Hi all,

This is my first GBT post.  I'm a relatively saavy DIYer.

10 second question:
I'm considering installing an interior to foundation metal stud wall and filling it with foam board (probably 2.5" of poly iso but still debating).  I'm surprised I don't find lots of people doing this system as a basement insulation retrofit.  What am I overlooking?  Oh, I'm in Colorado. Zone 5->4

Details:
I have a 700 sf 'daylight basement' (grade is ~42").  It's not comfortable in the winter because the concrete portion of the wall and slab are uninsulated.  The houses sits on the partial height foundation wall (2x6 stick).  At that height and up, I have 5.5" of fb batt and 1" of XPS outsulation.  Below 42", I have nothing.  The slab is also uninsulated.  Most of the slab has a redguard moisture barrier that probably never worked.  I tried an epoxy paint in part of the basement and you can actually feel the humidity difference in the room that has it (lower) but it isn't holding up well due to not grinding off the old redguard.

What I was thinking of doing, having little to no ability to insulate the slab (shout if you have a good retrofit option (oh 92" ceiling so anything involving a head height consuming structural floor is out), was to install metal floor and ceiling stud track and then place 2.5-3.5"of foam board in the track (building code here says R-15 cont or R-20 stud for basement foundation walls).  Heck, if I could come up with a good wall finish (remember must be fire barrier with all that foam!) other than drywall, I'd consider skipping the metal studs themselves and just affix the finish to the foam.  So for I haven't talked the gf into floor to ceiling corrugated metal interior walls...

What I'm wondering is am I missing something on moisture management?  Why don't people do this?  There are some many examples of wood stud basement construction on-line and it doesn't seem like a lot of people doing this kind of work.  I am planning to 'paint' the concrete walls with drylock first.   I realize it might be better to dig the perimeter and install outsulation/dampproof but that's really a chore.  We are talking about quite a bit of earth moving for that ....

Oh, trusses in the ceiling that clear span the level so no need to bear weight with these walls.

Thoughts?  Ideas?

Here in Minneapolis we start a basement remodel by removing old concrete, 6 mil, 2" of XPS, PEX and concrete over all. If new concrete is not the budget we use an insulated aluminum panel grooved for the PEX and add 3/4" to the floor, plus covering; wood, tile and so on. Most basements can give up an inch and half for perfect comfort and a 100% vapor barrier that can mitigate radon as well.

We use 2# foam, first at the rim joist, most important place to insulate in any renovation save the attic and 2" on the walls with 2x4 studs to cover with electrical and other utilities therein. Ventilate with ERV for IAQ. Remember to set dry wall a 1/2" minimum off the floor.

The metal studs make for a pretty good thermal short circuit, which negates much of the benefit of the insulation in the cavity between them.

If you are going with metal studs on the interior, you really should install at least 1" of EPS or XPS between the studs and concrete with all insulation edges sealed.

A simpler approach that consumes equal or less interior space would be to glue 1" or 2" of EPS or XPS rigid foam against the concrete with sealed edges (two layers of 1" foam with seams offset would be my preferred option), then use tapcons through the foam to secure 2x4 (or 2x3 or ripped 2x4) vertical furring strips every 2' to the concrete. Then fill the voids between the vertical furring with 1.5" EPS, XPS, or Polyiso.

You are still going to have a huge amount of heat loss through the slab if it is uninsulated. You might consider gluing down 1" of XPS or 2# density Type IX EPS (25 psi compressive strength), then glue down 3/4" tongue-and-groove plywood decking on top of the foam with at least one tapcon into the concrete in each corner. Just make sure the type of glue you use is foam board compatible. This would reduce your ceiling height from 92" (7'8") to ~90" (7'6") depending on finish floor thickness. Installing something like vinyl or cork flooring would further reduce heat loss and minimally reduce ceiling height.

for DIY, I would consider insofast for the walls and possibly even for the floor. www.insofast.com Cheers, Eric

I have wood studs & fiberglass + gypsum and I know people who have gypsum screwed and glued over foam (no studs). Neither had water concerns. I prefer the latter.

If water was a problem, I'd consider rigid foam + MGO or other waterproof panels. I wouldn't consider steel studs & inset rigid insulation plus gypsum.

Basement wall insulation is much more important than floor insulation.

Badger,

Thanks for the description. I'd love to take this gold standard approach.  I am a pretty savvy DIYer (I do enlarged window openings, electrical, gas ...) but flat concrete work is beyond my current skill set (I've poured some deck piers and patched a pool).  I think pricing for the kind of work you are talking about probably runs $15/sf here?  Guessing a bit but I know what the concrete stainers charge to blow out an old slab and put in a new one which doesn't include the insulation or pex: $10-$12/sf.  For my ~750 sf level, we'd be talking $10k minimum.

Keith

Arkie,

Good ideas re negating the thermal cost of the metal studs.  I'd love to find (but haven't yet) something that quantifies the impact on the total wall system of using metal against the also highly conductive concrete ... hmmm, I think you just convinced me I need at some foam between the concrete and the metal studs.

Ok, here are the numbers: (click for reference)

Looks like ~0.40 correction factor for metal studs.  Yikes.  Thanks for pointing this out!


Re: metal studs:
I've been trying to stick to stuff that has zero ability to mold and could be wetted if we get a monster flood event (there is a creek behind my property and my water is off a shallow well (<~30' deep). At many times of the year, the water table is probably only a handful of feet lower than the basement. And yet there is no sign the basement has flooded in 37 years.  I'll have to re-evaluate based on those correction factors.

Keith

Eric,

I looked at insofast and one other (can't remember the name).  None of them achieve the R required by energy code in my area: R-15 continuous or calculated system; R-20 for stud and batt.  So they aren't options based on performance.  I suspect I wouldn't like the convenience cost either but they do offer what appears to be a fast installation.  Probably a great option for somewhere with less aggressive energy code (basement: R-20 batt; walls: R-19 + R-10 outsulation; attic: R-38).

Keith

MGO: I've looked for a supplier here.  No luck so far (and thus no idea on cost).  Seems like some of the anecdotal evidence is mixed to boot.

Wood studs: It seems like several people think I'm overreacting to the moisture value of metal vs wood studs.  Hrmmm.

I'm not planning on gypsum to the floor level.  Right now, I'm consider corrugateding metal to the concrete foundation top/sill height (i.e. a wainscot) (I want an industrial-modern look) with gypsum above, though I've consider T&G above or a decorative plywood material (bamboo?).  I'm unconcerned about bulk water above the sill level (lots of disasters if that ever happened).

Ok, thinking outloud here based on the ideas above (close to Arkie's description)

PT 2x4 at floor adjacent to foundation sitting on 24" o.c. 1/2" redwood lath (to allow a drain space at floor level in the event of wetting). Tapcon to floor.
2x4 screwed to truss bottom (easy in one direction, nuisance in the other two possibly).
2x2s set 24" o.c. at the interior edge of the 2x4s. Provide nailer for metal panel/drywall/decorative panel
Allows 2" of continuous foam board against the foundation (glued?) + an additional 1.5" in the mini stud bays. Should yield (with polyiso) something like 100% x 2 x 6.5 = R-13 + 75% x 1.5 x 6.5 = R-7.3 = R-20 for the bottom off the wall?

Seems like a lot of carpentry compared to just attaching directly to the foam board. 3" of XPS (R-15) continuous with wall surfaces attached directly...

I'm not a huge fan of tapcons (not an expert either). I tried using them on a garage project on another house (45 year old slab). They pulled. Ramsets did not. Obviously an option.

Posted By keithhoffman22 on 10 Aug 2013 11:53 PM
Arkie,

Good ideas re negating the thermal cost of the metal studs.  I'd love to find (but haven't yet) something that quantifies the impact on the total wall system of using metal against the also highly conductive concrete ... hmmm, I think you just convinced me I need at some foam between the concrete and the metal studs.

Ok, here are the numbers: (click for reference)

Looks like ~0.40 correction factor for metal studs.  Yikes.  Thanks for pointing this out!


Re: metal studs:
I've been trying to stick to stuff that has zero ability to mold and could be wetted if we get a monster flood event (there is a creek behind my property and my water is off a shallow well (<~30' deep). At many times of the year, the water table is probably only a handful of feet lower than the basement. And yet there is no sign the basement has flooded in 37 years.  I'll have to re-evaluate based on those correction factors.

Keith

There is a big difference if you are designing for dampness or flood!

If you are designing for flood, make your basement as cheap to redo as possible. While many of the products you talk about are very good from the point of preventing mould, most of them leave gaps. If your basement is flooded with contaminates i.e. sewage, silts, fine clays (mud) these contaminates will find there way into any gap or crack and remain behind the foam products causing a stink. You have to strip out any and everything that can get contaminates behind them.

From the flood zone in southern Alberta.

I'm more concerned about my sewer line.  It likes to backup (36 years old, trees over the line that I should remove).  No sign that the basement has actually flooded in 36 years.  I'll think about your gaps comment.

First, don't use XPS (ever!), unless you really have to. It's blown primarily with HFC134a, with something like 1400x the global warming potential of CO2, and loses most of it over the first 50 years (taking a ~15% loss in R-value in the process), and in 100 years it'll be pretty much identical to the R-value of EPS. Since both polyiso or EPS could be used on the wall application, they're a far better choice, being blown with pentane (@ 7x CO2 GWP), and have a stable long-term R value. On the slab you can't use poly iso, but you can use EPS.

You've apparently discovered the folly of using foam behind steel studs due to the extreme thermal bridging, but wood is a thermal bridge too, and it wicks moisture. Lose the concept of using PT in direct connection with the slab- slip some foam under the bottom plate as both a thermal & capillary break, and use standard lumber.

In many places it's possible to buy reclaimed fiber faced roofing polyiso from commercial re-roofing & demolition at less than the cost of batts per unit-R. I did my place in reclaimed 3" iso (R18) held in place with 1x furring through-screwed to the poured concrete foundation with TapCons, mounting the gypsum to the furring. But it's fine to trap it to the wall with a 2x2 mini-stud wall too. When using iso in this application it's important to keep the cut edge at the bottom off the concrete slab or it can wick moisture over time, but if you ran 1.5-2" EPS under the bottom plate of the studwall & wall-foam all the way to the foundation wall, that wicking issue is taken care of. But there are still steel-stud options.

In the event of a flood the iso would take on some water, but if you pumped it out within a few days the amount wouldn't be much. But EPS has a much higher moisture tolerance (they float docks with it, after all), and may be the best choice for the bottom half of the wall.

Corrugated steel wouldn't likely meet the letter of code for a thermal barrier on the foam by itself in a fully conditioned space (but would meet ignition barrier requirements for crawlspaces etc in locations without ignition sources and only accessed for service, not storage.) If not gypsum, you'll still need something. Using 3.5" of unfaced rock wool insulation would be legal, and you could cut back to 2" of iso or EPS and have R20+ for whole-wall performance. To minimize the thermal bridging of steel, you could turn the steel studs sideways, since it's not a structural wall in any way, and only needs to keep the insulation from wandering, and hold up the gypsum.

You don't need anything like 25psi goods for the foam under a subfloor- even the cheapest Type-I EPS would be good enough, so long as you stagger the seams of the foam & subfloor. Some of the Huber subflooring materials will tolerate a pond-soaking, and would be the preferred way to go if flooding is a real issue.

Dana,

Thank you for the thorough reply. I've been learning about the non-greenness of XPS lately and will avoid its use. EPS really doesn't meet my goals in terms of R per inch so I'm unlikely to utilize it.

I'd love to use mineral board but it is simply unavailable in my area (greater Denver). I can get batts from either Roxul or Thermafiber but then I'm obviously back to building a true stud wall and the net R-value will not meet energy code. I can only get boards if I order a truck load. While I might be willing to consider that when I eventually re-do my stucco and increase my outsulation (not relevant to these foundation walls), this project is ~2400 sqft-in (eg 600 sf x perhaps 4 inches of insulation). I forget what a truckload is but I believe it is more like 1000s of square feet. To add to the annoyance, even if I wanted a truckload, I can't get it direct from Roxul. They would truck to a distributor, who would mark it up and won't deliver it. So mineral board, for all its appeals, is essentially impossible. IMO, Roxul doesn't care about penetrating the US market.

Recycled PolyIso: great idea but haven't had any luck here. We have a building material recycling program. They don't get foam board. I'm not sure if we don't have enough commercial space demo going or if they are reusing that on site (there are credits for that in our area) or what.

In terms of wall covering, I'm a lot more willing to take chances on code compliance with the finish surface than I am with inner work for the obvious reason that re-working it isn't so bad. I find it interesting that metal might not meet the letter of the code. I'm pretty sure the drywall has burned through long before 24 ga steel ignites. It seems like ideal material for below grade work doesn't exist? Finished look, meets ignition/barrier standards, wettable and dryable. In addition to not being able to get rock wool board, I understand from my research that the project is messy. I can't imagine that as a finished surface though it would deal with the letter of the code and allow my metal or whatever finished surface.

Yes, research resulting from the great comments here has lead me to ~25-30% corrective factor assumptions for wood stud and ~40% corrective factor for steel stud. Both are not great. I'm currently leaning towards direct application of the foam boards to the wall followed by a metal wainscot to sill height and either drywall or t&g pine board above. I doubt it is for everyone, but I might love the look and I don't remodel for resale. (It amuses me when people buy their own home so they can do what they like and then remodel so that the stereotypical buyer will purchase it).

Other posters have suggested that pumping out the basement immediately post-flood would leave a smell issue. I'm uncertain how to address that issue. As my understanding of options advances, I'm willing to drop flood-resistant (technically my house isn't in, though it is very close) in favor of tolerant of a sewer line backup. Man, maybe I just need to address that too. Project dependency chain! Ugh.

I've also talked to the basement finishing companies (not exactly the lower cost DIY option I'm looking for). They really don't have better solutions. Lower R value, stud walls, some still want to build wood studs with fiberglass against the concrete slab and drywall on top (talk about mess in terms of post-flood).

You guys have me wondering if I should be trying to do the outsulation project instead. Dig out the 42" of dirt adjacent to the foundation, probably end up replacing the footer drain (after 36 years, it is probably clogged since I'm not aware of it going to daylight), install 2 layers of 2" polyiso on the exterior from footer over the basement/first floor truss rim, re do the stucco, replace the dirt. But man, that is a ton of work for DIYer. It is also very hard to find foundation type contractors here so there is little to no hope of contracting that out. Beating the first freeze here might be tough at this point.

I very much appreciate the discussion. Currently opening more questions than it is answering but helpful for thinking through the project.

pumping out the basement immediately post-flood would leave a smell issue. I'm uncertain how to address that issue.

A certain amount of "basement flooding" can be pumped out faster than it can come through the soil. But if it's really going to flood, then I'd use screwed on, removable, waterproof materials.

Jon:

Yeah, what I really wanted was removable wall panels that just screwed on. But TMK, there aren't any good options: no high R-value, most of them use some kind of manufactured wood panel or support that would never tolerate flooding, and they all still expect drywall finishing.

It's probably not going to flood as in big event despite the creek and the proximity of the sill height to the 100 year baseline.

But it probably will back up (as I said before, I'm starting to think that dealing with the sewer line is now a pre-project dependency). As it has done every ~6 months. But the back up really is in the <1" of water category so some gapping at the bottom of the wall might be adequate as long as my baseboard and flooring choices are water-resistant. Meh. Remodeling and actually fixing problems either requires a big stomach for work or a big wallet. Makes me want to just build new square footage but then the house is (mostly) big enough and has good bones.

I would guess that someone makes an alarm that can detect sewer backup early enough that you can stop putting more down it before it floods into the basement. Or just dig up the yard and put well sealed PVC pipe in.

Several of the foam-reclaimers in my neighbohood advertise primarily on craigslist, eg:

http://worcester.craigslist.org/mat/3994014004.html

You can't get away with using rigid rock wool in a basement app anyway, since it's both extremely vapor permbeable & somewhat air-permeable, and you'd end up with moisture/frost at the concrete/rock wool boundary in your climate. If you put a vapor barrier on the interior side you end up trapping ground moisture in the fiber- it's a no-win proposition.

You only need to hit R15 whole-wall after thermal bridging, but you need at least R5 of something air-impermeable for dew-point control with 3.5" of fiber, in a zone-5 location to skip interior vapor retarders on the above grade section ( http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm ) but somewhat vapor permeable between the fiber layer and the concrete. With 1.5" of EPS and 3.5" of rock wool batts you're at R20, as long as you don't thermally bridge it fully with steel. You can get the full-R out of it if you used 1/2" deep steel furring 22.5" o.c. to trap the batts, using standard steel framing top plates rather than turning steel 2x4s 90 degrees. but either WOULD meet the R15. Just because the batts are cut to fit 24" stud bays doesn't mean you can't just butt them up and trap them in place- they don't actually need side compression to stay put- just some amount of face pressure is good enough.

R15 rock wool batts are fairly air-retardent, but also extremely vapor permeable- it's an assembly that would have some haze condensation on the foam at the upper part of the wall during the overnight hours, re-evaporating during the day, but there would be no mold-inducing moisture accumulation.

The difference between 1.5" EPS + 3.5" of rock wool and 3" of XPS or 3" of polyiso is 2" of depth, and an improvement of about R5.

If you really want the 2" back, go for polyiso, and use fire-rated Thermax to get around the thermal barrier issue:

http://blog.energysmiths.com/2011/08/basement-insulation-part-3.html