Code conflicts with Building Science

phoenix2000January 24, 2008

I just spoke to my township about finishing my basement. I want to put 2" XPS on the cinder block walls, with the wood frame built a few inches away from the wall filled with unfaced insulation and then sheetrocked.

The township requires that I use FACED insulation as a vapor barrier. AND they require fireblocking with gypsum board (or fiber board, batts or glass fiber) at 10 feet intervals horizontally and vertically within the wall space. This seems to me a sure fire way of trapping the moisture and providing extra organic food for mold.

What recommendations do you have for mitigating the potential damage of these requirements? Should I use purple board for the fireblocking? Should I use it for the internal walls, too? Should I pay extra for the yellow board instead (the paperless kind)?

Should I hire a contractor who is experienced with using galvanized steel frames?

I should mention that I live in Central NJ and have a very high water table. The previous owner drylocked the walls and the floor, so there doesn't seem to be a lot of moisture coming through (though the drylock won't last forever, and I haven't done the plastic moisture test to confirm, yet). We have gotten flooded due to sump pump failure, but we plan to replace our current 2 pumps and include a back-up water powered sump pump. We also have a perimeter drain in the basement. We do use dehumidifiers.

I wasn't planning on getting mold-resistant drywall or look for steel framing, but with these code requirements, should I?

Any thoughts?

thanks in advance,


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There should be no space between the XPS and any wall being built.

Once XPS is on the walls, whether or not there is a vapour barrier doesn't matter. (Though Dr. Lstiburek says better not to have it at all.)

AND they require fireblocking with gypsum board (or fiber board, batts or glass fiber) at 10 feet intervals horizontally and vertically within the wall space.

There must be some confusion here. First off, fiberglass is not a fireblock. Secondly, since you are proposing a sheetrock wall covering the XPS, I don't see why they would want another wall within a wall. And with ten foot gaps in it??

In light of all that potential for moisture, I would avoid galvanized material, which rusts once the sacrificial layer of zinc is gone.

The best thing on the interior walls is a crystalline waterproofing. However, there will be a lot of prep work removing the drylock. The walls may have to be acid washed, too, so you may prefer to use a co. specializing in this.

Be sure to place the plates on top of strips of XPS to help keep moisiture from wicking into the framing and wall.

    Bookmark   January 24, 2008 at 4:42PM
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A contractor first told me about the extra fire safety precaution of building walls within walls which I thought was crazy.

It is confirmed by a document that the township referred me to and it says "Fireblocking shall be provided in concealed spaces of stud walls and partitions, including furred spaces, at the ceiling and floor levels, and at 10 ft intervals both vertical and horizontal." It also lists "glass fiber" as an "acceptable fire block."

Why can't I have a space between XPS and the frame? I thought the spacing wasn't necessary, but is it potentially damaging?

Unfortunately, my cinder block walls are not flushed horizontally. In a couple of places, there's a cinder block column that extends out 1 ft. So, building a frame out 12 inches or more will allow me to have a straight sheetrocked wall.

The potential good news is that I talked to another contractor today who said that the township had accepted the seamed XPS boards as the vapour barrier which contradicted what the township told me on the phone, but I'll call and ask again.

By the way, you indicate that once XPS is on the wall, it doesn't matter if there's another barrier - I don't understand. I thought the double vapour barrier construction was to be avoided. I thought that XPS is a vapor retarder, so some vapor will come in and with the presence of faced insulation, won't that moisture build up in the cavity due to the double vapour barrier present?

    Bookmark   January 24, 2008 at 8:07PM
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If that's the "fireblocking" they insist on, that's what you'll have to provide. Since at least 1990 fiber glass has not been an acceptable fire stop material in Ontario. Other jurisdictions, such as Connecticut, have made the same decision. Also see here on the suitability of fiber glass as a fire stop.

Putting a vapour barrier on the interior of a basement wall is a proven mistake that still persists as Code. The only time I've been exempted is for a basement that has been spray foamed. However,Yost and Lstiburek note on page 9 of "Basement Insulation Systems" that walls built that way can still work efficiently as long as they are mechanically dehumidified in the summer.

    Bookmark   January 25, 2008 at 12:38AM
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Why can't I have a space between XPS and the frame

To hold the fiberglass in place on a freestanding wall you'll have to secure it on the foundation side using, say, galvanized mesh. Also, I don't like idea because of the likelihood of convection currents within the wall reducing the effectiveness of the batts and perhaps cooling that space enough to support mold growth on the back of the fiberglass.

I would prefer to just insulate the foundation wall with sufficient foam board or spray foam to at least meet Code, then build the gyprock wall. That's what I've done in the past. In that case, authorities will likely not insist on a poly vapour barrier.

    Bookmark   January 25, 2008 at 10:30AM
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Further to paragraph one, once you separate the insulating materials from each other--in this case up to 12"--you can't add the R values together. A wall of, say, R5 XPS, a 12 in. air gap and then R11 Fiberglass (3.5ins.) is not R16. (Leave aside the fact that "commonly installed" fiberglass batts lose 27% of their labelled R values.)

    Bookmark   January 25, 2008 at 5:39PM
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I am confused. I used some glue to hold the thick XPS pink boards to my block wall. I then used 2x4's layed FLAT as framing for my sheetrock. Is this OK? I did not want to turn the 2x4's the conventional way due to space restrictions. So do I need insulation in the space between the XPS and the void the 2x4 makes layed flat (I guess we are talking about an inch and a half)? I plan on sheetrock the framing.

    Bookmark   January 25, 2008 at 9:22PM
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It doesn't matter if the only insulation is the foam board. However, the framing should have been done conventionally to minimize the effect on the drywall if/when the wood warps. Or you could have just used furring strips laid flat against the insulation.

    Bookmark   January 26, 2008 at 8:37AM
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I was considering using furring strips, but that would not accomodate wiring and electric boxes. The extra thickness the 2x4 provide this room. Also, I did not find any fasterners (Tapcon) that would provide the length to get throught the wood, and the foam, and into the concrete block. Won't furring strips warp also?

    Bookmark   January 26, 2008 at 11:20AM
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If you've used sufficient concrete screws or powder charge fasteners you're fine!

    Bookmark   January 26, 2008 at 12:55PM
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I attached the 2x4's layed flat to piece of wood anchored to the floor and another at the top. Is this OK? Are you saying because it is layed flat it will warp and mess up the drywall? Do I really need to undo this and install the correct way?

    Bookmark   January 26, 2008 at 1:19PM
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Is this OK?

Guaranteed to warp.

Since you say you need space for electrical, I'd build a conventional 2x4 or 2x3 wood or steel wall. The 2" of depth of the 2x4 and drywall laid flat won't give you enough depth for electrical boxes anyway. Remember to put the baseplate on top of at least 1" of XPS.

    Bookmark   January 26, 2008 at 3:05PM
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