Roger Lin is planning to use insulated concrete forms in a house he hopes will meet the Passivhaus standard of 0.6 air changes per hour at 50 Pascals (ACH50). ICFs are rigid foam building blocks stacked like Legos and then filled with concrete.
Lin has been told by ICF manufacturers they won’t need air-sealing, but he’s not so sure.
“I understand that ICFs with the concrete will be pretty airtight,” he writes in a Q&A post, “but I still see the joints where the ICFs meet as potential weak spots. Will I need to air seal them and how would you do it?”
The conversation that follows is as much about the Passivhaus standards themselves as it is about leaky ICF walls. Are the standards arbitrary and unreasonable rules promulgated by zealots? Or sensible construction standards that can work anywhere?
First, the air leak question
John Semmelhack is first out of the box with an assurance that a Passivhaus home in Wisconsin built with ICFs tested a very low 0.36 ACH50 with a preliminary blower-door test. And that was without any additional air sealing of the forms.
GBA editor Martin Holladay agreed with Semmelhack, noting that “ICFs have a continuous monolithic concrete core that is an excellent air barrier.” He added a caveat: “Of course, there are many locations where careful air sealing is still required, including: around windows and doors, at the intersection of the walls and ceilings, and at ceiling penetrations.”
J Chesnut adds that a “very diligent” site foreman had a lot to do with the good airtightness results in Wisconsin, but it’s not so much the walls that may be leaky but other parts of the structure.
“An important aspect to the air tightness…
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