Box in the heating duct and nail the top plate into that.

Current basement insulation thinking is to cover the walls with a continous layer of extruded polystyrene foamboard (the pink stuff). Then do your framing inside of that. The foamboard eliminates the need for drylock.

I would use pressure treated bottom plates, and regular kiln dried wood for the rest. I haven't had good luck with the nail guns that use the powder charges. I prefer to drill into the concrete floor and use expanding anchors such as Redhead.

Thanks...what about those tapcon scews...I guess they work too? Do you glue the pink foam board onto the walls...how do they stay in place? Do I need a poly vapor barrier too? Do you tape between the foam board panels to seal it? I thought the R value was very low on foam boards.

IÂm not sure about tapcon screws, donÂt they seem a little flimsy? You should check the instructions that come with them to see if they are rated to secure walls to concrete.

IÂve glued the pink foam board to the concrete walls using an adhesive that comes in a tube that is applied with a caulking gun.

You donÂt need, or want, a vapor barrier. A barrier could cause condensation, which could cause mold. The foam board is a vapor retarder, which is what you want. See Building Science CorporationÂs article on insulating below-grade masonry wall basements. (link below)

2 inch thick extruded polystyrene is an R-10. Additional insulating, if needed, could come from installing unfaced batting between the studs.

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

Tapcons (or equivalent) work fine in concrete. I like them better than explosive charges because you can remove and reset the plate easily if you need to.

If your walls are uneven--poured rather than unit masonry-- the adhesive may not work well. In that case, use tapcons and fender washers. Use the shiplap XPS, but still tape the joints with Tuck tape (or equivalent). Very important: place the bottom plate on at least one-inch of XPS and keep any drywall at least an inch above the floor. This will prevent wicking up of moisture into the plate and drywall.

Worthy...if the wall has some damp spots on them...is XPS OK to cover the wall with? I have never had free running water, despite the hardest of rains ever, but does water some how get out the the wall? Is this kind of like that Ownings Corning system?

Nice link brewbeer

XPS is unaffected by water.

Still, I would try to find and eliminate the source of water that's causing the damp spots.

BTW, attach your new framing to the floor putting the fasteners through the plate and the XPS--not the foundation wall.

I was not aware of XPS being used in basements..I am doing mine now and was framing for regular insulation that comes in a roll and then install poly over that. Is it too late, can I install XPS between the studs? Or does this defeat the purpose. When you say put XPS under the bottom plate...does it "squish down" and deform being it is sytrofoam like making the bottom plate "unlevel/bumpy."
Also, it seem no one sells unfaced insulation anymore for 2X4 framing. Can I just slit the facing to allow air through?

Forgot to add...on one side of the basement, there is a concrete block wall that is not below grade (walk out basement side)...can this use regular roll batt insulation. I am gutting the basement, and can leave this wall alone if this is OK. I currently has batt insulation with a poly vapor barrier clear plastic sheeting. Do all this moisture issues go away for a wall that is not below grade? Also, can a hole be cut into the XPS if you need to install an electrical box? How is this done if furring strips or something like a 2X2 is used on top of the XPS? Can drywall screws go through the wood and into the XPS? The one post recommended 2" thick XPS..I would hate to have to use a 2X4 the regular way, I would like to lay it flat or use something thinner to keep the wall thickness down. I guess the stud can be screwed into the wall through the XPS. How many tapcon screws should be used per stud...3?

Worthy/Brewbeer...should the walls be drylocked also to prevent water entry?

mat r

Poly covered batt insulation in the basement walls can fail in as little as a year, saturated with trapped water vapour feeding mould in the drywall. (See brewbeer's link above.)

The bottom plate does not deform when it's on top of XPS. Just score and snap sections wide enough to fit under the plate. (I used to use Super 6 vapour barrier; this keeps vapour from wicking up, but doesn't do anything if you get standing water.)

XPS between the studs isn't ideal, but it sure beats most anything else. If there is space between the foundation wall and your stud wall, it would be better to slip in even half inch sheets along the wall. In that case, and that case only, you could safely use batts on the warm side of the basement wall with or without a facing.

Just slitting the facing on the batts and putting them in by themselves is inviting trouble. (Following "best practices" at the time, I used to put a poly sheet on the concrete wall and leave it loose at the bottom; the idea was that the moisture on the wall would roll down and somehow get into the room and evaporate. A lot of hoping! And by slitting the warm side barrier, you'd be counting on the vapour going through the insulation, condensing on the warm side of that same poly and running back into the room as water. Long term, I bet it will fail too.)

Above grade, Building Science Corp. says unfaced batt insulation is acceptable. (See Link above.)

Electrical boxes. Just mount them on the 2x4s as on any interior wall.

Do not attach the 2x4s to the walls; use plates (2x4 laid flat) top and bottom. If you have a concern about the wall thickness, you can use 2x3. Or even 2x2 light steel; but only where there is XPS on the wall and as little air movement as possible, as the steel will condense any water vapour that gets into the wall.

Number of fasteners for the floor plates--every three to four feet.

While XPS is my preference, Building Science Corp. says that EPS is also acceptable. But it has a lower R value per inch.

Finally, be sure to provide mechanical dehumidification during the summer months to keep the moisture levels down.

If I were to treat the walls first, I would use a permanent product, preferably a crystalline waterproofer in a slurry coat.

Instead of using foamboard (XPS or EPS) you might want to get a quote on spraying closed cell foam onto the walls. I used this on the last home I built. No cutting, no drilling, no taping, no fiddling. And it works better than boards. It was worth it to me.

Wow...what a wealth of good info. One last issue...in the earlier message, I have a heating duct flush with the wall I need to XPS and stud out. I have not access to the joists above. One mentioned to box out the duct (which I need to do regardless to finish the room), but I am still unsure how to anchor the horizontal running 2x4 to the wall. It ultimately needs to attach the the wall, no? To me, something needs to attach to the wall...either the stud running vertical from the floor, or the box around the duct.
Could use a bracket and attach the stud from the floor to the wall, or lay sideways and screw to the wall?

It ultimately needs to attach the the wall, no?

If there's no access to the floor joists to anchor a plate, maybe so.

I don't like using the foundation wall as an anchor for the finished wall for several reasons: it may not be square to the other walls, thus leaving the finished wall out of square as well; it may be quite irregular--especially a poured wall in a production home where the forms weren't lined up all that carefully; putting holes in a block wall, you might go right through the web, thus breaking the air barrier effect of the wall and inviting vapour leaks.

(There really should be an editing function on this site!)

Just thought of one approach we've used in the situation you describe. Build the stud wall on the floor. That way it's square. Then use a few blocks on the wall as anchors and anchor your complete wall to those, using shims if necessary. Anchor the wall to the floor as well.

High folks!
New member of group.

Brewbeer says: "...The foamboard eliminates the need for drylock."

I am concerned about that! I would sure hope that is true as I would need to strip whatever has previously coated the basement walls to apply drylok or Thoroseal (what I think is better). I would appreciate greatly what your experience has been using this as from what I've read in yours and others postings, along with an exhaustive research of basement insulation solutions, using XPS in direct contact is the "preferred" method. The added use of drylok/thoroseal was for me added insurance, not to mention a huge labor and cost addition I do NOT want to do if I don't have to.

Thanks in advance

Doesn't drylock with XPS kind of create a double vapour barrier which is a no-no.

XPS is semi-permeable unless it is sealed with polyethylene. (PDF file)

XPS is not a substitute for Drylok, whose manufacturers claim it stops even water under pressure.

I had a waterproofing system put in the basement this past winter and it has a sort of vent against the wall at the floor around the perimeter of the basement. The vent is there to route condensation in to the system's gutters which lead to the sump pump and would, therefore, allow ventilation. Should I still use the polystyrene sheets or can I get away with using 4 mil plastic as I initially intended?