Hardie Plank siding - Is a water resisting barrier required?

[Jerry Peck - Codeman]

(From a phone call question.)

Codeman,

Does HardiePlank siding require a weather resisting barrier be installed behind it?

The local building department said "A weather resisting barrier (WRB) did not used to be required by code, however code now requires a WRB."

I ask because builders have been installing HardiePlank siding directly over OSB sheathing without installing a WRB over the OSB and behind the HardiePlank siding.

Jim G., Builder

[Jerry Peck - Codeman]

Hi Jim G.,

The short answer is 'Yes.', however, that short answer is complicated by the vague references to the weather resisting barrier in the HardiePlank installation instructions, which the codes clarify with a "Yes, but", there are limited exceptions.

The HardiePlank installation instructions state "A water-resistive barrier is required in accordance with local building code requirements. The water-resistive barrier must be appropriately installed with penetration and junction flashings in accordance with local building code requirements. James Hardie will assume no responsibility for water infiltration."

The code I will reference is the International Residential Code (IRC) as most AHJ (Authorities Having Jurisdiction) have adopted the IRC for residential construction (construction of one-and two-family dwellings and townhouses) 3 or fewer stories high.

The 2006 IRC, in section R703.2 Water-resistive barrier, states that a water-resistive barrier (WRB) be "free from holes and breaks" and "shall be applied over the studs or sheathing of all exterior walls".

There is an exception which states:
- Exception: Omission of the water-resistive barrier is permitted in the following situations:
- - 1. In detached accessory buildings.
- - 2. Under exterior wall finish materials as permitted in Table R703.4.
- - 3. Under paperbacked stucco lath when the paper backing is an approved weather-resistive sheathing paper.

The 2003 IRC has the same requirement, as does the 2000 IRC, the first year of the International Codes after the model codes merged into one code. Thus, that requirement has been in effect since 2000 in the International Codes, possibly before 2000 in one or more of the previous model codes.

As I said during our conversation, everyone is aware that wood and water do not go together well, that wood decays when it gets wet and does not dry out quickly enough, that basic construction practices, as well as common sense, dictates that all wood sheathing which is not designed to be left exposed the weather (i.e., which is not 'pressure treated') needs to be protected from the weather and from water which penetrates through non-water-tight siding systems, such as HardiePlank fiber cement lap siding systems.

Within a few years you can expect to find decaying (rotted) OSB sheathing behind HardiePlank lap siding if the OSB sheathing was not protected with a WRB behind the HardiePlank siding at the time of construction. How many years this takes will depend on the amount of water and moisture getting behind the HardiePlank siding and how long the water and moisture stays there before drying out, if it dries out.

What is the solution?

Remove the HardiePlank siding, make sure that all penetrations, including windows, doors, etc., are properly flashed, then properly wrap the house in a WRB (a "building wrap" or "house wrap" product is typically used as the WRB, building felt is not recommended as it does not have the necessary water resistive properties needed for a WRB).

After properly flashing and wrapping the house - flashing and building wrap go hand in hand and must be done in accordance with the manufacturers' installation instructions, the next step is to properly install NEW HardiePlank siding over the WRB.

It is as simple as, and as complicated as, doing that.

Codeman