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Trying to make final selections on materials

ZDude | Posted in General Questions on

Need some advice. Got some before from here but can’t recall login so posting new thread to ensure I am covered.
– I am in zone 6. Minimum code R-15 required.
– I am finishing a walkout basement.
– Have poured concrete walls with exterior R-5 XPS rigid on walls.
– No history of water, never damp basement
– Was encouraged to do R-13.1 foil faced on interior walls and frame in front. Problem – cannot find foil faced anywhere local that is 2″ thick R-13.

– Can I do R-10 XPS frame in front
– Should I put roksul or batting or some other unfaced batting to get R value higher. I understand r-10 plus R-5 = R-15 BUT I have also come to learned R goes down over time so wanted to go a bit higher on R scale
– There are utility pipes and other items which are secured to walls – do I need to somehow get insulation under these items? Most are in middle/back part of basement which is surrounded by ‘earth’ outside.
– Sump pump pipe is on a wall that will be framed and be part of a bathroom. Pipe is somewhat tight to wall. Do I just spray some foam under it and rigid on either side?

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  1. GBA Editor
    Martin Holladay | | #1

    You can find answers to your questions about insulating basement walls here: How to Insulate a Basement Wall.

    You have two choices when it comes to pipes: (a) You can provide less insulation in these areas -- just slip as much insulation as possible behind the pipes, or (b) Move the pipes.

    Moving pipes isn't that hard. Plumbers do it every day. So do some homeowners.

    -- Martin Holladay

  2. Expert Member
    Dana Dorsett | | #2

    Does the exterior XPS extend all the way to the framed wall, or is it trimmed to grade, with concrete exposure to the exterior? (It makes a difference on how much interior foam is required for dew point control)

    If the XPS runs all the way up to the foundation sill, an inch (R6) of foil faced polyiso and a batt-insulated studwall will provide sufficient dew point control for a 2x4 batt insulated studwall with no interior vapor barriers, and the assembly would have the necessary whole-wall-R / U-factor to meet code from a thermal performance perspective.

    If the XPS stops at grade, you'll need a minimum of 1.5" of polyiso (R9) on the interior, trapped to the foundation wall with the 2x4 batt insulated studwall.

    If the plumbing can't be moved further into the interior, put as much insulation as you can on the exterior side of the plumbing (spray foam is fine), but don't insulate between the plumbing & drywall.

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