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Best insulation solution for Pacific Northwest crawlspace

uqKAabigt6 | Posted in General Questions on

Hi and thank you for looking at my question.

I have a newer built house (1993) that is built on a dirt crawlspace with no current insulation either on the concrete walls or in the first floor joists. due to the high water level in the soil and the awkwardness of excavating the current dirt, I am stuck with what approach to take in light of cost and longterm efficiency.

Beyond this, here’s some information that might help you when answering: I live on Vancouver Island, BC on the landward side, winters are marine and mild (coldest generally is -5°C), summers are the same (warmest is 30°C, generally more like 25°C).

The solutions I think might work (and just to clarify things, I am a DIYer, not a building trades professional) are:

1. insulate walls and floor joists
– with rigid insulation (Extruded Polystyrene Rigid Insulation (most likely at R5/inch) to the concrete walls
– spray foam the rim joists to air and vapour seal them
– add rock wool to floor joists to fill the space fully (I don’t have the exact measure; but it’s at least 8″ of depth)
– to hold the rock wool in place, I was originally thinking of using 1″ rigid insulation (along with tuck tape) to both stop thermal bridging from the conditioned part of the house to the crawlspace and also seal moisture from getting into the insulation from the crawlspace. But this has been replaced by the idea to use reflective bubble wrap in lieu of the rigid insulation (both for cost savings and the ease of installation); I plan on stapling/nailing whatever becomes the strapping to each joist for better longevity and ease of replacement, should it be needed.

2. encapsulation of the basement

– evacuating by hand 8″ or so of dirt from the crawlspace; then putting down 6mil poly (and running it up the concrete wall to about 18″ from ground), 2″ of rigid pink insulations (so about R12) and then 4″ of concrete
– once the above is done, then insulate the walls to R20 or so with either rigid insulation or spray foam
– once the above is done, putting a heat dump (from my fireplace (the house’s main source of winter heat) into the crawlspace to keep it warm and drive out moisture

What do you think? I can’t find information that either is bad; but which is better for longterm viability and cost benefit? I am leary of using spray foam for reasons noted in other GBA articles and for reasons of off-gassing. Rock wool would be my preferred solution for insulating the joists for ease of installation and the fact that I am likely doing the work myself.

Cheers,

Anthony

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Anthony,
    Consensus has been reached about the steps required to create a sealed, conditioned crawl space, so you don't have to reinvent the wheel.

    In general, you are on the right track. However, you don't need to insulate the crawl space ceiling (unless you abandon the idea of creating a conditioned crawl space), and the crawl space may not need any added heat.

    Here's more information: Building an Unvented Crawl Space.

  2. user-1037041 | | #2

    Hi Martin --

    I have an addition/remodel job starting soon and have been researching crawlspaces. There's a ton of great information here, and many of my questions have been answered. I'll be adding an addition to a single story midcentury home here in Portland Oregon (4c) and retrofitting the existing house. I think my best option for the addition foundation will be to pour a concrete foundation and have a crawlspace underneath. I'd like to seal and condition the crawlspace as GBA recommends. My question concerns your above answer to Anthony. You say that insulation on the ceiling (or the floor of the living space) of the crawlspace is unnecessary. If I'm not installing a rat slab (or the insulation under it), is the ceiling insulation still unnecessary? At this point, I'm planning on leaving the "floor" of the crawl dirt, covered with poly as recommended.

    I guess the real question here is: Is the Earth acting as part of the thermal envelope in this scenario?

  3. uqKAabigt6 | | #3

    Martin,

    Thank you for the information; that said, though I appreciate what you say, if creating a sealed and conditioned crawlspace isn't being considered (or practical, as in my case), would what I suggest the best solution? I ask as bringing the crawlspace in my house's instance into the conditioned envelope is both cost prohibitive and space-wise is foolish. I say this as the head room in the crawlspace is at most 30"; I can't excavate down as the water table in the soil on my property is only 18" - 24" below soil level. So from what I can find, the insulating of the crawlspace ceiling and putting in a vapour barrier is both better from a cost point of view and also in that the extra area of the crawlspace isn't heated where there is no use.

    As an aside, I may put in a concrete skim coat over the poly; as well, I am going to be putting in hydronic radiant floor heating in the joists and then insulation; ending with the vapour barrier (something like Tyvek or a similar breathable membrane).

  4. GBA Editor
    Martin Holladay | | #4

    Jeff,
    Q. "If I'm not installing a rat slab (or the insulation under it), is the ceiling insulation still unnecessary?"

    A. Yes. In your climate, insulating the crawl space floor would have a very long payback period.

    Q. "At this point, I'm planning on leaving the floor of the crawl dirt, covered with poly as recommended. Is the Earth acting as part of the thermal envelope in this scenario?"

    A. Yes. The heat loss through the dirt won't amount to much. However, if you are aiming to achieve the Passivhaus standard, it's always possible to install rigid foam and a rat slab if you want to.

  5. GBA Editor
    Martin Holladay | | #5

    Anthony,
    You wrote, "I am going to be putting in hydronic radiant floor heating in the joists and then insulation; ending with the vapour barrier (something like Tyvek or a similar breathable membrane)."

    Just so you know: a breathable membrane like Tyvek is not a vapor barrier; it is an air barrier.

    It sounds like you have a plan, so I'm not sure what your question is.

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