Finished Wall Against Cinder Block in Basement?

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ManCave

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I am finishing my basement and have an issue I'm not sure how to tackle. The one side of my basement stairs is right up against the cinder block wall. This leaves me no room for furring strips and drywall as that would be as wide as my stair stringer and leave no room for baseboards down the steps. (I should note that my stairs are the kind where the stringer is not cut, the steps and risers are routed into the stringer).

My question is: what are my options if I want a finished (drywall or drywall like) wall against a cinder block wall with no room for furring strips?

Any advice would be greatly appreciated. Thanks!
 

Vegas_sparky

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If the block is good you can prime, and skim coat it with mud. This is time consuming, and messy, but a flat wall can be floated out.

Otherwise 1/4" DW with adhesive, and ramset or countersunk tapcons is about as shallow as you can build it. The DW will be as wavy as the block, so it may need attention after the rock is up.
 

Dana

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For the section with the stairs right up against the foundation Vegas_sparky's solution is about as good as you can do.

But for the rest, in a Columbus OH climate it's well worth insulating the wall rather than just drywalling it in. A CMU wall is worth at-best R3 (on a good day, with hollow cores and no wind, counting on the interior & exterior air films). Add to that the air films created by the sheet rock plus the sheet rock itself brings that to maybe R4 at best, which is a really CRUMMY R value, with a fairly high risk of condensation and mold in the cavity between the gypsum & CMU. When finishing a basement room it's an opportunity moment- if you don't insulated it then, you won't ever, unless it eventually turns into a mold farm or the furring rots. But adding the insulation NOW prevents those conditions.

Even 1" of EPS would roughly double that R-value, but 1.5" of foil-faced polyiso (R9-10) trapped to the wall with 1x furring would be even better, delivering about R12 performance due to the foil facer in the cavity. Subtracting off the thermal bridging of the furring & TapCons, and adding on the R-value of the CMU it performs at about R13-R14, which is about what a 2x6 wall with R19s performs at with all thermal bridging accounted for.

If using polyiso it's best to keep the bottom edge of the foam off the slab, since that type of foam will wick moisture, reducing it's performance. (EPS is fine resting on the slab.) Tape the seams with 2" FSK tape (purpose made aluminum duct tape) and seal the foam to the CMU with can-foam. If EPS, tape the seams with housewrap tape, then trowel on duct mastic over the tape to ensure the edges don't peel off the EPS over time.

It's also worth air sealing & insulating the band joist and foundation sill before you close it all in, if it isn't already. The air leakage between the foundation sill and CMU is usually a lot higher than you would expect just looking at it- more than all the window & door leakage in the rest of the house combined in most houses, a really long-skinny hole with a big cross sectional area.

To meet IRC 2012 code min for Columbus (US climate zone 5A) the foundation would need R15 continuous insulation, but the CMU adds a bit, as well as the air-gap with the foil, so you'd be close. If you wanted to take it the whole way to that code min, 2" foil faced polyiso with a 3/4" air gap between the foil & gypsum is pretty close R15. With 3/4" thick furring that would take 3.5" TapCons 24" o.c. (or tighter, if the wall would look too wavy) to mount the foam & furring, which are available at box stores. Any thicker on the foam and it's sometimes hard to find the right screws. With thinner foam you can use shorter screws, of course, but try to get at least 3/4" screw penetration into the CMU.

With an insulated wall the wall temps will be warmer & more comfortable, and there is no condensation/adsorption potential on the furring or paper backing to the wall board. Even 1" EPS (R4) would mitigate the mold issue, but from a comfort and energy use point of view it's well worth taking it to at least R8 continuous insulation, if not the whole R15.
 

ManCave

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Thanks for the reply guys.

Dana, thanks to your advice a few years ago I have 1.5" EPS against the walls everywhere in the basement with unfaced R13 in the stud bays. I've got 1" XPS on the floor. Plus 2 inches of XPS on the rim joists and sealed the edges. So I think I'm good everywhere else. This wall by the stairs has be stumped. I've attached a picture below. The cinder block wall is below the garage so it (in theory) shouldn't see as much ground moisture. As you can see I have the stair stringer right up against the wall and I can't put any furring or else the finished wall will be as thick, or thicker than the stringer. The drywall at the top of the picture is where the garage is on the other side. The drywall stops at the sill plate. I would like the finish wall against the block to be a somewhat smooth transition to the existing drywall.

IMG_3531.jpg


So now that you can somewhat see what I am working with. Here are two ideas that I've thought about...

The first one was to use half inch Hardie cement backer board. It is nearly as smooth as drywall and takes paint. I assumed it would hold up well against the CMU wall from a moisture and mold perspective. I would glue/screw it to the wall then finish the joints like drywall, prime and paint.

Idea two was to use something like this (LINK) against the wall. Then put drywall over it. The product in the link would be a vapor barrier between the cinder block and the drywall but also allow some air flow to dry up any potential moisture issues.

Thoughts?
 

MrStop

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I'm assuming you don't want to rebuild your stairs? Using a "cut" stringer instead would give you a 1.5" between the stair and the wall. However, that's a big commitment.
 

Dana

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"The cinder block wall is below the garage so it (in theory) shouldn't see as much ground moisture."

I was assuming that the wall adjacent to the stairs was an exterior wall rather than below an enclosed but not fully conditioned space. If the garage walls are insulated the temperature of that stairwell wall is never going to be cold enough for wintertime condensation, and the heat loss to the ground will be quite low. Five to ten feet of dry dirt has a reasonable R-value, and there is probably more than that between the CMU and the outside-outside (outside the garage) everywhere.

This can be verified by measuring it's temp with an infra-red thermometer on a really cold day, comparing it to the temperature of the insulated basement walls. It'll probably be pretty close. If that's the case, a hard-plaster directly on the CMU is fine. Since it's under the garage it's pretty protected from bulk-water incursions, and the ground water moisture coming through the wall will be slow enough to not blister the paint on latex painted plaster, nor will it present a large latent cooling load.

If you're still concerned, and want a vapor barrier there, you can use an epoxy-based floor leveler material troweled smoothly onto the CMU. But when you do that it increases the moisture drive that the stair stringer would see (but that's sill probably low risk here.) It's probably going to be safer better to just keep it semi-permeable, using cementious/plaster finishes and latex paint.
 

ManCave

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"The cinder block wall is below the garage so it (in theory) shouldn't see as much ground moisture."

I was assuming that the wall adjacent to the stairs was an exterior wall rather than below an enclosed but not fully conditioned space. If the garage walls are insulated the temperature of that stairwell wall is never going to be cold enough for wintertime condensation, and the heat loss to the ground will be quite low. Five to ten feet of dry dirt has a reasonable R-value, and there is probably more than that between the CMU and the outside-outside (outside the garage) everywhere.

This can be verified by measuring it's temp with an infra-red thermometer on a really cold day, comparing it to the temperature of the insulated basement walls. It'll probably be pretty close. If that's the case, a hard-plaster directly on the CMU is fine. Since it's under the garage it's pretty protected from bulk-water incursions, and the ground water moisture coming through the wall will be slow enough to not blister the paint on latex painted plaster, nor will it present a large latent cooling load.

If you're still concerned, and want a vapor barrier there, you can use an epoxy-based floor leveler material troweled smoothly onto the CMU. But when you do that it increases the moisture drive that the stair stringer would see (but that's sill probably low risk here.) It's probably going to be safer better to just keep it semi-permeable, using cementious/plaster finishes and latex paint.

Yes, the wall on the left of the stairs is under the garage. It is also the side of the garage that is shared with the house. It should also have R-5 Warm N Dry insulation board on the exterior from the Tremco water proofing that the builder installed. The garage is mostly insulated. I still have a few spots that I haven't gotten to. And yes, that wall next to the steps feels fairly warm, all things considered. I'll see if I can get my hands on a laser thermometer.

If that's the case, a hard-plaster directly on the CMU is fine.

What do you mean by "Hard Plaster"? Drywall? Green board? What about using Cement backer board?
 
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Dana

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By "hard plaster" I mean Vegas Sparky's solution- skim coat it with a traditional plaster. The CMU itself may may not be sufficiently rough to use as a scratch coat, and using a cement plaster scratch coat could also be useful for making the mortar-lines disappear, and it will be both thinner and better than using cement backer boards,

You can probably find tips for details on what materials work best using web searches. I dug this one out without spending any time:

http://www.stuccoguru.com/resources/article.cfm?articleID=B-06150&resourceID=2

If the CMU has both waterproofing and R5 foam on the other side the moisture drives are miniscule, and it's an insulated wall (R5 foam + R2 CMU + Rxyz of dirt). No need to seal the interior with anything low permeance or insulating there.
 

ManCave

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By "hard plaster" I mean Vegas Sparky's solution- skim coat it with a traditional plaster. The CMU itself may may not be sufficiently rough to use as a scratch coat, and using a cement plaster scratch coat could also be useful for making the mortar-lines disappear, and it will be both thinner and better than using cement backer boards,

You can probably find tips for details on what materials work best using web searches. I dug this one out without spending any time:

http://www.stuccoguru.com/resources/article.cfm?articleID=B-06150&resourceID=2

If the CMU has both waterproofing and R5 foam on the other side the moisture drives are miniscule, and it's an insulated wall (R5 foam + R2 CMU + Rxyz of dirt). No need to seal the interior with anything low permeance or insulating there.

Ok, this is probably out of my skill level and out of my budget... Given everything you've heard would I be ok gluing/screwing drywall or even Georgia Pacific's DensShield or DensArmor tilebacker to get away from paper faces? The tile backer is more readily available in my area, but seems to be the same product, just with an acrylic water barrier.

http://www.buildgp.com/densarmorplus-high-performance-gypsum-panels

http://www.buildgp.com/densshield-tilebacker-board

Thanks so much for your help.
 

Dana

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A tile-backer would be better if you're going to plaster it, drywall if paint.

Tape a sheet of 6-mil poly to the wall and leave it there for a week to see if there is any moisture of any consequence coming through the the wall, but I'd be surprised, assuming it's been waterproofed on the other side.
 

ManCave

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I haven't tried taping any plastic on the wall yet. Does it have to be 6-mil? I think I have some thinner stuff around the house...

I did finally get my hands on an IR thermometer. It was about 25 degrees outside this morning. The coldest it has gotten for about a month. I took some readings of the walls in the basement. I currently have the heat dampers closed in the basement so it isn’t really seeing any heat.

Finished exterior walls (painted drywall, with R11 fiberglass, and 1.5” EPS foam) measured around 64 degrees.

Unfinished cinderblock walls that are on the exterior of the house (no insulation on the walls) measured around 58-59’ish.

Unfinished cinderblock wall shared with the garage (the wall in question in this thread) measured 61’ish, maybe a little colder near the top where the cinder block is exposed within the garage.

I will keep an eye on this as it gets colder. But you still think I am ok gluing/screwing drywall to this cinderblock wall?

Thanks again for the help!
 

Dana

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The moisture trap test is important for figuring out if you can just screw & glue the drywall to the cinder block without risk of mold or blistering paint. If you can withstand another 1/4"-3/8" of thickness of the stackup, a layer of UNPERFORATED fan-fold XPS siding underlayment would be sufficiently vapor-retardent that you can skip the test, and would add another R1-1.8.

Short term instrumentation drift and differences in material emissivity makes the 3F delta between the insulated wall and the CMU wall "in the noise", but the fact that the exposed CMU measures 3-4F below the dirt-backed CMU is meaningful, since it's the same material, same emissivity. I doubt you really need to insulate between the CMU and gypsum in the stairwell- the dirt insulation is doing it's job, but if you added 1/4" fan-fold under the gypsum it wouldn't hurt, and would limit the mold potential from both a groundwater and temperature point of view.
 

ManCave

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The moisture trap test is important for figuring out if you can just screw & glue the drywall to the cinder block without risk of mold or blistering paint. If you can withstand another 1/4"-3/8" of thickness of the stackup, a layer of UNPERFORATED fan-fold XPS siding underlayment would be sufficiently vapor-retardent that you can skip the test, and would add another R1-1.8.

Short term instrumentation drift and differences in material emissivity makes the 3F delta between the insulated wall and the CMU wall "in the noise", but the fact that the exposed CMU measures 3-4F below the dirt-backed CMU is meaningful, since it's the same material, same emissivity. I doubt you really need to insulate between the CMU and gypsum in the stairwell- the dirt insulation is doing it's job, but if you added 1/4" fan-fold under the gypsum it wouldn't hurt, and would limit the mold potential from both a groundwater and temperature point of view.

That was another question I meant to ask you. Can I use fan-fold XPS behind drywall? I think I might go that route then. I can probably give up the 1/4" for peace of mind. Covering up the cinder block will make my wife very happy. Thanks!

EDIT, ok it looks like finding UNPERFORATED XPS could be a challenge...
 

Dana

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Dow, Owens-Corning, and Pactiv all make it. Some of the orange or blue box stores carry it, others don't, as does Menards. You may have to order it if the local stores don't carry it.

Lowes-branded Pactiv fan-fold goods are usually perforated. That's OK if you're willing to put a sheet of poly between the wallboard & foam, which is a PITA but do-able.
 

ManCave

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Dow, Owens-Corning, and Pactiv all make it. Some of the orange or blue box stores carry it, others don't, as does Menards. You may have to order it if the local stores don't carry it.

Lowes-branded Pactiv fan-fold goods are usually perforated. That's OK if you're willing to put a sheet of poly between the wallboard & foam, which is a PITA but do-able.

Ahh, found it. My (orange) store doesn't carry it so it wasn't showing up in my searches. Good thing is other stores in town do. I will check it out. Thanks!
 

Curt Pasawicz

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I am finishing my basement and have an issue I'm not sure how to tackle. The one side of my basement stairs is right up against the cinder block wall. This leaves me no room for furring strips and drywall as that would be as wide as my stair stringer and leave no room for baseboards down the steps. (I should note that my stairs are the kind where the stringer is not cut, the steps and risers are routed into the stringer).

My question is: what are my options if I want a finished (drywall or drywall like) wall against a cinder block wall with no room for furring strips?

Any advice would be greatly appreciated. Thanks!
 

Curt Pasawicz

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Any material you put up against a cinder block will more than likely absorb moisture and the minerals in the concrete. You will probably notice lime stains after a period of time. Painting the block with a stain cover or a water proofer will help a little. Use some type of barrier at least. Plastic should do it. Maybe a piece of cedar(used in decking). Get creative and use "Trex", an engineered outdoor product used for decking and around pools. You can adhere it with liquid nails.
 

Curt Pasawicz

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Thanks for the reply guys.

Dana, thanks to your advice a few years ago I have 1.5" EPS against the walls everywhere in the basement with unfaced R13 in the stud bays. I've got 1" XPS on the floor. Plus 2 inches of XPS on the rim joists and sealed the edges. So I think I'm good everywhere else. This wall by the stairs has be stumped. I've attached a picture below. The cinder block wall is below the garage so it (in theory) shouldn't see as much ground moisture. As you can see I have the stair stringer right up against the wall and I can't put any furring or else the finished wall will be as thick, or thicker than the stringer. The drywall at the top of the picture is where the garage is on the other side. The drywall stops at the sill plate. I would like the finish wall against the block to be a somewhat smooth transition to the existing drywall.

View attachment 27473

So now that you can somewhat see what I am working with. Here are two ideas that I've thought about...

The first one was to use half inch Hardie cement backer board. It is nearly as smooth as drywall and takes paint. I assumed it would hold up well against the CMU wall from a moisture and mold perspective. I would glue/screw it to the wall then finish the joints like drywall, prime and paint.

Idea two was to use something like this (LINK) against the wall. Then put drywall over it. The product in the link would be a vapor barrier between the cinder block and the drywall but also allow some air flow to dry up any potential moisture issues.

Thoughts?
 

Curt Pasawicz

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I saw your picture. You do have room for firing strips. I did it in my basement. Put the drywall over the strips so that the finished wall is flush with the stringer. The drywall will continue right down to the wood on the staircase. After painting it will appear that there is no stringer. It will look like a floating staircase. Problem solved.
 
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