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I have a 5-story residential multi-family stick-built structure on a concrete podium in the DC Metro area. The building is fully sprinklered including interstitial spaces. The pre-engineered roof trusses are 30" deep with 6" of R-30 rigid insulation above the top chord of the truss. Based on the image below, can the space be protected with a wet system or would a dry system be necessary? 
6 Comments
Franck
4/10/2020 10:31:37 am
Can you expect freezing temperatures during wintertime if the area below is unoccupied ?
Casey Milhorn
4/10/2020 10:40:12 am
Unless you want to stand behind the insulator, GC, etc... should not be your call to make. Write an RFI to the GC for them to forward on to the design team. You don't want to be the one doing heat loss/transfer calculations and making the call on this. If you are doing a design build and are the engineer of record, then you will want to have some type of backup to prove wet pipe won't freeze. Even if its commonsense that wet would be okay, I would be very careful sticking your neck out without backup.
Pete D.
4/10/2020 11:06:37 am
Maintained at 40F or above is the NFPA qualifier for wet systems. Since 40 is 8 degrees from freezing I've used a looser interpretation of this qualifier, that the return air would be 40 and the inflow would be some lesser temp say 38 by design.
sean
4/10/2020 01:52:51 pm
I would CYA and put in an RFI but in reality plan on a wet system because it is fully withing the thermal envelope
Mike
4/10/2020 05:41:05 pm
Good question and the above code answers are correct. If it were me, I'd send an RFI to the mechanical engineer since they're responsible for the building energy calculations which would include the building envelop design. CYS
Mike
4/10/2020 06:07:43 pm
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