# XPS for interior (above grade) basement wall?



## jklingel (Dec 21, 2008)

markley said:


> Anyway, our basement consists of cinder block walls with 2 of 4 walls completely above grade. •• Are the blocks and the 2x4's somehow firred out to be in the same plane, or offset?
> 
> My plan is to attach either 3/4" or 1" XPS to the block walls... •• Why not insulate why you're at it? Use 2"; what is code R value in your area? (Exceed it.)
> 
> ...


 See after the bullets, if I am following you.


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## markley (Aug 4, 2009)

Thanks for the response! Regarding the code R value of the area...I am unsure. I live in western NC, and based on what I can find online F(NAIMA), the recommended value is R2.5 to R6? 

The price of the 2" XPS is a little much for my budget (would need approx. 35 4x8 sheets), which is why I figured I would insulate between the studs as well. 

Also, i have read that it will take entire tube of pl300 to attach the foam board..is that true?


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## jklingel (Dec 21, 2008)

markley said:


> Thanks for the response! Regarding the code R value of the area...I am unsure. I live in western NC, and based on what I can find online F(NAIMA), the recommended value is R2.5 to R6? •• Sounds very low, but I live in an ice box. I'd call a local building code official.
> 
> Also, i have read that it will take entire tube of pl300 to attach the foam board..is that true? •• No idea, but a second tube around is better that running back to the store when you are in the middle of the job.


 Bullets, again. Good luck w/ the job. Insulating is almost always a great thing to do.


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## markley (Aug 4, 2009)

Thanks again..yeah, i'll check the local codes. One more thing, would it be at all beneficial to apply drylock to blocks prior to putting up the XPS? I understand that all the drylock doesn't fix a water problem, it's just that one wall used to have a moisture issue due to improperly installed gutters. This problem was fixed and have not had any moisture over the past 8 months. However, in the event that say the gutters are clogged with leaves and overflows, i was thinking it may be beneficial to have an additional line of defense.
Thanks!


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## Gary in WA (Mar 11, 2009)

Are you in Zone 5? http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_par002.htm

If so- or Zone 4, 6, 7, lol, your f.b. requirements are R-13 cavity or R-10 continuous. http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002.htm

With R-13 fiberglass or Roxul and 2" fb (R-10 XPS) the dew point for your location is 41%RH at 70* room temp.

1-1/2" fb (R-7.5 and fg= 36%RH

1" fb (R-5) and fg= 32%RH

3/4" fb and fg= 30%RH

Gary


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## markley (Aug 4, 2009)

Thanks for the info and links Gary! Not sure if I am understanding this correctly though. Following your example, if I went with 1" XPS and R-13 fiberglass, I would have moisture issues if the humidity reached >32% (inside finished basement?) at 70 deg?

Thanks again!


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## markley (Aug 4, 2009)

Also, I am in Zone 4.


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## Gary in WA (Mar 11, 2009)

Yep, those are the dew points for your annual average low temperatures for your city, not Zone. The inside of foamboard would have condensation if you exceed the relative humidity (inside the room at 70*) stated with the thickness given. Foamboard allows moisture through slowly over a long time period, as it collects. Dryloc stops the moisture where it is applied. Once the concrete is saturated and cannot weep through the wall, it follows gravity (wicking down in the wall like a sponge) and collects/pools at the bottom of the wall, usually coming in at the slab/wall joint area or just to surface through the slab there. Instead of smaller amounts of moisture through a large area to diffuse through the wall all-over, now it will be collected/concentrated at the bottom in a smaller area (along the bottom of the whole wall), depends on the wall height, thickness of the application of waterproofing, capillary drive, hydrostatic pressure in soil behind wall, drainage of back-fill material, exterior waterproofing/moisture proofing, exterior drainage, temperature of wall/soil, downspout deposits, slope of exterior grading, type of soil, etc.

Gary


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## markley (Aug 4, 2009)

So it sounds as though I may be better off not applying Drylock? That will save me a few hundred dollars as well, which I can put towards going with the 2" foam board.

Thanks again Gary! Your comments throughout this forum are much appreciated!!


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## Gary in WA (Mar 11, 2009)

Thank you for those kind words. That is why are here, to help you DIY!

Gary
PS. Thanks also, JK.


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## remodler (Nov 26, 2012)

Just reading through and I'm confused. Your saying there would be moisture issues using the xps of 1". Where would those moisture issues be if the humidity is too high. Would it be in the concrete wall, or in the xps? I thought the purpose of the xps was to move the condensation, if any, to the interior face (room side face) of the xps. That way if there is condensation it is formed on the interior face, not the inside, of the xps where it is on a material that cannot rot and can evaporate. The xps drying capabilities is for moisture that moves through the concrete wall, not condensation... I


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## Gary in WA (Mar 11, 2009)

The moisture (from the room) would condense on the inside face of f.b. next to the f.g. batt insulation, wetting it for a loss of R-value: http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/90-controlling-heat.html

The f.b. can take the wetness, and the f.g. (only the resins to mold that hold it together) because it will later dry with elevated temperatures, but not the wood framing (organic) and could mold. If the moisture condensation is short-term, the basement may be able to continuously dry the cavity insulation so long as the humidity in the room is not excessive and temp. is warm enough.

The insulating sheathing warms the cavity, just as it does above grade. My dew-points were for basements at the "frost-line" and above grade part of the wall with local temps. F.b. above grade warms the wood sheathing and prevents condensation, page 6; http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

Notice the wall permanence, and the f.b. thickness; dictates if a vapor retarder is needed or not.

Gary


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## remodler (Nov 26, 2012)

I gotcha. Thanks for the help. Amazing how much goes into the insulation process. When I first started remodeling I didnt pay that much attention to it. Now it's the part I try to research most...


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