Greetings from the Northeast,
I have a unfinished basement room 14×25 that had a french drain installed which has a 3/8 inch thick plastic membrane that is exposed 5 inches above the floor. I plan on installing a steel stud wall against the outer basement wall but how to do it. Do I leave this 3/8 inch membrane and the same amount up the entire wall for air to circulate? If so, then do I need a vapor barrier on this side and after insulation another vapor barrier against the green board sheetrock of the finished wall? Am I getting pushy here? How does the wall meet the top of the cinderblock wall and the rim joist? Thanks for any insights here!
Replies
That 5" of membrane is designed (in theory) to make any water that weeps through the wall run down the wall behind the membrane and into the drain system. So my inclination would be to keep at least that much air space and maybe more, use metal studs to avoid any rot problems, and put a good continuous vapor barrier on the side of the stud facing the living space.
But I can easily imagine others disagreeing. I'm wondering about air circulation behind the studs, and the possibility that metal studs will get too cold and moisture condensating anyway. I wonder if some sort of exhaust fan would be warrented?
This may or may not help you, it depends on some of the other conditions in your basement. After lots of research (on this forum and others, buildingscience.com, and other misc. sources) this is the basement finishing system I think makes the most sense. I'm assuming since you needed a french drain system around your perimeter (exterior walls not waterproofed) your slab floor probably doesn't have a vapor barrier under it either. So even if you've dealt with the issue of liquid water leaking through your walls, you will still have a significant amount of water vapor passing through both your walls and through the floor. With your basement unfinished, any of this moisture dries quickly to the inside. Your other moisture source is going to be humid summertime air condensing on the cold concrete surfaces. My belief is this (and of course some will disagree), even with careful attention to detail, it is not likely that you can obtain perfect air and moisture sealed finished walls in a basement. Therefore it is better to assemble a finishing system that allows for the inevitable entrance of water vapor and condensation and deal with it by isolating your framing and flooring and allowing the walls to breathe and dry to the inside. So here it is...
Insulate by using foamboard insulation against the foundation wall. (I typically use 2" foamular). Seal the top edge and exposed side edges with a foamboard adhesive. Tape all seams and corners. Do not seal the bottom edge. In your case, start the foamboard just above the exposed membrain. This keeps the dew point on the foundation wall side of things, and provides a thermal break between the foundation wall and framing.
Use a floating floor system (there are many types that would work). I think this years' Tools and Shops issue of Fine Woodworking compare a few. I personally like Dri-Core. This will give you an airspace under your flooring and isolate it from any water that condenses on the concrete floor. It has a pretty high bearing capacity, so all basement perimeter and partition walls can be constructed on top of it.
Frame your walls however you wish; I use wood because it is now completely isolated from any moisture sources and I just like working with it better. 2 x 3's are adequate because you don't need insulation in the wall cavity and it helps with the space you lose with the foam against the wall. Don't use a vapor barrier when you sheetrock. Any moist air that gets into the wall cavity isn't any different than that same air on the outside of the wall cavity. It doesn't have a surface to condense on that will support mold or mildew growth.
Good luck!
I did that on a basement this year.In addition to the weep channel the foundation had cracked was leaning in 4" at the top. The REAL CAUSE, surface grading and drainage issues, had been fixed. And vertical I-beam post had been installed to support the foundation.So I used 2" XPS and lapped the bottom over the weep channel (it was easy with the 4" lean in the wall). A few daps of adhesive to hold it in place. Then sealed top, bottom, and edge with Great Stuff.The built a plumb 1 5/8" steel wall infront of it.
I did the same thing this year in a basement at home. XPS foam on the walls and 1 5/8 steel studs. I am going to put ceramic tile on the floor, with radiant heat underneath and Ditra. The Ditra is a crack suppression membrane but it will also keep the moisture from coming up through the slab.
Billy
Thanks Billy for the help. My question is does this finished area provide comfortable (heated in the winter) living space? With what I assume will still be cool walls with sheetrock and heated tile under foot ( or none at all but added furnace vents) am I creating a problem of moisture by adding more heat to accomodate again a comfortable room? JP
Yes, it will provide comfortable heated space in winter.
The coolness of the walls depends on the thickness of the foam insulation that you install on the walls and the quality of the installation (no gaps, etc.). Since most of the basement walls are underground, they will be warmer than the upstairs exposed exterior walls with the same amount of insulation...
The heated floor does not solve the moisture problem, but it can help dry out a floor that is otherwise moist. The foam walls greatly reduce moisture instrusion, as does the Ditra under the floor tile.
See http://www.buildingscience.com for more info on construction techniques and basements in particular.
Spend some time with this article, newly updated in December 2004:http://www.buildingscience.com/resources/foundations/basement_insulation_systems.pdf
Billy
Edited 12/21/2004 2:42 pm ET by Billy
Edited 12/21/2004 2:46 pm ET by Billy
Thanks Billy for the information and your rich knowledge that you have shared! JP
I would not think that Ditra avoids the problem of condensation on the floor. For that you need 1" foam insulation or something like Dri-Core on the floor, but then your floor is no good for tiling.....
I agree that Ditra does not solve the problem of condensation on the floor, but it does stop (or reduce) the moisture migration into the basement living space from the ground. The heated floor raises the surface temperature of the tiles enough to prevent condensation. It would be best to have foam under the slab, but it is an existing slab...
Billy
Is 2" of polystyrene foam really necessary. The 1" foam here in Detroit is toung and grove so it seals at the edges better. The thicker foams have square edges requireing more sealing. Remember with foam there is no air to air transfer so the effective R value is much higher than that of lets say fiberglass. Even the best sealed walls allow some air circuilation in the fiberglass.
Let's do the math. if the foam is lets say R5 per inch (I think this is pretty close). The concrete block was lets say R0.5. The heat loss with 1" of foam is 10 times less or 1/10th (0.1) of the original heat loss. Adding 1" of foam for a total of 2" gives R10 total for 20 times less heat loss or 1/20th (0.05) the heat loss. What is the difference between 0.1 or 0.05. Not much (0.05 actually). I don't see a real advantage of adding the extra inch with a lot of extra cost. Given that the original concrete block had almost no insulating value the 1" of foam is infinately better and allows no moisture or air transmission. Also if you use an interior framed wall (1-5/8" as described above) and air is trapped between the drywall and the foam you will get a little extra insulating value.
What do you think?
Also how do you attach the foam to the concrete walls. Construction adhesive or fasteners of some type. I put a post last year about gluing the foam to the wall and then the drywall to the foam. I recieved some replies that this method was being used at some commercial sites. What are your opinions on this glue/glue attachement method?
Tim
"Remember with foam there is no air to air transfer so the effective R value is much higher than that of lets say fiberglass. Even the best sealed walls allow some air circuilation in the fiberglass."Not exaclty.The most common test for R-value puts a heated plate on one side of the product under test and another plate on the otherside and measure the heat transfer.So it does not have anything to do with air leaking in the wall, because the wall is not tested.Now there have been some studies of RLW (real life walls) and if you do some searching on the DOE sites you can probably find those. Also a basement won't get the extreme cold where internal air currents will be formed within the FG which causes it's effect R-value to decrease in cold weather.
Bill,
Your right. I guess I should have said a better more effective R-value.
Still looking for an answer to whether the extra 1" of foam is worth it or not.
Tim
I used 1 inch foam. Remember, most of the basement is underground. 2" is better but I suspect you get most of the improvement in insulation & moisture control with 1".
Billy
I think the way to decide on the amount of insulation depends on the temperature difference across the wall and the cost of energy. For simplicity, only consider conductive losses and you can estimate them with this relationship among the variables:
BTU/hour = Area (ft2) * (T1 - T2)(oF) / R (hour*oF*ft2/BTU)
If your natural gas supplier bills in therms or dekatherms, there are 100,000 BTU/therm.
As an example, consider a square foot of concrete 6" thick, with an R value of 0.08/inch. If you hold the inside of the concrete at 60oF and the outside at 30oF, the heat loss is 62.5 BTU/ft2/hour.
Suppose you have 150 ft2of such wall above ground and your power company gets $0.892/therm (like mine did last year):
150 ft2 * 24 Hours * 6.25 *10-4 therms/(ft2*hour) * $0.892/therm = $2.01/day.
Now, glue on some R6 beadboard and the energy loss goes to:4.6 BTU/hour, thus the R6 in this case yields a 93% reduction and the cost drops to about $0.13/day. But if you double the R factor, you'll cut that cost in half. So, area, R, and temperature difference all come into play when you are making the decison. I cut some of the above out of a web page I worte after insulation my basement and cutting energy losses by about 30%:
http://www2.umt.edu/Geology/faculty/sheriff/Sheriff_Vita_abstracts/Basement%20Insulation%20Energy%20Savings.htm
Steve,
By your calculations adding an extra 1" of insulation would only save $9.50 a heating season in Michigan. I did the calculations then my computer locked up so I'm not going to do them again. What you proved to me is it would take a long time to recover the cost of adding an extra 1" of foam to that basement wall. I'll stick to 1" T&G extruded polystyrene.
Tim
Steve,
(Different, Tim) You said,
"As an example, consider a square foot of concrete 6" thick, with an R value of 0.08/inch. If you hold the inside of the concrete at 60oF and the outside at 30oF, the heat loss is 62.5 BTU/ft2/hour."
Your calculations are wrong. If you account for only the resistance to heat transfer of the concrete, you are right, but that's not how it works in the real, physical world. You have to account for both surface effects. With air on both sides you have to add 0.17 for the outside surface (assuming certain design conditions) and 0.64 for the inside surface. R=1.29, not 0.48 as you calculated. If one side is below grade, R=1.76, for the first 3' below grade.
For a 30 deg DT, adding extra insulation is a waste. In areas of the country where heating costs are significant, an 85 deg temperature difference is more the norm (it was -5 F this am), and going from R-1.3 to R-6.3 is a significant improvement.
Edited 1/14/2005 11:24 am ET by Timbo
typically... 2" foam costs twice as much as 1"... BUT..
there are also some practical considerations..
1) use EPS instead of foamular or XPS.. it costs a lot less with only minimal redcution in R-value.. and contrary to popular mythology.. it does not absorb water
2) 1" foam is almost impossible to protect when prepping & pouring a slab.. two inch is thick enough that it can usually be walked on if it has a level base under it.. but 1" just breaks up and makes a mess..
if you want more compressive strength than you usually get with EPS, spec a higher density.. typical is 1 lb/cf.. but you can also get 1.5 lb/cf & 2.0 lb/cf
i base the above on pouring slabs on foam since 1976
but , hey, whadda i no ?
Mike Smith Rhode Island : Design / Build / Repair / Restore
"use EPS instead of foamular or XPS.. it costs a lot less with only minimal redcution in R-value.. and contrary to popular mythology.. it does not absorb water"No, EPS will absorb water.I have fought enough waterloged EPS to know.But it does not absorb it easily.It has to be under "pressure". In my case they where used for floation devices. On on a float, which was a no, no.The other in a sailboat that had a thin plastic shell over it.Over the years the shell started cracking and taking on water.
the studies i saw were scandinavian.. and pretty much refuted the idea that eps absorbs water
as to your flotation device.. it sounds like the SHELL that contained the eps is what absorbed the water, not the eps itself
for about 20 years we would NOT use EPS because of what you are saying... that it absorbs water.. the testing that i read convinced me otherwise.. and
since eps is one of the few foams that is availabel with borates, that clinched it..
no absorption... no vermin... costs less... available in higher densities .... hmmmmmmMike Smith Rhode Island : Design / Build / Repair / Restore
No, there where two different cases. One the eps was used as floation on a small lift that held a sailboat.Just the foam in the water.The 2nd case was the saiboat that had the shell.Drain holes where put in the sail boat and after setting for months no signficant amout of water came out.However, foam should not be used in building construction under those kind of conditions. In fact one should not be building a house under those conditions.But the fact is EPS will absorb signficant amounts of water.
<<<<But the fact is EPS will absorb signficant amounts of water.>>>>>
actually bill, that is not a fact....
http://www.epsmolders.org/4-ruling.html
if it were a fact.... we couldn't build with SIPS, or ICFs, or GEOFOAMS...
i think you are confusing the " minimal amounts of absoption"
with "degrading from significant amounts of absorption"..
they are not the same thing
Mike Smith Rhode Island : Design / Build / Repair / Restore
<<EPS is non-hygroscopic and does not readily absorb moisture from the atmosphere.>> http://www.epsmolders.org/4-ruling.html
You guys are talking about two different things -- water absorbtion from the atmosphere and water absorbtion while immersed in water. Hmmm... which one should apply to foam under slabs?
I'm the first to admit that I don't know the answer. Just adding my 2 cents.
Billy
Edited 1/14/2005 11:19 pm ET by Billy
"i think you are confusing the " minimal amounts of absoption" with "degrading from significant amounts of absorption".."No you are the one that is confused.In http://forums.prospero.com/tp-breaktime/messages?msg=51493.19You said. "1) use EPS instead of foamular or XPS.. it costs a lot less with only minimal redcution in R-value.. and contrary to popular mythology.. it does not absorb water"If it does not absorb water how can you have "degrading from significant amounts of absorption".And your link talks about absorbing moisture from the air. Not ABSOORBING WATER which is the term that you used.And if absorbing water is not a problem why do they add this; "Although EPS provides a high level of moisture resistance and breathability, recommended design practices for walls and foundations should be followed in the selection of vapor and moisture barriers for severe exposures."
bill... the question is what to put under a basement slab...
the answer is EPS
here's the properties of the material:
<<<
R-Control EPS Properties
Property
TypeXI
TypeI
TypeVIII
TypeII
TypeIX
Nominal Density
lb/ft3(kg/m3)
0.75(12)
1.00(16)
1.25(20)
1.50(24)
2.00(32)
Density1, min.
lb/ft 3(kg/m3)
0.70(12)
0.90(15)
1.15(18)
1.35(22)
1.80(29)
Design Thermal Resistanceper 1.0 in. thickness
75ºF
ºF•ft2•h/Btu (ºK•m2/W)
3.22(0.57)
3.85(0.68)
3.92(0.69)
4.17(0.73)
4.35(0.77)
40ºF
ºF•ft2•h/Btu (ºK•m2/W)
3.43(0.60)
4.17(0.73)
4.25(0.75)
4.55(0.80)
4.76(0.84)
Thermal Resistance1,min.,per 1.0 in. thickness
75ºF
ºF•ft2•h/Btu (ºK•m2/W)
3.10(0.55)
3.60(0.63)
3.80(0.67)
4.00(0.70)
4.20(0.74)
40ºF
ºF•ft2•h/Btu (ºK•m2/W)
3.30(0.58)
4.00(0.70)
4.20(0.74)
4.40(0.77)
4.60(0.81)
Compressive strength1@10% def., min.
psi(kPa)
5.0(35)
10.0(69)
13.0(90)
15.0(104)
25.0(173)
Flexural strength1,min.
psi(kPa)
10.0(69)
25.0(173)
30.0(208)
40.0(276)
50.0(345)
Water Vapor Permeance1of 1.0 in. thickness,max., perm
5.0
5.0
3.5
3.5
2.0
Water Absorption1by total immersion,max., volume %
4.0
4.0
3.0
3.0
2.0
Oxygen Index1
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 1/15/2005 6:45 am ET by Mike Smith
Seems like you would want to use rigid foam, and keep any moisture activity sealed behind it. You actually want as little air circulation as possible (and no exchange with the living space), while allowing a drainage plane. The foam will also prevent moisture from condensation. Then build a wall in front of the foam.
How will you treat the floor? That part can be tricky...