# Doubling up of XPS foam panels



## wasjr (Oct 25, 2015)

I am going to start building out the basement (about 28' X 36') to be mainly used as the laundry and rec areas. There currently is only some poorly installed paper faced insulation stuffed in the rim joists and no other insulation on the block walls. Living in the midwest near Chicago, I need to start with insulation since I melt the snow all around the house for 2' out during the winter.

I have read a bunch and decided that (to me) XPS panels are the way to go. I want to slice up and foam seal XPS panels fit into the rim joist cavity and glue XPS panels on the walls. In pricing the panels out, I see that doubling or tripling up 1" panels is much cheaper than buying a solid 2" or 3" panel. Assuming I stagger the joints and tape them, would that be better than installing 1 thick board? Even doubling the cost of the tape saves money using two 1" panels over one solid 2" (I most likely will go only 2" thick using square cut panels).

Also, is there any worth applying RadonSeal (about $300) or Drylock (about $450) waterproofing as added insurance? I've lived in the house for over 12 years and never have had any water problems and want to keep it that way. 

Lastly, is it better to frame right up to the foam panels, or leave an air gap in front of the insulation? I've read that 1/2" air gap might be good but doesn't seem to be very popular. To me, loosing an inch or so of width is worth it if keeps it more comfortable. Thanks.


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## Windows on Wash (Aug 30, 2011)

No issues with using multiple layers of 1" foam.

I would just caulk the joints as it will hold up better over time. Be sure to caulk between the boards (i.e. glue them together) as well This will eliminate any air movement around the panels. 

I wouldn't bother with the Drylock or RadonSeal. The foam is air impermeable. 

Frame so the walls are plumb and straight. You will have some gaps in some places and none in others. 

I see so benefit in a purposeful gap being left. Be sure to shim the gap between the stud and the foam to help with the clamping force.


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## wasjr (Oct 25, 2015)

Good point about glueing the panels together ... I didn't think about that. Thanks.


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## Fix'n it (Mar 12, 2012)

why not = 1' xps, framing, roxul, drywall ?


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## Windows on Wash (Aug 30, 2011)

More is more. Minimum I would want to see in Chicago would be 2".


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## Gary in WA (Mar 11, 2009)

Drylock is for water (and will direct it to the wall/slab joint to puddle inside- if it works at all; http://neutocrete.com/drylok-an-honest-review, the XPS will slow that enough as a "throttle" to use the HVAC for control. If you have Radon (under slab), handle that separately; http://www.propex.com/C_f_env_radon.htm

The rigid foam board panels shrink in use, butt them together and use furnace cement/mesh tape to cover the joints, use canned foam under them for air-sealing. Apply the boards with a 1' adhesive grid pattern to limit air movement; http://joneakes.com/jons-fixit-database/743

Use foil-faced XPS or plastic faced for stopping summertime moisture ingress through the rim joist. Then the wood will dry out rather than allowing more moisture in the FB to keep the wood wet longer. Caulk the wood joints first, before adding foam w. foam from can to air-seal from infiltration.

For Zone 5 (Chicago) R-5/13 cavity FG gives safety to 36% Relative Humidity, R-7.5/13 gives 39% RH, R-10/13 is 44% RH at 68*F for 3 coldest months per location. Use a dehumidifier or go with thicker FB. Code requires R-10, footnote "c"; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Illinois

Though from an insulating view, R-5 works with a 2x4 wall especially as the earth tempers 6' below grade to +11*F higher than ambient air plus blocks the wind-chill factor above grade wood walls have... if you control the moisture from inside sources.http://buildingscience.com/document...ulating-sheathing-vapor-retarder-requirements

Gary


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## wasjr (Oct 25, 2015)

So if I understand everythong correctly, I will go with 2" XPS foam board glued in a 1' grid pattern directly up against the bare block wall, with the edges taped with furnace tape, and sealed at the bottom with expanding foam. I will then build the wall directly in front of the XPS FB with no air gap.

For the rim joists' cavity, I'll caulk the wood joints first, then use foil faced XPS (neither Menards nor Lowes has it here so I may have to use regular) foamed sealed in.

I hadn't planned on adding any insulation in the wall stud cavity, just drywall covering the outside.

Thanks for the advice. All I have to do now is contact the building inspector and show him the plans.


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## Gary in WA (Mar 11, 2009)

No problem using multiple layers to get code thickness. Just make sure the first layer on the concrete wall is in a grid pattern as air movement defeats it all causing condensation. Air gaps are fine on the room side= frame wall/foamboard but insulation requires an ignition barrier and fire-stopping of fiberglass; (one example);http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_6_sec002_par018.htm 

every 10' horizontally; http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_6_sec002_par017.htm

Local interpretation may require studs against the concrete wall (with a vapor barrier at edge and FB BETWEEN the studs) at every 10' rule for fire-stop; *#7 here*; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_3_sec017.htm

Use some foamboard scraps under the wood wall bottom plate so the wall is not a "heat sink" 24/7, it will also act as an air/capillary draw in addition to a thermal break; http://www.greenbuildingadvisor.com...ressure-treated-sill-plates-and-building-code You could just use plastic sheeting at the rim and under unfaced 1" foamboard under the bottom plate. IMO, run a bead of caulk to help air seal the wood plate at slab/FB/plate joint.

Plastic sheeting wrapped around unfaced XPS on the rim stops the foam from absorbing moisture from interior/exterior humidity which would prolong the drying of the rim. Last paragraph; "Thus in these cases, the alternative is to install rigid insulation (such as foil-faced polyisocyanurate) flush against the interior side of the sheathing. This still allows air drying of the rim joist cavity but reduces the potential for interior summer condensation by decreasing the condensation surface to the bare minimum of the interstices of the insulation/ sheathing interface. In the winter, the insulation is protected from interior-sourced water vapor by the foil-facing. These considerations are reflected in the recommendations of section F." from; http://www.buildingfoundation.umn.edu/RimJoist/rimjoistphenom.htm

You don't need cavity insulation if meeting code for "continuous" on concrete, new code allows mixing the two types for total R-value. Any air gap on the warm side (at FB/Frame) doesn't effect the R-value.

Gary


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