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John Wagner Posted: Feb 18, 2020 05:04 PM	Spray Foam Directly on Fiber Cement siding Hi Everyone, I was noticing that some retrofit contractors will fill cavities of wood clad houses with the drill and fill method. They will pump closed cell into wall cavities which essentially causes the foam to be applied directly to the exterior siding. The question I have is why is this not industry practice in new construction? I read an article by the USDA/Forest Products Laboratory titled "Contribution of Gypsum Wallboard to Racking Resistance of Light-Frame Walls". It turns out that horizontally applied 12ft. sheetrock, applied only to interior walls, provides in excess of 8.8X the necessary racking resistance of light-frame structures, irregardless of any other form of wall bracing. In other words, let-in bracing or full house sheathing is a complete waste of money, time, and resources. This was calculated by a standard nailing sequence of every 7 inches and then taped. Normally these days, a lot of sheetrock is put up with screws and glue. The sheetrock failed at the nailed joints. I would love to see the safety factor completely glued sheetrock would provide. These tests were also done on 24" centers, not 16". Granted the racking resistance goes to zero if the sheetrock gets wet but if sheetrock is soaked there are some other building issues much more serious than racking resistance. So, here's the point. If sheathing is practically useless in providing any additional racking resistance over internal nailed and taped sheetrock, why not forego the sheathing and put up fiber cement siding. If siding was installed with a small gap between the siding and wall studs, we could use closed cell to permanently adhere the siding to the studs while nixing the requirement for sheathing and building wrap. Even if the siding/foam combination was not taken into account to calculate racking resistance, the building would still be fine. Heck, put a couple of metal straps in if you wanted to in order to satisfy the inspector. What problems would this cause long-term? Paint issues since the siding is always drying to the exterior? Thoughts?

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John Wagner

Posted: Feb 18, 2020 05:04 PM

Spray Foam Directly on Fiber Cement siding

Hi Everyone,

I was noticing that some retrofit contractors will fill cavities of wood clad houses with the drill and fill method. They will pump closed cell into wall cavities which essentially causes the foam to be applied directly to the exterior siding.

The question I have is why is this not industry practice in new construction? I read an article by the USDA/Forest Products Laboratory titled "Contribution of Gypsum Wallboard to Racking Resistance of Light-Frame Walls". It turns out that horizontally applied 12ft. sheetrock, applied only to interior walls, provides in excess of 8.8X the necessary racking resistance of light-frame structures, irregardless of any other form of wall bracing. In other words, let-in bracing or full house sheathing is a complete waste of money, time, and resources. This was calculated by a standard nailing sequence of every 7 inches and then taped. Normally these days, a lot of sheetrock is put up with screws and glue. The sheetrock failed at the nailed joints. I would love to see the safety factor completely glued sheetrock would provide. These tests were also done on 24" centers, not 16". Granted the racking resistance goes to zero if the sheetrock gets wet but if sheetrock is soaked there are some other building issues much more serious than racking resistance.

So, here's the point. If sheathing is practically useless in providing any additional racking resistance over internal nailed and taped sheetrock, why not forego the sheathing and put up fiber cement siding. If siding was installed with a small gap between the siding and wall studs, we could use closed cell to permanently adhere the siding to the studs while nixing the requirement for sheathing and building wrap. Even if the siding/foam combination was not taken into account to calculate racking resistance, the building would still be fine. Heck, put a couple of metal straps in if you wanted to in order to satisfy the inspector. What problems would this cause long-term? Paint issues since the siding is always drying to the exterior? Thoughts?

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