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XPS as a bottom plate?

Posted by engineer_ryan (My Page) on
Wed, Sep 10, 08 at 14:42

I'd read worthy's suggestion in other threads to use XPS as a bottom plate for basement walls, and I have questions.

I suppose you score and snap to 3.5" widths - and use 1" thick material? Any thicker and I'd think you'd risk not having a good nailer for base trim.

If using tapcons to secure the plate, do you simply bore through the bottom plate of the wood framing, through the XPS plate, and into the floor? Or do you somehow secure (glue?) the XPS to the concrete floor first?

I can think of lots of variations here, but I'd like to hear a working method from someone that's been there.


Follow-Up Postings:

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RE: XPS as a bottom plate?

Just drill through and use selftapping concrete screws or powder-actuated fasteners such as Ramset.

BTW, it's not my idea, but the suggestion of Building Science Corp.

Here is a link that might be useful: Building Science Corp.


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RE: XPS as a bottom plate?

Why use 1 inch material? I can understand it being under there if you're insulating the whole floor, but if you are only looking for a capillary break, just use a 3.5 inch polystyrene sill gasket:

http://owenscorning.com/worldwide/admin/tempupload/pdf.pdf.OCPinkExtrudedPolystyreneInsulation.pdf


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RE: XPS as a bottom plate?

A capillary break is all that is required by Building Codes; it's the way I used to do basement renos and new homes and is all you will usually see. It's easier to install and cheaper than using XPS.

However, the XPS also provides a thermal break.

As well, by lifting the framing, wood or metal, an inch off the floor you're preventing contact of the framing and drywall with water from the flooding that is almost inevitable in basements from a variety of sources--broken supply pipes and burst connections, overflowed laundries on upper floors, overflowed sinks, stoppered toilets, clogged window wells, unusually heavy storms, burst hot water tanks etc., etc. Water and wood in a basement leads to mould; water and steel leads to rust.

XPS under plates is best practice, not required practice.


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RE: XPS as a bottom plate?

A capillary break is all that is required by Building Codes; it's the way I used to do basement renos and new homes and is all you will usually see. It's easier to install and cheaper than using XPS.

However, the XPS also provides a thermal break.

As well, by lifting the framing, wood or metal, an inch off the floor you're preventing contact of the framing and drywall with water from the flooding that is almost inevitable in basements from a variety of sources--broken supply pipes and burst connections, overflowed laundries on upper floors, overflowed sinks, stoppered toilets, clogged window wells, unusually heavy storms, burst hot water tanks etc., etc. Water and wood in a basement leads to mould; water and steel leads to rust.

XPS under plates is best practice, not required practice.


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RE: XPS as a bottom plate?

Thanks, all.

I would use just a capillary break, but I think worthy's (BSC's) reasons are compelling. I wanted to avoid using a pressure-treated bottom plate, and with a thinner sill gasket I'd feel like I would still need PT.


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RE: XPS as a bottom plate?

Yes, Worthy's reasoning is good and falls in line with BSC's practices, but seems to be more of personal technique than "best practice" considering that I haven't seen it mentioned in the BSC research reports nor have I seen it in their detail diagrams. While the polystyrene sill gasket is cheaper, it is not inferior. It is the same material as the panels, and one of its design functions is provide a capillary break to keep moisture from wicking into the wood.

As far as elevating the wall from possible water breaks and such, these incidents are few and far between and the materials will dry once the problem is alleviated. The point of the sill gasket is prevent prolonged moisture exposure. Besides I could argue that if elevating the wall 1" is good, why not elevate it 4", wouldn't that be even better? The drywall and trim are still going to be down in the water even with 1/2" of water....


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RE: XPS as a bottom plate?

Besides I could argue that if elevating the wall 1" is good, why not elevate it 4", wouldn't that be even better? The drywall and trim are still going to be down in the water even with 1/2" of water....

BSC RR-0509c (Renovating Existing Basements) p. 2 advises that drywall should be kept at least 1/2" up from the floor. Using XPS under the baseplate keeps water sensitive framing (steel or wood) 1" from the floor and drywall even further up than the minimum. (Only the trim is at risk and it's a lot easier to repair or replace than the other wall components.) If the floor has more than 1" of water, your basement drains are not working. Or you have not installed backflow preventers. Or there is catastrophic flooding. An inch of XPS won't help if the Mississippi River is coming your way.

If using XPS weren't so easy and cheap I wouldn't mention it. After all, you can protect 48 feet of base with just one 2'x8' sheet; just score and snap.

But if you use poly or gasket under the base, you'll still be ahead of most DIY renos, at least judging from those I've pulled apart rife with mouldy baseplates and drywall and rusted away steel framing.

Here is a link that might be useful: Renovating Existing Basements


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RE: XPS as a bottom plate?

I am still not sold on using the XPS on walls assuming you have no mositure issues in your basements. I am leaning towards keeping the framing an inch or two from the wall and using fiberglass. If the XPS were cheaper in my area I would consider it.

I WILL consider using it under the 2X4s...I like the idea of elevating the base a bit though I would still likely use a pt base. I would think if you then affixed the drywall and inch above the baseplate you should pretty safe. If you get a flood your molding may be shot (though I wonder how that plasticy moulding would hold up). I think the pt base would dry w/out mold and the drywall would be 2" up from the floor.

Anyone see that basement renovation show on the DIY network. They just nailed a regular 2X4 right to the concrete. I could not beleive it!


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RE: XPS as a bottom plate?

It is interestin reading the posts. Being a building official from New Zealand, we see 99% of buildings being designed and built to the minimum or below.
It is obvious there are professionals posting above who take pride in there work and have based their recommendations on their experience. Then there are the people who are rife over here, price conscious value ignorant.
Whether the odd flood will or won't cause mould is dependant on the area you live in. In the Nevada, you could drown the bottom plate regularly or attache the bottom plate to the concrete without any harm. In Florida the use of the XPS is in my mind totally advisable. Basement do not mormally ventilate naturally and the higher the humidity, the greater the risk of problems.


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