Extruded Polystyrene, T111 and Fiber Cement
Hi all,
I am preparing to reside my house in central South Carolina. I currently have the original T111 siding installed over 1/2″ extruded polystyrene (blue Dow) without an internal vapor barrier (the eps is also untaped). I would guess that the eps was meant to serve as the weather resistive barrier, although it is untaped.
I would like to install fiber cement siding over the t111 since the t111 serves as the sheathing (I also plan to replace all windows and doors). Should I use a house wrap over the t111 before installing the fiber cement or should I omit the house wrap due to moisture problems. Or I should I remove teh t111 and insulation, then reinstall a sheathing substrate and place eps over the new sheathing followed by house wrap and siding (a lot of work, so would like to avoid this option)
Thanks,
Zach
Replies
You live in a cooling climate, therefore your main source of moisture is going to be on the outside of your house. You fight the vapor battle there. Up north, all those walking talking meatsacks keep releasing water inside the house - the outside air is very dry in comprison, so the vapor battle is fought at the drywall line there.
Here's an idea: If the T111 is in good shape, you could cover it with a new layer of foam board (and tape the seams this time), along with rain screen detailing. This gives you the benifit of an exterior water barrier that beathes - which you need for FC - along with the added insulation. Two bangs for One buck!
Fiber Cement over T111 and Foam
Hi Zach.
I have worked for two fiber cement manufacturers. Fiber Cement is warranted based on the following:
1) Must be installed over a Weather Resistive Barrier. Foam constitutes a WRB if it is taped at the joints.
2) Fasteners must penetrate wood or steel framing member 1" minimum. This applies to all sheathing including foam (maximum 1" thick foam allowed for Hardie and Nichiha.)
3) Of course, foam has no sheer value so if you haven't picked up that engineering value in the wall design you may need metal strapping or cut-in wood braces.
UV breaks down foam. If your foam has turned porous, you will need to replace. In that case, a rainscreen as mentioned by the other responder is a good call.
Cheers,
Twitter @fibercement
Interested in your response, since this question came up a while back on another forum. Hardie instructions say 1 3/4" roofing nails into OSB is also acceptable.
" OSB minimum 7/16"
• 11ga. roofing nail (0.121" shank x 0.371" HD x 1.75" long)
• Ribbed Wafer-head or equivalent (No. 8 x 1 5/8" long x 0.375" HD). "
If the T1-11 meets the thickness requirement, and is in good shape, why would it not be as good as 7/16 OSB?
Thanks!
The foam still seems ok as viewed from the inside of the house and most of the t111 is still in good shape (previous owner was meticulous about the z flashing and keeping things sealed). I am leaning toward rain screen over the t111 and then fiber cement using 2.5" stainless nails (this would leaving 1.25" penetration into the studs) . Do you think this would work?
Sure, just like water could be trapped between two layers of foam board. Lets just assume it did, and the worst happened:
The downside of trapped moisture in an insulation cavity like a stud wall is that it will support mold growth and rot, leading to potentially dangerous mold entering your breathing space OR your stud walls rotting out - both are serious consequences.
On the other hand the T1-11 is not an an area that could lead to mold spores entering the house nor is a structure that supports the house. All the T1-11 will be is a nailer - thats it. Sealed between two layers of foam, very little transmission of vapor will occur. Since you have a clean outside face to work with on the outside foam, you can more accuratly seal the seams to prevent exterior water and vapor intrusion. This is a little different from the stud bay with an interior vapor barrier, where there are potential penetrations allll over the place.