Fire Stop Question
If you have a double-stud wall for extra thick insulation, is there a code requirement for vertical fire stops to prevent fire from spreading horizontally through the openings between the double-studs? If so, how far apart can the fire stops be?
When this question came up in regards to a tall wall that separated a living space from nothing but 15' of verticle open space under a roof and along one side of the house. we were required to block off the stud cavities to provide fire blocking between floors (vertically), but not horizontally withing that space. In our case that amounted to nothing more than adding plywood to the exposed side of the 2nd floor stud cavities.
The situation you mention is also common when liveable spaces are built in what would normally be attic space - the side walls make a space between them and the rafters that is open and fire could spread horizontally - same for attics in general. I've never seen or heard of a double wall construction needing fire blocking, but I can imagine some places probably require it.
That detail shows a draft stop to make a vertical separation between the stud bays at 10' intervals. I am working on a double stud design where the studs will not be offset, so I guess if a code in my area requires this detail, I can just close off the opening between the pair of studs (that make up a double stud) with solid plywood. Thanks for posting that information.
Draft or fire stop?
If it is just a draft stop to reduce air movement within the insulation bats I would just use sray (canned) foam between the studs. If fire stopping is the goal I might still use a foam product formulated for fire stops (red colored stuff, ICR the correct name).
Sometime the purpose of blocking within a wall is not clearly understood. In ballon framing it is called, and is for, fire stopping. That term is also used in standard platform framing when it is in fact a structural requirement to keep tall studs in walls from bowing under loads. IIRC our building code in KY requires blocking in stud walls over 8' tall. So 9 and 10' walls that are becoming more common in new home need to having blocking. I don't have a new IRC book so some one who does can correct me here. Sheathing material also effects when blocking is used. Plywood or osb reduce the need for blocking on many assemblies, while the use of non structural foam panels as sheathing requires blocking.
I'll look in my 03 IRC when I get to my shop and see if I can futher muck the topic up:)