How important is a thermal break between a house foundation and an attached garage foundation?
Our footings were poured today for a new house and attached garage. The walls of the basement foundation (9′) and garage foundation are scheduled to be poured next week.
Looking over the plans, our concrete sub is worried about the 7″ thermal break between the basement and garage foundations (no physical connection) as currently drawn on the plans. The separation is there in order to allow us to attach 5″ of Roxul Comfortboard 80 to the basement foundation (we’ll be doing the same for the whole exterior perimeter of the basement foundation).
In Passive House terms, this thermal break makes sense (trying to eliminate every last thermal bridge), but the concrete sub is worried about movement and then eventual separation developing between the basement and garage foundations over time (even though they will be physically tied together at the framing above). He’s reluctant to follow the drawings as shown. He really wants to pour a connection between the two foundations. He’s been doing this for decades, so it’s difficult to discount his concerns.
How much of an energy penalty is there if the basement and garage foundation walls are tied together in one continuous pour? There will be only two points of contact between the two foundations, otherwise we will still be able to attach the Roxul as planned between the basement and garage foundations (we would just install the Roxul around those two points of contact where the basement and garage foundations would meet).
As a compromise, since rebar will tie in the connection between the house and garage foundations, what about using a piece of 2″ thick rigid foam set directly in the form at the point of thermal bridging?
Not enough R-value for the effort? Better than no thermal break at all? Or is this area of thermal bridging not worth worrying about?
We’re trying to follow Passive House science as much as possible (in terms of air tightness and level of insulation), but we’re not interested in official certification, so the ultimate goal is to achieve the Pretty Good House concept. It seems like the only real penalty for a direct, uninsulated connection between the two foundations would be a slight increase to overall heating/cooling demand. Is this incorrect?
Any information, opinions, or suggestions are welcome.