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Community and Q&A

Insulating and Fireblocking when finishing a basement

leeken | Posted in GBA Pro Help on

I live in northern Iowa (Climate Zone 6) and am finshing my basememt. The basement walls are 8″ concrete with Owens Corning Fold Form insulated concrete forms that have 2″ rigid foam insulation both outside and inside of the concrete wall.  I have insulated the rim joist areas as well as the area above the sill plate on the ends of the house where the floor joists are parallel to the wall, with 4″ of rigid foam sealed with great stuff spray foam. I am installing 2 X 4 stud walls with 1/2″ sheetrock inside the rigid foam insulated walls. I plan to add 1/2″ sheetrock above the space between the 2 X 4 stud wall and the sill plate on top of the concrete wall. I had already built the stud wall along one of the walls where the floor joists are perpendicular to the wall. Therefore it will be necessary to place individual pieces of the 1/2″  sheetrock between the floor joists for each bay of the floor joists. For the other wall where the floor joists are perpendicular to the wall I will be able to place the 1/2″ sheetrock by fastening strips of sheetrock wide enough to bridge between the inside face of the 2 X 4 stud wall and the inside face of the sill plate that sits on top of the basement wall. The strips will be fastened to the bottom of the floor joists before the 2 X 4 stud wall is placed below the strips of sheetrock. For the walls where the floor joists are parallel to the wall, I will be fastening strips of treated wood that sit on top of polyethylene sill sealer on the concrete wall to the inside face of the sill plate so they are 1/2″ lower than the top of the sill plate and then bridging the 1/2″ sheetrock from the top of that strip to the bottom of the nearest floor joist. The 2 X 4 stud wall will be placed under the the 1/2″ sheetrock. The GBA blog on Three Ways to Insulate a Basement Wall shows a detail that has rigid foam insulation on top of the concrete wall between the sill plate and the stud wall. In my case there will be an air void as I will not have rigid foam in this space. I plan to use caulk to seal the intersection of the bottom of the sill plate/sill sealer with the top of the concrete wall. The sill plate sits on a polyethylene sill sealer. I plan to use 8 ft.  fiberglass batts every 10 ft. along the length of the exterior walls to fireblock the space between the 2″ rigid foam insulation and the 2 X 4 stud of the stud wall. I will also fireblock the utility soffits using fiberglass batts to surround the utilities every 10 ft.
The area I live in does not have codes requiring building inspections other than for electrical work. Therefore I do not have any local codes to refer to or local government building inspectors to answer questions about codes. Based on the above lengthy explanation of what I’m doing is it appropriate in terms of insulating and fireblocking? Additionally I have some specific questions. 
1. The 2″ rigid foam insulation on the exterior walls is flammable. Does something additional need to be done to protect that from fire?
2. Inside one of the utility soffits there is an iron pipe gas line. I have been told that if a gas line is in an enclosed space it has to be continuous with no joints. The gas pipe has a couple of threaded elbow joints where it comes into the house and further along it also has a threaded union. My plumber tells me that even though the threaded elbows and union are within the soffit, as long as the soffit is open to the utility room further down the line, it is OK because the gas would be able to migrate to the utilty room where you would be able to smell it. However if the soffit is fireblocked every 10 ft., the gas would remain confined and not able to migrate to the opening into the utility room. My thought is to place a cold air return grate in the soffit within the 10 ft. fireblocking areas where there are joints in the gas line that would allow the gas to migrate into those rooms. Would that be acceptable with regard to the codes or rules regarding gas lines?
3. My basement has a wall that divides the basement with 16 ft. width on one side of the basement and 12 ft. width on the other side. Once the basement is finshed with a sheetrock ceiling, it will be left with  28 ft. long closed spaces between floor joists. I have viewed videos on you tube regarding fireblocking but none of them talked about fireblocking every 10ft. between the floor joists. Is it the intent of codes that there should be fireblocking every 10 ft in the enclosed spaces between floor joists? If so, I presume that can be done by inserting fiberglass batts every 10 ft. between the joists. The videos do show how vertical holes for wires and pipes that go up to the first floor should be sealed with fireblocking foam or caulk, but do not show anything about sealing horizontal holes for wires and pipes that go through the floor joists. Do they need to be sealed with fireblock foam or caulk also?

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  1. Expert Member
    NICK KEENAN | | #1

    First, it sounds like you've done a great job with your insulation.

    Fireblocking is needed in concealed spaces, but there is disagreement whether an area filled with foam board is a "space" or not. If you feel that it is, then fireblocking at the top of the wall should go from the edge of the mudsill to the interior edge of the top plate of the wall. If you feel it isn't, it sounds like the foam fills the space behind the studs and the top plate fireblocks the stud cavities.

    Fireblocking should be 1/2" drywall or 5/8" or thicker wood.

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