Fireblocking with Roxul

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  #1  
Old 09-18-15, 04:28 PM
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Fireblocking with Roxul

I am insulating and fireblocking my steel framed basement myself. I am planning to use XPS rigid foam on the concrete, and I want to use Roxul (Roxul ComfortBatt 5-1/2 in. x 15-1/4 in. x 47 in. R-23 Fire Resistant Stone Wool Insulation (8-Roll)-RXCB551525 - The Home Depot) to insulate the above grade portion of the wall, as well as the rim joists and ceiling. Can I use this stuff for fireblocking? How would I attach it to the frame and wall? What else can I use if this isn't a good solution?
 
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  #2  
Old 09-18-15, 08:04 PM
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Can you describe where you need it for fire blocking? From the description you give you are planning on some thickness of foam over the entire wall and then framing a 2"x6" wood wall over that.

Mineral wool, which generally includes the rock wool as well as fiberglass material, serves as a fire blocking material. If you are planning on using the wood stud size you describe, you would be achieving an acceptable fire blocking material and the material would be "friction fit" into the cavity between the studs.

If you are only using it at the top of the wall then putting a piece of solid wood blocking in below it would be the best added support there is as well as providing an acceptable fire block material to separate the lower portion of the wall from the upper. This would also provide an air barrier to prevent vertical migration of cold air from the upper part of the cavity to the lower.

It can be used in the rim area as well but I think that using foam, either spayed in closed cell or rigid board cut to fit and then sealed at the edges with a high quality caulk or can foam would be best, especially if you have an osb rim board. You can add the roxul in front of the foam for fire protection.

Maybe a little more information would be helpful. I do believe that you may be getting a little overboard with the r-values you are intending to apply. In your zone, if you cover the wall with a continuous layer of insulation, such as your foam, you only need an R-10. Not that the extra insulation isn't helpful but the payback period will be considerable.

All of the foam issues can perhaps be better handled, from a fire rated perspective, by using "Thermax" brand foam, the core of which, is made to be a rated foam for situations where it may be left exposed with the foil facing it has.
 
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Old 09-18-15, 08:22 PM
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I need to add fireblocking at the top of the frame to prevent vertical fire. I'm not going to use roxul from floor to ceiling, only about 12 inches down from the ceiling because that's about how much of the foundation is exposed outside. From what I've seen on building science this is a good practice in areas where there are heavy winters.

But for my main question, it seems like most people use a piece of wood or drywall installed at the top of the frame to cover the gap from the framing to the concrete. Just wanted to know if Roxul is acceptable for this, since my framing is going to be steel I would rather avoid wood.
 
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Old 09-18-15, 09:24 PM
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If you use steel studs and have a top track, I'm not sure you need any fire blocking at the top since you will prevent fire spread with the top track.

If you really need to use the roxul, you should be sure to get the full 16" material. The material sold at the big box stores is 15" wide since it is designed for wood framing which has bulk. The steel studs do not have the 1-1/2" thickness of the wood so you need the full 16" width. If they don't have it in the big box stores then you may have to get it at a drywall supply yard since they sell the products that go hand-in-hand with steel framing systems.

As I stated earlier, fiberglass is also considered mineral wool and can also be had in the full 16" width for steel framing. It would most likely be obtained from a drywall supply yard or an insulation contracting co..

If you were thinking of running the steel studs down along the floor joists in the basement and eliminating the top plate, you would be better off with the top plate.
 
  #5  
Old 09-19-15, 05:26 AM
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Depending upon how thick your rigid foam board is, you may need to run that Roxul (or other) all the way down to meet current insulation requirements. You didn't mention the thickness.

As for just insulating the top area where the foundation is exposed, if the rigid meets the code, then you still want the extra insulation to go a couple of feet below outside grade.

Virtually every basement wall that leaves the insulation on the back or top (or any area) exposed will eventually fins mice using that space for their very convenient home. The resulting smell can be terrible, When I do infrared inspections I can see their trails, nests, and potty areas and it is always difficult to explain this to the home owners. Be sure yours does not provide mouse access.

Bud
 
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Old 09-19-15, 06:02 AM
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Hey Bud, thanks for the feedback. The rigid foam will be 2" thick so I'll get R10 on that. Is the Roxul enough for firebloacking, or should I install something else to cover the gap between the frame and the foundation wall? I can't find any specific information of what's acceptable in code, all I know is you need to have some sort of fireblocking.
 
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Old 09-19-15, 07:05 AM
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There are 3 climate zones in PA:
https://energycode.pnl.gov/EnergyCodeReqs/
This is the 2009 IECC, but your local authority may be using older or newer codes. The two larger zones require R-10 continuous so I suspect you are ok.

Just my opinion, but I agree with others that Roxul has not been approved as a fireblock. It might help, but it would need testing and approval to be used as such.

Be sure to air seal that rim joist before you insulate it and cover all fiber insulation to prevent nesting.

Basements are tough as you can never eliminate potential water issues. Just bought a new set of reinforced washing machine hoses to replace the one that busted and sprayed water for several hours. Fortunately it was the cold water as hot water can be much worse.

In addition to hoses i have seen copper pipes just spring a leak in places you would never imagine. And pex is nice, but it can also fail and fill that bathtub with water.

I have a floor drain to daylight so my flood was minimal and I don't have issues with power failures. Sump pumps are a must.

Before the 2" rigid goes up, be sure you have eliminated all water issues and minimizes all moisture vapor intrusion. I would most likely avoid any interior vapor barrier and allow whatever comes through the rigid to handled on the inside if needed.

Bud
 
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Old 09-19-15, 07:37 AM
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Thanks Bud! Good information to know.
 
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