# Basement Ext Wall - Will Rigid Insulation allow my wall to dry inward?



## Windows on Wash (Aug 30, 2011)

The short answer to you question is...depends.

As you have identified, it will depend entirely on the permeability rating of the foam.

There never would be a vapor barrier applied to the exterior wall (given the lack of moisture in the air at that altitude and even less likely given the climate region).

If the issue is bulk moisture, that is what needs to be accounted for and revisit the foam/wall insulation question once you have corrected that issue. A bit of efflorescence (especially with high mineral content in ground water) is completely normal and to be expected if the home is not brand new. 

Class II or III vapor retarder rated foam will allow the small amount of moisture to dry to the inside. Fiber faced iso will give you a good permeability rating without trading off R-Value drops of EPS at a per inch comparison. EPS is cheap and well proven though in moisture exposure areas. 

How old is the home and how much efflorescence? If it is just a bit, make sure the grade is proper and you should be fine with rigid foam (proper perm rating). If it cannot dry to inside, it may run down the foam and pool on the floor where it will dry out shortly thereafter. If you are concerned, I would beta test the wall assembly with just EPS/ISO foam and furring strips and drywall. Look for moisture accumulation and adjust your plans accord to visual results.

I am not a fan of the waterproofing paints out there and don't think they are that effective.

Leaving and air gap to the basement somewhat defeats the purpose of the insulation schedule. Convection will be great for drying the wall when necessary, bad for accomplishing the goal of warming the space.

The floor would be the same schedule as the walls in most cases. 

If you don't see water, you are probably fine. 

Make sure the exposed CMU block is painted on the exterior and moisture is being diverted from the home. If you are worried about the creek and water table, a french drain around the home should de-water it effectively.


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## COLORADOMTNMAN (Jan 13, 2012)

Thank you for the reply! 

Unfortunately, even though we have high altitude, our area is actually quite damp. We are on the north side of the hill, all summer long, nothing really dries, it rains each day. Moss is growing everywhere, the ground is always wet when I dig it up. Winter of course, is quite dry and everything is frozen. Spring we get major runoff the hill behind the house. 

The mountain behind the house is very steep, probably 35 deg and basically slopes directly into our foundation wall. There is no perimeter drain that I am aware of. I know that it would be ideal to dig up the whole back of the house and, install a perimeter drain. However, due to the exterior landscaping, wood storage lean to, and most importantly the inability to get machinery back there (it would have to be dug by hand) it would be quite expensive and not too practical. I am hoping to find a way to do this remodel without digging up the back of the house.

Maybe that's the key, we haven't had any "bulk water issues", so all we need to do is use a foam that won't act as a total vapor barrier, and let the wall dry both to the outside, and the inside. The part the keeps troubling me though, is that they say in "very cold" regions, you are supposed to go with a Class I vapor retarder. Looking at that map on the BSC website, it looks like we are in that little blue bubble west of the flatlands in Colorado and falll into this region type.

Besides, if the concrete is "wicking" won't it pull water through the concrete as it passes down by the wall on the way to the perimeter drain?

House was built in 1981, and at that time, our County didn't care much what was going on "up there" in the hills and nobody built anything too well. Actually, we suspect the foundation may be from an earlier "hunting cabin" from the 30's or 40's. A neighbor remembers another house there before our current 2 story house. 

Nonetheless, there really isn't much efflorescence, except in a corner by where the well water line comes in, which will be made into a small wine cellar, off to the side of the finished room.

I'm just nervous to introduce drywall into this basement without a vapor barrier. If I do a Class II or III and water is drying inward, am I going to get moldy drywall?

You say the convetion behind the wall would be good for drying, bad for heating. I don't know that it would be *that* bad for heating would it? I was thinking of using 2 layers of foam plus unfaced batt in the stud bays. That's a lot of R value.

I can't do a test, its winter. We have a baby on the way, and need to get the toy room relocated to the basement, can't wait for summer to start the remodel.

If water can't dry inward and runs down and pools under the floor (which will be insulated in the same way) don't you think I'll get mold eventually? I've read it can grow on just about any surface, regardless if the surface is providing "food" or not. I am worried about mold because we've found it in other places around the house doing remodel work.

Thanks!!!


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## Windows on Wash (Aug 30, 2011)

COLORADOMTNMAN said:


> Thank you for the reply!
> 
> Unfortunately, even though we have high altitude, our area is actually quite damp. We are on the north side of the hill, all summer long, nothing really dries, it rains each day. Moss is growing everywhere, the ground is always wet when I dig it up. Winter of course, is quite dry and everything is frozen. Spring we get major runoff the hill behind the house.
> 
> ...


As I stated previously, if you are concerned, you would be well advised to set up a test wall and see what it yields.


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## Gary in WA (Mar 11, 2009)

No Tuff-R, no air gap, no concrete sealer. http://www.buildingscience.com/docu...merica-high-r-foundations-case-study-analysis

http://joneakes.com/jons-fixit-database/743

Thickness of foam board: http://www.buildingfoundation.umn.edu/FinalReportWWW/Chapter-5/5-optimum-main.htm

Fire-block at top (floor joists) and every 10' lineally; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec002_par031.htm

Gary


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## COLORADOMTNMAN (Jan 13, 2012)

thanks for the links. The one on the air gap is great. Makes sense, air gap is bad.

In the BSC case studies, what I found interesting was in Case 6 (CASE 6 : 3.5” 2.0 PCF CLOSED CELL SPRAY POLYURETHANE FOAM), where they said:

"Because closed cell spray foam is an air and vapor barrier, there are no risks to air leakage or vapor diffusion condensation. The concrete is unable to dry to the interior through closed cell spray foam, but concrete is generally not affected by a high moisture content. Figure 21 shows the relative humidity in the middle of the foam does not exceed 80%, which means there are no moisture related risks from vapor diffusion."​ 
This is interesting, because this seems to indicate that it is better to use a vapor barrier up against the concrete to prevent "vapor diffusion". They say there is no problem with the concrete being unable to dry to the inside.​
So, why would I not want to use DOW Tuff-R since it is less permeable than other types of XPS rigid insulation according to my research on manufacturer websites? 

But then, in the conclusion it says:

"Besides bulk water movement, which is not specifically addressed in this report, there are two modes of wetting in the foundation; vapor diffusion and capillary wetting. The exterior surface of the below grade portion of any foundation wall is maintained at approximately 100% relative humidity so moisture movement below grade is always to the interior and drying is not possible to the exterior. The IRC has been modified to reflect this, not recommending a Class I or II vapor control layer on the interior of any below grade wall."

Doesn't this directly contradict what they said in case 6? The do not recommend a Class I or II vapor control layer on the interior of any below grade wall, because drying to the interior is not possible, and it can't dry to the exterior. This is exactly my concern!

So, then based on that, maybe a vapor permeable XPS foam is better?

In the Jon Eakes article on the air gap, it references his article on moisture proofing the basement wall. He says:

"Insulation pushed directly against the basement wall will effectively prevent these air convection loops.

Heavy condensation discovered behind insulation that is snug against the basement wall is a sign of either a very poor job of sealing the warm side of the wall or of a need to damp proof the foundation wall."

If you look up the article on moisture proofing the wall that he references, it says:

"If there is no water leakage problem, but the basement walls are a bit damp, you must cover the walls before insulating. Waterproofing coatings are not really necessary for moisture proofing. Polyethylene plastic sheets or horizontal strips of building paper are sufficient.

If the moisture is very minor it can usually be ignored. If the wall has a history of always being dry, you can ignore this complication all together."

My walls have never been "wet" but the basement defintely is musty. Guess I can skip the vapor barrier?

according to the case studies, case 8 is most like what I was planning to do. 

8. CASE 8 : 2” XPS, 2X4 FRAMING WITH FIBREGLASS BATT

"The water vapor diffusion and capillary wicking are controlled by 2” of XPS insulation assuming that the XPS is well sealed to the concrete.​ 
(This case) had some condensation potential but improved performance with a vapor retarding paint. There was some potential for air leakage condensation at the above grade section of the wall in the winter alternating with drying periods."​ 
I don't know if I'm going to sleep soundly at night knowing that I have a "condensation potential" behind my walls. Its frustrating you can't get a consistent recommendation.​


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## COLORADOMTNMAN (Jan 13, 2012)

I was just reading this book:
http://www.amazon.com/Insulate-Weatherize-Tauntons-Build-Like/dp/1561585548

They recommend XPS which allows the walls to dry inwards. This seems more logical to me than trapping any moisture that comes through the cmu wall back behind the insulation. They also say that you could consider using cement tile backer instead of drywall to avoid mold. Sounds like a winning combinations to me.


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## Gary in WA (Mar 11, 2009)

Here is more reading for you.
Now we’re getting into it….. ESP vs. XPS water retention and loss of R-value; http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0568/0901b803805681b5

The debate of a vb, or not, and the answer is: https://docs.google.com/viewer?a=v&...Gcfigu&sig=AHIEtbRnzexpOuy2DmtpoAYQmvK_bHtRKg

Show the moisture and drying of both boards in a different application, real-world test; http://docserver.nrca.net/pdfs/technical/317.pdf

If you don’t mind the XPS at 80% RH most of the time, leave out the poly: http://www.buildingscience.com/docu...ing-hygrothermal-modeling-basement-insulation

In a nutshell (even though from the manufacturer); https://docs.google.com/viewer?a=v&...cpdHN-&sig=AHIEtbTrXqQSudoHqqBg3V4gVambxIzD3A

And finally, the actual test by BSC (my favorite): http://libdspace.uwaterloo.ca/bitstream/10012/3242/1/Ueno MASc Thesis.pdf


Gary


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## COLORADOMTNMAN (Jan 13, 2012)

thanks!
I couldn't get the dow link to work. But I read this instead:
http://www.dow.com/PublishedLiterature/dh_013e/0901b8038013ede3.pdf?fi

they say EPS can trap water in the foam (4%) and cause mold growth. 

So, Ill go XPS. 

The university test confirms my decision. In their test, they used a vapor permeable foam (open cell), with no additional vapor retarder or barrier and got the best results.

This just makes more sense to me. Let the wall dry out from both sides. I plan to use all materials to build the wall that are unaffected by moisture, and that will let it dry out. Then, I should have less chance for mold. Just need to select the most permeable closed cell, XPS foam.

DOW lists their "blue styrofoam" XPS as 0.3 perm. (Vapor Semi-Impermeable, or Class II Vapor Retarder)
http://dow-styrofoam.custhelp.com/a...-ratings-of-various-styrofoam™-brand-extruded

Owens Corning Formular 150 XPS is 1.1 perm (Vapor Semi-Permeable, or Class III Vapor Retarder)
http://insulation.owenscorning.com/assets/0/428/429/431/e4fb716b-6f2b-46e5-83f6-2da29e2fd5bc.pdf

Owens Corning Formular 250 XPS is 1.1 perm (Vapor Semi-Permeable, or Class III Vapor Retarder)
http://insulation.owenscorning.com/assets/0/428/429/431/5fb32eea-abcf-4c3a-8d0a-b6d32d27dec3.pdf

Owens Corning Insulpink is 1.1 perm (Vapor Semi-Permeable, or Class III Vapor Retarder)

This is the stuff you can buy at Home Depot I think.
http://insulation.owenscorning.com/assets/0/428/429/431/53d1812d-4056-47d3-91ba-9b3b3c8bd1e7.pdf


anyway, not sure I follow that other test case. But it seems they are saying the same thing, that EPS will hold more water.

link doesn't work for that last one either (your favorite). Thanks
Lee


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## COLORADOMTNMAN (Jan 13, 2012)

So what do you guys think about the floor? I was planning to insulate with XPS and then put a bamboo floor over it. Do all of the same rules apply? Most of the sites I've seen tell you to put a vapor barrier down first.


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## COLORADOMTNMAN (Jan 13, 2012)

Holy smokes. wish I would have read this document to begin with:

http://www.buildingscience.com/documents/reports/rr-0309-renovating-your-basment

This sums it up, right on page 2/3 of the document:

"Moisture tolerant insulation installed in an airtight manner on basement walls​and floors warms the first condensing surface above the dew point of the interior air. This reduces the probability of warm moist air condensing on cool surfaces. The best insulation to install on basement walls in many homes is extruded polystyrene that comes in
sheet or panels, 2-feet or 4-feet in width by 8- or 9-feet high. Extruded polystyrene (XPS) provides an R-value of 5 per inch, is moisture tolerant, and does not absorb liquid water, but does allow water vapor (water molecules in the gas state) to slowly pass through it. The semipermeable nature of XPS allows some drying of moisture through the panel but at a
rate that is slow enough to prevent wetting of materials on the dry side of the assembly."

This document also confirmed my question on the floor. It says "Basement floors may appear to be dry but only because ground moisture passing through the slab is evaporating into the interior air space. If this is the case, installation of impermeable flooring may lead to the accumulation of water on the surface of the slab or in the flooring material."
​
"Properly insulating a concrete slab can provide a warm dry surface on​which carpet or other flooring can be installed. If a concrete slab is in good repair and does not have significant water problems (liquid water on the surface) sheets of extruded polystyrene can be installed directly on the slab. The joints between sheets should be sealed with tape or mesh imbedded in mastic. A plywood floor can then be “*floated*” over the XPS insulation in either of two ways. If sufficient room height is available wood sleepers (2X4 or other dimensional lumber) can be installed to which the flooring is mechanically fastened. This is illustrated in *Photograph 3*. If room height is minimal, tongue and groove plywood with the butt ends of the plywood joined can “*float*” above the XPS insulation. Wood biscuits are one method for jointing the butt ends so that one sheet cannot ride up above the next sheet."​


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