roof and attic floor. I´ve followed the recommendation 2 - tag the attic floor as “floor”, while keeping the flat roof above it, and then setting the attic as “unconditioned” space. The fact is that changing Roof u-value has effect only on attic´s temperature but no effect at all on room temperature. Why is it so? Doesn´t Seifara consider heat tranfer between the attic and the room beneath?
@niraj.poudel Just by dowloading hourly data (xlsx file) in Comfort tab! It shows zone´s temperature hour by hour. A simpler way is by comparing zone´s unmet hours under comfort tab. Changes in roof U-value affects attic´s percents, but nothing happens to zone underneath. I´m using Nat Vent only.
Ah okay. That makes sense to me now. Let me check how the energyplus model is set up for heat transfer between floors. Would you mind sharing the .idf file for the project in question with me here?
The model is pretty big, so I made up a simpler one, but with the same issue. The idf file extension was changed to txt, so that I could upload. Thanks in advance! (please consider “Attic Test - correct.skp”. The other file was missing the attic´s floor)
@Geo Hey there. I’ve seen you around a lot in the forums. I know this project is over for you but I figured I’d mention two things. Firstly, radiant infloor heating becomes less and less effective when you start dealing with energy efficient homes. Also, as I’m sure you know, SIP panels had a lot of problems in the past due to 3 dimensional air flow networks casuing thousands of homes to fail. I imagine this is a reminder for you, but really important that you understand how that works if you’re specing a run of the mill sip panel. Joe Lstiburek has great articles all about that. Second, although I do know a ton about the building science, I don’t know jack about the energy analysis stuff. I hire an energy advisor for that. My energy advisors dad actually started passive house canada, so I kind of lucked out meeting him and he’s also a builder. This guy has done more good for me than any program will ever do. I would highly suggest finding a good energy advisor if you don’t already have one. Unless you have a foundation in energy analysis and understand the science of it, the right energy advisor will no doubt save you more time (make you more money) and give you better results on your projects. Not saying hire an energy advisor for everything, but for any new home project, or energy efficient remodel, certainly worth it in most cases. Nice to see some building science stuff and btw, you are definitely referencing THE legend of building science, good to see Joe Lstibureks name being mentioned here. He’s the man.
P.s., are you familiar with Deisgn PH or the PHPP programs, design ph is a sketchup plugin for energy efficient home analysis and it’s coupled with PHPP…
Yea @db11 has got it. Design ph is meant to be used with the PHPP program for the most part. For this, I would again reference my comments I made to geo above about energy analysis knowledge, I would say the same thing applies to use of design ph and PHPP. If you have any questions about those programs RT, I took my passive house consultant course awhile back implementing design ph and PHPP, I’d be happy to help, just fire me a message if you’re looking to save time.
