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How would you insulate this crawlspace?

barnabaas | Posted in General Questions on

Hi all.  Hoping you can help me here.  We want to insulate or encapsulate our crawlspace.  Research leads me to either spray foam or encapsulation in this case but I’m not sure which would be better.  Here’s some info about the house.

– Home was build in 1952 in Northern California (climate zone 3)
– Stucco siding with very little or no insulation in the walls.  I presume original insulation if there is any, most of which has probably deteriorated.  The walls are super cold or warm to the touch depending on the weather.
– Pier and beam foundation with a vented crawlspace that’s slightly on grade (do I need a sump pump bc of that?)
– No insulation under the house anywhere (see pics) – only insulation is in the Attic
– Plumbing is under the house in the crawlspace
– HVAC and duct work located in the attic (I know, not ideal)
– We recently had old Attic insulation removed, then air sealed and had R38 blown in cellulose.
– New windows were put in last year to replace old single pane.  Vinyl windows with non-metal spacers.

Pictures of crawlspace here

Big question for me I guess is, if spray foam is the suggestion…should we update the plumbing now due to the fact it’s old and most likely deteriorating in places.  I imagine it’ll be a hassle to deal with after spray foam.  Or do we not worry about that and just encapsulate the crawlspace?  We plan on adding onto the house in the future but leaving this floor plan as is.  

Thanks in advance

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  1. Expert Member
    Dana Dorsett | | #1

    The cut'n' cobbled XPS on the pony wall was a lot of work for minimal gain. A full cavity fill of R15 rock wool batts with a broadsheet air barrier over the interior would have been cheaper and more effective. Local codes may now require an interior side ignition barrier for fire protection on the foam, where it wouldn't if rock wool.

    There is no "payback" to insulating the crawlspace floor in a DOE zone 3B or 3C climate, but a vapor barrier is necessary. If the budget allows, a 6-10 mil polyethylene vapor barrier with a 2" concrete rat slab would be a good idea, but you can skip the rat slab if it's cost-prohibitive.

    Lap the vapor barrier at least a foot up the foundation wall and seal it to the foundation with duct mastic.

    The IRC calls out R5 minimum for crawlspace wall insulation, which could be fire rated 3/4" Dow Thermax extending from the floor vapor barrier (overlapping where the VB is on the wall), all the way up to the under-side of the floor joists, including covering over the pony wall sections with the XPS cut'n'cobble. It can be installed with blobs of foam board construction adhesive every 16-24", with a full bead at the vapor barrier and anogher along the top; of the foundation to make it air tight. Tape the seams with a temperature rated foil tape. It's common to also install 1x4 strapping through-screwed to the foundation wall along the bottom (clamping the foam board over the vapor barrier) as well as another course higher up, but that's not completely necessary.

    Using a fire-rated foam board over the whole thing will probably appease the inspectors regarding the XPS, otherwise you may be required to install half-inch wallboard or another timed thermal barrier over the XPS.

    1. barnabaas | | #2

      Hey thanks for the reply. I guess my main thought of insulating or spray foaming the floors was to create an air barrier to help improve indoor air quality while also making the home more comfortable in regards to floor temps. Makes sense that you might not see a return energy efficiency though. Do you think wall insulation would be more cost effective as far as energy efficiency is concerned then? Also In regards to the XPS - is it too late now to add the additional rockwool or will the fit just not work out as well?

      Thanks for the info regarding the vapor barrier, any suggestions on 6-10 mil products?

      1. Expert Member
        Dana Dorsett | | #3

        Sheet polyethylene in various thickness and sheet widths are ubiquitous. EPDM sheeting (membrane roofing) is a bit more rugged if it's going to be left exposed (no rat-slab).

        Insulating the foundation walls will take less material than insulating the floor, and have a much bigger impact on energy bills. Most of DOE zone 3 in California has deep subsoil temperatures in the 65-70F range, which has almost zero increase on the space heating load, and a slight reduction in the sensible cooling load if the thermal envelope of the house is brought down to the crawlspace floor level.

        What's the subsoil temperature in your area(?):

        Insulating the crawlspace floor isn't necessary for indoor air quality or air tightness- an air tight vapor barrier is the key thing. Protecting it with a rat slab tightens it up further even the seams (though you should tape them anyway), since concrete is fairly air tight, and guarantees pretty good air tightness over the long term.

        1. barnabaas | | #4

          Soil temp ranges between 50-60 degree's. (average is 50 at the moment). What your take then on blown in insulation for exterior walls. We have fire blocks across each stud bay so we'd need to have 2 holes top and bottom that would need to be patched. I guess I'm worried about just blowing in wall insulation and just covering up mold or some other potential mess I'm unaware of vs. just taking the walls down to the studs and putting in some batts or something. That's the ideal situation I know but also the most invasive and costly. Again I'm not super familiar with the in's and outs of this so any info is appreciated. Thanks for the help and info thus far.

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