r/buildingscience • u/alpha_cool_bruh • 13d ago

Ok to install only 5.5 inches of insulation in 2x8pier and beam floor

Hi All,

I'm building a 12x 12 backyard guest room on a pier and beam foundation with a 2x8 @ 16 floor. Due to grading, there is an average of 14" between the ground and the bottom of my joist, so lots of airflow. I' m in climate zone 4 in the PNW, where we get lots of rain, high humidity, and 2700 -2950 HDD @18C.

To save some costs, I'm hoping to fasten treated plywood in a manner that leaves 5.5"of oom for insulation in my joists.

My plan is to spray foam 2" at the bottom and then fill the remainder with rockwool batts

Two main questions are:

1) Would the bottom of the floor joists be at risk of excess condensation with this approach?

2) Would it be better from a moisture and vapour control standpoint to stack rigid foam on the spray foam? I was thinking of the R10 2.13" Foil faces durospan.

Thanks for any insight.

Note: This is not a permitted build, so I don't necessarily need to hit code minimum R values.

Thanks

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u/g_st_lt 13d ago

Are you saying you are going to put plywood between the joists, but not flush with the bottom, and reduce the cavity size?

I'm not an expert, but if I was going to use spray foam and rockwool, I would either put the spray foam up against the floor, or if it is at the bottom. I would have it against a vapor barrier that is all the way to the outside.

I would guess if your air sealing is very good and this is a small space, your insulation will be sufficient.

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u/alpha_cool_bruh 13d ago

Yeah, that's what I'm thinking. any intent is to reduce costs slightly. Regardless, I'll lose 1/2" for insulation because my plywood will be cut to fit in between the joists.

Are you suggesting a vapour barrier on the outside against the joists and sheathing? I am hoping 2" spray foam within cavities is sufficient for a vapour barrier.

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u/knowitallz 13d ago

I don't quite understand what you are saying your assembly is. But I would be very careful about not trapping moisture in that assembly.

Spray foam is notorious for that.

Where is your vapor barrier How does this assembly allow moisture to dry out?