Subject ICF house under construction in Florida has a flat roof of 3" of poured concrete held up by Hambro metal joists whose top element is embedded in the concrete.

We'll spray 3-5" (haven't decide on precise thickness yet) of closed cell spray foam on the bottom of the concrete roof. That has an aged R-value of about 6 per inch.

However, I don't think I dare  feed the full R-6 per inch into my load software because I expect substantial thermal bridging via the steel joists. The upper element of the bar joist sits in poured concrete, which, though covered in a white poolside-type cool deck material, will still get quite hot on a summer afternoon.

About every linear foot or so a 1/2" tube of steel welded to the top chord will emerge through whatever thickness of sprayfoam I specify. There are a total of 400+ such penetrations, and intuition suggests that on a hot day they'll conduct heat enough to feel warm to the touch.

I need a number for that load, either a way to correctly derate the insulation or as an added sensible load on a design day.

I surfed then called Hambro - they expressed curiosity and asked that I call them back should I reach a conclusion, in other words the guy I talked to hadn't the foggiest notion, and no suggestions as to whom I might call.

Anyone here ever run into anything like this?

seems to me the only way to do it right is look at each particular situation as individual loads and add .
In other words the total area of pipe (OD) penatration = x
the total area of all metal ie. I beam linear times .75 or whatever = y
and the vast majority of sealed fully insulated ceiling =z
x plus y plus z= total load

seems like a long way to go but the only way that I can see to get the right number

Others might have an idea how to get you close enough with out going crazy and the insulation may be derated for timber bridging allready so that may have to be corrected

Using estimated cross-sectional area of the steel penetrating the foam, its conductivity and a guess at likely roof deck temperature I worked out an approximation that resulted in adding 1 btuh / SF on a design day.

Better than nothing or ignoring the issue completely, I hope.

Do what I do....ask an engineer.....
I'll ax around.
J

Thanks!

Is spraying  foam on the top of the roof a possibility? I know it is done for commercial flat roofs, with a waterproof elastomeric coating applied over top of the foam.

The family engineer provided some thoughts that might be helpful.
Perhaps Curt could give us some cliff notes (my eyes glaze over with engineer speak.
j

I'm not sure if this qualifies as "cliff notes", but the concept of calculating the "composite U-factor" for the constellation of diverse materials in a particular wall, ceiling or roof assembly was what came out of the "family engineer" spreadsheet you emailed me, and that was very helpful.

I have the choice of staying with the additional back-of-envelope calculated sensible load and assigning full credit to the installed insulation thickness, or calculating the composite U-factor, take its reciprocal for R-value, and plugging that into my load calc for the top floor ceilings. I'll run loads for both and pick the one I like best.

A more typical and perhaps easier to understand example might be a wall insulated to R-20 but whose assembly includes wooden studs that rate only R2, and the studs are 10% of the wall area. The net effect would be to reduce the whole wall to about R18. The studs act as thermal bridges, bypassing the insulation. If steel is substituted for wood, the bridging effect is greater.

Sorry, my eyes glazed over again, but I think you came up with a tangible R value.
Honestly Curt one of your challanges in the Engineer V Contractor world will be making sure you are "right +".
As a young contractor in the early 90's with fairly early and archaic load software I had 2 air conditioners that fell short of homeowner satisfaction and paid to upsize them. A very bright friend of mine pointed out that no one ever called and bitched that it was "too cold in here" in July. Curiously I've had 1 customer in 22 years complain about RH in the house.
Lesson; people are buying air conditioners from me to cool their homes and dehumidifiers from Kenmore.
Nothing costs a contractor more than an unhappy customer.
No one has preached "right size" more than me, but I have also recently commented that a little oversized only costs the customer (not the contractor) a little extra once.
Only another Engineer will appreciate a system that gets the job done most of the time. As a contractor you'll want to make sure to deliver all of the time.
When in doubt "right +".
J

I generally agree with all you wrote. The house in question is looking to come in around 3.5 tons but it'll have 2 x 2.25 ton units. My own came in at 30k btuh but I have an 038 - I didn't dare risk an 026. My 038 has been locked out of Y2 almost from day one.

RH is a bigger deal down here owing to the frequent condition (continuously for 3 months) wherein it is 75 deg and 100% RH outside from midnight until sunrise, and the occupant response is often to drop the t-stat setting so as to create sensible load to combat the latent load. Add in some hacked ductwork and iced evaps and moldy walls easily result. In a notorious sick building case the Polk County courthouse constructed for $20 million had to mold remediated for nearly that same sum...at least the building provided a convenient venue for all the resulting litigation.

Engineers are generally guilty of oversizing nearly everything because no one much notices when we oversize but we get in terrible trouble when something is just a bit undersized - Google Minneapolis I-35 bridge gusset plates for an example. Gusset plates are small, man-portable components, but those used were 1/2" thick and failed, contributing to the catastrophe.

I will consciously oversize to provide a margin of comfort, but I want to do so in a controlled, educated manner. I want to be 20% oversized, not 50%.

The value in determining that the trusses leak 1500 btuh into the top floors does not lie in giving me license to shave a few CFM or fractional tonnage, but rather in the due diligence peace of mind in determining that I haven't overlooked a 15,000 btuh load.

Bin data reports are helpful in calculating dehumidification.
Look at % of hours equipment runs, it is easy to see if you will adequately dehumidify.
j