UL Listed, 1-Hour Fire Rated Double Stud Wall
cody_fischer
| Posted in Building Code Questions on
I am building a low embodied carbon, passive house multifamily building in Minneapolis. We just determined that our wall assembly must be 1-hour fire rated due to proximity to the lot line.
Our preferred assembly is a double stud wall insulated with dense pack cellulose (for embodied carbon reasons). There does not appear to be a multitude of UL-listed double stud assemblies to draw on.
Any suggestions? Know anyone that’s done it?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Does your assembly need a UL listing, or are there acceptable solutions within the code you can use?
I don't know and am not the best at interpreting code. My architect just indicated that we had to have a 1-hour rated assembly. Are there code provisions in the IBC that you suggest I have them research for a potential solution?
Im not sure, I do not use the IBC. Maybe Malcolm or Armando could chime in?
If you have an architect then they should be spec'ing anything of this nature, they will have access to to many different sources of information for designing wall assemblies. That is one of the jobs you are paying him/her for.
plumb_bob,
I poke around in the IBC and IRC out of curiosity when questions arise here on GBA, but I'm under the BCBC like you.
Cody,
I suspect you will also need a rated assembly for any party walls, and floor/ceilings between units. The fire rating for all these situations will be pretty easy to achieve. What you need to look out for is an assembly that also has the required sound attenuation necessary to meet your code. Our has a very useful table that includes both ratings for a variety of walls that you may find helpful.
https://free.bcpublications.ca/civix/document/id/public/bcbc2018/bcbc_2018dbp9rt
All these assemblies are the responsibility of your architect to specify on your construction drawings.
The usual "1 hour wall" is a layer of 5/8" type X drywall on either side of a regular studwall. It's the drywall doing all the work in terms of fire resistance. I'm pretty sure I have seen some double stud assemblies in the book too, but I've never build a fire wall that way so I'd have to double check. Technically, to be a UL listed wall, you have to build EXACTLY what is shown in one of the listed assemblies. this means if the stud spacing was 17-5/8", you couldn't change it to 16" and still have the listing -- any changes and you'd no longer be building a listed assembly.
For exterior walls I've always used masonry, usually boring ol' concrete block (commercial buildings don't care as much about aesthetics :-). For anything with siding, your architect should be able to specify an assembly that will work.
Bill
Exterior walls are rated from the inside, so either one or 2 layers of 5/8 type x will certainly get you there. I would guess that you will also require noncombustible cladding. Insulation can also add to the rating.
Bill is correct about the details for UL listings- they are specific and they matter.
If you have a friendly conversation with your inspector they could probably give you a few options.
plumb-bob,
When you say exterior walls are "rated from the inside"...do you mean two layers of 5/8 type X just on the inside might be worth running by the inspector?
The hygrothermal modeling (WUFI passive) of our assembly indicates we need to be more vapor open than gyp on the exterior. Basically insul web with Tyvek stapled over the top and taped.
Thank you all for your input here. I'll be sharing the key takeaways with my architect.
I believe most of the major codes look at assemblies the following way:
Floors and roofs rated for fire exposure from below
Interior wallls rated for fire exposure form both sides
Exterior walls rated from the inside. Basically, we protect our neighbours from fire, and they protect us from their structure fires.
I would think the inspector would accept 2 layers 5/8 type x on the interior for an exterior wall rated for 1hr, and noncombustible cladding, but always best to ask! It would at least start a conversation, and if that proposal is not acceptable to the inspector maybe they will suggest modifications.
I think what you'll run into is most UL rated double stud walls are chase walls, not load bearing. You can try searching, maybe one of the insulation suppliers has tested a double stud assembly in this application.
I would also check your local code as typically on multi family the exposed building face needs to also be non-combustible construction.
One option is to include a UL listed wall inside your wall assembly. Basically have a layer of TypeX sheathing in the middle against one of the studs something like this:
-UL assembly
-gap with insulation
-interior stud wall
P.S. WUFI models on thick walls tend to overestimate moisture issues. People have been building them a long time and they work.