What are my vapor-barrier options under Hardie siding?

Question

I'm currently in the process of replacing all the siding on my 1940s home. It was originally wood dutch-lap and that was covered with felt paper and asbestos siding. I'm taking all the siding off, repairing what I can, insulating, and installing Hardie siding. I'm at the point of beginning to consider what to put under the siding, specifically as a vapor barrier.

I plan to add OSB of some sort, rather than just nailing the siding to the studs. The house needs a bit of structural help, and I think it'd make installation easier.

As a long-term DIY project, I like the idea of the HuberZIP because it seems easy and seems like it'd be harder to damage as I'm finishing the rest of the house. But, it's AWEFULLY expensive, at first glance.

How would that compare to standard Tyvek house wrap? Are there other options that I can consider?

I've also heard a little about Hardie-backer, but that sounds more like an interior sheetrock replacement. Is that intended for this purpose?

So the question is, what are my options, and what kinds of things do I need to consider when making that decision?

EDIT: In case climate matters, I'm in South/Central Texas, just NW of Houston.

Answer 1

You have asked several questions at once so a little difficult to give you the answers you want. First, Hardy backer board is a newer, lighter, substitute for concrete board and usually used in bathrooms and under tile installations. I have never seen it used on exterior walls as a sheathing.

Second, the huberZip system is from Advantec. I love Advantec sheathing and subflooring. I'm sure it does what it says, but it is pricey compared to OSB sheathing and tyvec. Selecting a product because of price is not what we do here. An OSB, or any T&G exterior sheathing with properly installed and taped Tyvec or a house brand wrap also works very well.

Third, a true vapor barrier is installed on the heated side of the wall studs, not under or over the exterior sheathing. the wall cavities and insulation must have a a way to breath. They will breath and excessive moisture should escape through the exterior siding. Tyvec is not a vapor barrier, it is an air penetration barrier, just like the new Zip system. they both stop liquid water infiltration, but water vapor is a different story, it must be able to pass through. On wet, foggy or very humid days, humidity gets into the wall cavity, the vapor barrier keeps it out of the living space. When the weather conditions are better, the moisture migrates out of the wall cavity through the siding, thus drying the wall.

I hope this has answered some of your questions. Good luck on your project.

Answer 2

This is such bad advice! You NEED a water retardant barrier on the outside of your sheathing, which will be UNDERNEATH your hardi shingles. This goes on the OUTSIDE of the home (on top of/over the sheathing). Not inbetween a wall cavity or under drywall. If the water gets in any gaps from the outside, and you have made a water resistant barrier INSIDE the wall, the insulation will get wet and mold. The drywall may mold from within the wall (on the backside of the drywall) where you made it touch the barrier. It also won’t breathe. A barrier on the inside of your house sheathing (from the interior of the wall cavity) is pointless. I’d remove the exterior siding, wrap the exterior sheathing/plywood/shiplap/or whatever wood is there in a vapor retardant barrier, and THEN install the new cement (hardi) siding. You can then install insulation in the wall cavity and it won’t get wet! On the inside of the home, install drywall using the airtight method maki g sure to seal all outlet receptacles.

Answer 3

If it were my house, I would apply a redguard or some similar type of moisture barrier. Then I would apply double layer of black belt paper. Then I would install the hardy backer along with caulking at all seams, work butts together, and possibly even a Z metal if heights are an issue.

Answer 4

If it was my house? I would strip the siding and felt. I would replace the insulation (if any) between the studs with rock wool. If I can afford it, I would use huber zip and tape over the studs and if not, 7/16 OSB and then Hardi-wrap it all according to best practice. Then Hardie of choice over that. Follow Hardie instructions implicitly and seal it up with OSI- QUAD. Good luck!

Answer 5

Just NW of Houston is pretty humid, right? I don't know how different your location is from mine in Dallas, but a high end custom builder in Dallas TX told me that our region of the country is one which is relatively forgiving with regard to moisture control.

During the long a/c season the inside air is dehumidified by the a/c unit so there should not be a lot of water vapor generated inside and migrating from the living space into the walls. The problem would be water vapor from outside getting into the walls and condensing in the cool insulation. To the extent that does happen it could dry to the inside if there is no polyethylene or other air and vapor impervious barrier on the inside of the studs under the interior sheetrock.

This argues for putting up OSB sheathing directly on the exterior sides of the studs, bottom plate and top plate. The OSB would be covered with one of the tyvek housewraps. The Hardiboard is nailed through the sheating, into the studs and plates.

There are some housewraps which have raised bumps that allow water to drain down under the Hardiboard and allow air flow. Some custom builders go all the way and put up "rainscreen" which allows for 1/2" space behind the siding to promote water drainage and air flow to keep the back side of the siding dry.

A true rainscreen is more expensive and troublesome than using the special housewrap, but there are products which simplify achieving the spacing and have mesh at the bottom and top to allow airflow but prevent bugs from getting into the space between the back of the siding and the OSB sheathing convered with tyvek housewrap.

During Houston winters warm moist air from inside would migrate into the walls (assuming no vapor impervious barrier on the insides of the studs) and could condense in the insulation. However it should be able to dry to the inside or outside later in the year or as soon as the temperature gets above freezing each day.