Adding exterior foam insulation and HardiBoard
I live in north Florida (not along the coast). We have long, hot, humid summers and about 2-3 months of winter where nighttime lows are often in the 30s and sometimes the teens.
I have a house that is brick veneer on three sides and T-111 paneling on the back. The T-111 was installed originally with poor attention to water flow (for instance, a trim panel along the bottom edge is applied over the T-111 and depends on caulk filling in all the slots to keep water from going behind the trim. There is Z-flashing behind it which helps control any leakage, but even that is installed on top of the tar-paper behind the T-111 rather than vice versa and it slopes such that it holds water along the back edge of the flashing.) There are some rot problems along the bottom edge and in some of the window trim (whose installation is a story in itself, suffice it to say that the dependence on caulk as the primary water stop continues).
Anyway, I’d like to ask for comments on my proposed ‘cure’. I’m planning on removing all the trim, then securing 1.5″ polyiso foil-faced foam board directly to the T-111 (foil out). I’ll seal all joints with flashing tape and foam caulk. I intend to use HardiBoard vertical siding (the stuff that looks like T-111) over the foam, over 3/4″ x 3.5″ furring strips centered on the studs. The furring strips will be fastened to the studs with 4″ screws at 16″ intervals and the siding will be attached to the furring strips with 1″ nails at ?6″? intervals along the edge and 12″ down centers. I’ll screen the bottom of the furred space and the top will open into the soffit.
The edges of the HardiBoard will be set into a caulk bead on the furring strip, and the edges of the HardiBoard installed touching and without battens. Z flashing will be secured along the bottom edge, where it will be fastened to the T-111 and extend under the foam, furring, and hardiboard to sloped to provide drainage out over the foundation edge. The foundation appears to have been set up for a brick veneer on the back as well so it ‘sticks out’ beyond the exterior wall about 4″ now and still will be about 1.5″ after the above additions.
Thoughts, comments, warnings, dire warnings?
Many thanks,
Steve in Tallahassee
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Replies
Sounds like a good plan. But I would check with James Hardie about the advisability of bedding the vertical edges in caulk and about installation over furring strips.
Thanks Robert,
Their instructions call for caulking the butted joints, so I figured installing over a bedding bead just ensures a more sure seal. I saw reference to installing it over furring strips in some of the other articles but will check with Hardie.
Their instructions call for sealing the joint on the outside, and that requires leaving a gap between boards. With a drainage space behind, it's often better to forget the caulking and allow the siding to breathe.