# Rigid foam on basement walls - gap or no gap?



## Gary in WA (Mar 11, 2009)

They are wrong. No air space, do you need links?

Gary


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## spaceman spif (Jul 28, 2009)

Gary in WA said:


> They are wrong. No air space, do you need links?
> 
> Gary


Actually it was some of your previous links that caused me to question what the Drylok rep told me. My plan is to glue 2" XPS against the walls from top to bottom, seal with proper tape, put up a frame in front of that (2x4 studs turned sideways, with pressure treat along the floor), then drywall against that.


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## Gary in WA (Mar 11, 2009)

Some AHJ require foamboard fire-stopped every 10' horizontally rather than at the studs gap, and cover tops, check locally; http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_6_sec002_par017.htm

Per location for your Zone ?; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Ohio
http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements

Add a strip of fb under the plate (may not require pt now) for a thermal/air/capillary break from the slab/earth below; http://www.greenbuildingadvisor.com...ressure-treated-sill-plates-and-building-code

Gary
PS. Closest city?
2" fb may be overkill.....


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## spaceman spif (Jul 28, 2009)

Gary in WA said:


> Some AHJ require foamboard fire-stopped every 10' horizontally rather than at the studs gap, and cover tops, check locally; http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_6_sec002_par017.htm


I'll have to look more into this. Not sure about local code.



Gary in WA said:


> Per location for your Zone ?; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Ohio
> http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements


According to your link, Greene County, Ohio, requires an R value of 10 for continuous sheathing and 13 for cavity insulation. The 2" rigid foam provides either 10 or 13, depending on which brand I buy. 1-1/2 only gives me 7.5.



Gary in WA said:


> Add a strip of fb under the plate (may not require pt now) for a thermal/air/capillary break from the slab/earth below; http://www.greenbuildingadvisor.com...ressure-treated-sill-plates-and-building-code
> 
> Gary
> PS. Closest city?
> 2" fb may be overkill.....


I was wondering if I should lay something under the plate or not, since I was going to use PT. I couldn't read the link, so looks like I'll have to create an account on that page.

I kept wondering if 1-1/2 XPS would work "well enough" for me, especially since I'm on a tight budget and I'll likely have to do this in more rooms in my basement. But it looks like I won't meet code for the R value.


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## Gary in WA (Mar 11, 2009)

That is for one OR the other.... Fb alone, with furring strips (hard to wire outlets) = R-10* OR *meet code (R-13) cavity insulation) with frame wall and only enough foam to lower dew point in cavity against condensation. With 68*F room air, *R-5 *(unfaced XPS or ESP) Code suggested minimum; http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements to prevent cavity condensation above grade (below grade is way less due to air/water/wind blockage of concrete wall). 

Figuring dew point: 68*F room, Delta T at foam/cavity insulation is 42*F due to placement at 3/4 the way in wall from inside, with a dew point of* 39% Relative Humidity*. Safe at/below that percentage, if more humid and without a dehumidifier, thicken the fb to *R 7.5* XPS (unfaced) + R-13 cavity = *44%RH.* * R-10 = 49%RH.* 

If the fb. cost is prohibitive, use R-5 (1" XPS) to stop condensation keeping the RH in room at/below 39%RH with R-13 fiber or R-15 Roxul ($$$). Use fb under plate won't need pt ($), check with AHJ. It stops air, stops wicking water/moisture from slab/earth, and stops the "heat sink" effect (thermally) of wall on slab. 2" XPS is R-10, less after aging 25 years. Polyiso is R-13 or so, better because "greener" at manufacturing but don't use any with facer coverings to stop the moisture drive.


"Search" foam board, basement insulation, rim insulation, etc. for proper way to install in search box at top right side every page. Or just click on my *name *(in blue) and read posts answered by me (only about basements in title thread), lol. 

Gary


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## spaceman spif (Jul 28, 2009)

Gary in WA said:


> That is for one OR the other.... Fb alone, with furring strips (hard to wire outlets) = R-10* OR *meet code (R-13) cavity insulation) with frame wall and only enough foam to lower dew point in cavity against condensation. With 68*F room air, *R-5 *(unfaced XPS or ESP) Code suggested minimum; http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements to prevent cavity condensation above grade (below grade is way less due to air/water/wind blockage of concrete wall).
> 
> Figuring dew point: 68*F room, Delta T at foam/cavity insulation is 42*F due to placement at 3/4 the way in wall from inside, with a dew point of* 39% Relative Humidity*. Safe at/below that percentage, if more humid and without a dehumidifier, thicken the fb to *R 7.5* XPS (unfaced) + R-13 cavity = *44%RH.* * R-10 = 49%RH.*
> 
> ...


Man...you provide so much help on this site for free it makes me feel guilty sometimes!

From what I've read, 2" XPS moves me to a better class of vapor retardant (class II), than the 1" XPS plus unfaced batt. However, 1" XPS plus unfaced batt is much cheaper and would easily meet my R requirements for this region and still allow me to be able to feed my kids.

I see no signs of water issues on my walls (I know some water will always work it's way in, but my walls show no signs of excess water problems). Is the 2" XPS worth the extra cost in my case? Or would I be throwing money away for little to no benefit?

Next question - the rim joists. There is no insulation there, and in order to be able to reach in there and add insulation I would have to remove some of the drywall along the ceiling, creating more work. What's the best way to address that?


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## Gary in WA (Mar 11, 2009)

Lol, thank you for the compliment, we are here to help!

I forgot to mention the figures I used for your location specific dew-point calculations were taken from the three months lowest averages temps- both high and low;http://www.weather.com/weather/wxclimatology/monthly/graph/45385

So if you have a furnace with its own supply air and room so it wouldn't be adding extra humidity- or wood stove, clothes washer, deep sink, etc. that add humidity, quality windows, etc.... you may not need a dehumid. and rely on thicker fb to be safer. Check your RH in the room....with  hand=-held unit.

IMO, use the 1/2" with ADA drywall; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Rims should be f.b. first, then any fibrous cavity insulation to meet/exceed min. code for "floors" location specific per links. Canned foam the XPS (cut 1/2" smaller than needed) with a good bead all around to make *air-tight*. I'd go with 2" there because of the cavity fill added afterward puts the dew-point farther into the wall; (thicker cavity fill = thicker foamboard required) pp. 16-18, notice pp.7 also- thickness of fb are both same Class 2.... http://www.buildingscience.com/documents/guides-and-manuals/gm-guide-insulating-sheathing

IMO, cut the ceiling back 16" (to fit 16" rips from drywall 4x8 = times three pieces) that gives more room than 12" to work in. You need a fire-stop on the joist bottoms before adding frame wall to prevent fire from floor cavities running across ceiling, up a plumbing hole at vent you forgot to air-seal, and reaching attic, burning house down-and up same time. http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_6_sec002_par017.htm

Excellent read; #21, 22; http://www.diychatroom.com/f98/how-fireblock-framing-37190/index2/

As per your other thread on faced fb (PIC) or not... third paragraph below map; http://www.buildingscience.com/docu...ns?topic=resources/vapor_barrier_code_changes

Basically, FOIL faced will work fine at/below grade but cause condensation above grade areas with summertime moisture drives. With the ff (vapor BARRIER on all concrete wall, moisture has no outlet from inward vapor drive after saturation and wicks upward to rot the sill plate unless there is enough exposed concrete at exterior above grade to dissipate to outside air. Rather risky to figure percentages- beyond my expertise... safer just to use faced at grade and below to "frost-line" per location, and unfaced on rest of wall;* Figs. 11 and 12; *http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems

To me adding Dryloc just slows/stops the water, redirecting it down (gravity) on the exterior of wall to pool on the footing/wall joint (above drain-tile) where liable to enter there. Use a compatible foamboard adhesive, apply in 1' square grids pattern to limit air travel if you missed a 1/16" perimeter crack somewhere...lol. Budget wise, use some caulking where possible as part of the grid pattern other than adhesive requiring bond 16"oc. Make grids continuous; The air-sealing part is the most difficult of finishing a basement. *Pages 10-11;* https://docs.google.com/viewer?a=v&...iYVZpx&sig=AHIEtbRVXHNlbPlXiHMzGIwFLQF3YvmyFw

Foam board air gaps; http://joneakes.com/jons-fixit-database/743

Gary


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## Rowden (Sep 2, 2013)

Gary in WA said:


> Use fb under plate won't need pt ($), check with AHJ. It stops air, stops wicking water/moisture from slab/earth, and stops the "heat sink" effect (thermally) of wall on slab. Gary


 I've read this in several other occasions but I don't see how you can securely anchor the bottom of the wall. Do you Tapcon through the bottom plate & XPS ? These anchors would have to be 4+ inches for 2" XPS. Is 1" XPS sufficient? What is the minimum thickness of XPS to achieve the benefits listed?


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## Gary in WA (Mar 11, 2009)

Yes, long tapcons if going minimum code= R-10 first 2' in from exterior wall perimeter; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Ohio

Though, 1" of EPS would slow the heat sink loss through the wall/slab by 79%, XPS by 84% using R-3.6 per inch; http://www.google.com/url?sa=t&rct=...VjgAkW8XzKgQkGg&bvm=bv.44442042,d.cGE&cad=rja

Doubt you would get condensation: with 68*F inside temp and 55*F slab temp- with R-13 cavity, only 10*F difference between slab/room, as 6' below grade warmer by 11*F. http://www.epa.gov/athens/learn2model/part-two/onsite/ex/jne_henrys_map.html

Gary


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## Mike Aur (Mar 11, 2021)

spaceman spif said:


> *Do you agree with what the Drylok rep told me?*
> 
> I'm redoing the walls in my basement (vapor barrier + paper faced batt = yucky mold). I had to grind the existing paint/drylok off the cinder block walls because it was peeling off in big chunks (not from moisture, but because they painted the drylok on top of paint) and I am planning on gluing rigid foam against the walls.
> 
> ...


I went through the same thing with the customer service person and didn’t agree with her so I contacted DuPont maker’s of the rigid foam board I’m going to use. I told them I have 3 coats of Drylok on my cinder block wall. The said no problem as the rigid foam acts as a vapor barrier as well. The Drylok rep said you need to put some gypsum board in between the Drylok and insulation. Which to me sounds Totally ridiculous. I’m sticking to my plan but if anyone has gone further and had negative results please let me know.


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## Nealtw (Jun 22, 2017)

Mike Aur said:


> I went through the same thing with the customer service person and didn’t agree with her so I contacted DuPont maker’s of the rigid foam board I’m going to use. I told them I have 3 coats of Drylok on my cinder block wall. The said no problem as the rigid foam acts as a vapor barrier as well. The Drylok rep said you need to put some gypsum board in between the Drylok and insulation. Which to me sounds Totally ridiculous. I’m sticking to my plan but if anyone has gone further and had negative results please let me know.


Any one that tells you that water can be stopped by anything on the inside is full of it to start with.


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## J Briggs (7 mo ago)

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