BSP-020: Water Management Details for Residential Buildings
Section 2: Building Paper/Housewraps as the Drainage Plane
An integral part of many water management systems is the use of building paper or housewraps as the drainage plane in the wall system. The basic concepts with these products are that they should always be installed in a shingle lap fashion and have enough space to drain moisture down and out to the exterior. With house wraps, the joints are often also taped to provide increased protection from water entry and to increase the air tightness of the assembly. With no other factors to consider, following these basic ideas should result in long-term performance of the material. While the concept is simple, certain construction methods and details have interfered with the performance of the housewraps and building papers.
Figure 1: Shingle lap of drainage plane materials
Installation Concerns
Maintaining the shingle lap of the building paper or housewrap is generally pretty easy for the field of the wall. Problems usually appear at areas where there is a change in construction, specifically at window intersections and at roof wall connections. These are also often areas of higher liquid water concentration which further exacerbate the problem.
Figure 2: Improper installation of drainage plane materials
This occurs due to improper sequencing of the construction, or though the difficulty in installing the products at areas of more complicated geometry. Often at these locations the shingle installation ideal is exchanged for coverage (As long as we get the housewrap everywhere it should be fine, right? Wrong.). When roof overhangs, or other kick-outs in construction occur, the installation of the house wrap must be considered carefully to ensure that a reverse flashing is not created. While taping the joints of housewraps can reduce the risk of water entry, it is no substitute for proper shingle lapping of the product.
The use of smaller pieces of housewrap or building paper at these more complicated areas can be advantageous as they are easier to handle and install. The pieces can also be cut if need be into specific shapes to deal with the more complicated geometry. The development of standard details by builders for these situations can be beneficial in the long run, to provide consistent and reliable performance of the housewrap/building paper drainage plane.
Exposure Concerns
Prolonged exposure of the housewraps and building papers can be a concern. Building paper if left exposed to wind and debris can tear and get damaged before the installation of the cladding system. Even housewraps, with the increased tear resistance, will tear or the holes created by the fasteners will elongate if left exposed to repetitive wind loading for extended periods of time. Tears and elongated fastener holes will create discontinuities in the field of the drainage plane, reducing its effectiveness or creating a fundamental failure in the function of the drainage plane. Due to this, it is generally recommended that the cladding system be installed as soon as possible after the housewrap or building paper.
This may not always be possible, however, and in situations where the housewrap or building paper has been left exposed for a time, an inspection of the condition of the housewrap/building paper drainage plane must be done prior to installation of the cladding, with any torn or damaged housewrap/building paper repaired or replaced. This is not an ideal situation as the repaired areas are often not as effective as the original installation of the drainage plane.
In addition, the UV resistance of the products varies greatly and can affect the long term performance. Building papers are not very resistant to UV damage. Many housewraps are rated for certain amounts of UV exposure; however, this varies dramatically from product to product and should be considered if prolonged exposure is expected.
Problems Relating to Cladding
All cladding materials have affected in one way or another the long-term performance of building papers and house wraps. The issues were for the most part related to poor installation techniques and workmanship, or from unforeseen compatibility problems between materials.
The main fundamental problem is that water is often able to penetrate through a housewrap if the water is held against the housewrap for a given period of time. This is particularly true with all perforated housewraps, but it can also occur with non-perforated housewraps that have been exposed to surfactants.
Brick and Stone Veneers
It is generally considered a good idea to provide some amount of space behind a masonry veneer to allow for water to drain out. Brick veneer should be installed with a space between the brick and the back up wall construction, and this space is often considered to be cavity blocking the drainage out of the assembly or there is extensive bridging of the mortar to the back up wall construction trapping water up against the housewrap.
For brick and stone veneers one of the simplest solutions is to inspect the work done to make sure that the cavity is relatively free of mortar droppings. Some additional protection can be had through the use of some specialty products installed at the base of the cavity that prevents the drainage openings from being clogged up.
Other products are also available to prevent mortar bridges and ensure a drainage gap is provided behind the veneer through the use of a three dimensional drainage matt that creates the minimum gap requirements. These are likely the most effective products for long term performance and durability of the building paper or housewrap drainage plane.
Board Siding
Similarly to masonry veneers, some sidings installed directly against building paper or housewrap will also hold water up against them. The problem can be even worse with housewraps and wood claddings such as cedar or redwood, due to some water-soluble extractives in wood being a type of surfactant that can contaminate the surface of the housewrap. As the surfactants migrate into the house wrap, the house wrap looses its hydrophobic properties and begins to absorb moisture. This loss of water repellency of the housewrap constitutes a fundamental failure in its role as a drainage plane.
Figure 3: Hydrophobic and hydroscopic properties
Figure 4: Failed water drop test of a housewrap
For all sidings (except vinyl) it is recommended that they be installed over some form of drainage matt or furring strips in order deal with the problems of water retention and potential surfactant contamination. This spaces the siding off of the housewrap to prevent surfactant contamination, increases drainage, and often increases drying by ventilation as well. Furring strips are a more traditional method of moving the siding off of the back up wall construction, however from past experience both methods have been very successful at managing the moisture problems with board sidings and housewraps.
Figure 5: Drainage matt installed behind wood shingles
Hard Coat Stucco, Thin Brick, and Manufactured Stone Veneer
In the case of stucco, thin brick, and manufactured stone veneer claddings, renderings tend to bond to all plastic housewraps and some building papers. This reduces or eliminates the ability of the assembly to drain.
Figure 6: Stucco renderings bonded to housewrap
Stucco, thin brick, and manufactured stone veneer systems should always be installed over two layers of building paper, or one layer of building paper over a layer of housewrap. The first layer of building paper acts as a bond break, leaving the second layer to continue to perform its intended function of a drainage plane.
Other drainage plane concerns with stucco can be created when there is a change in the back up wall construction. In Florida it is common for the main floor of the house construction to be concrete block while the top floor is wood frame. Stucco is commonly installed directly onto the concrete block with no building paper installed behind the stucco. This barrier cladding approach can be used successfully due to the capacity of concrete block to absorb water and dry out under more favorable conditions. The problem occurs at the transition between the drained and barrier cladding approach. Standard stucco installation practices have created a reverse flashing at this location.The stucco control joint is traditionally installed over top of the building paper. This directs the water towards the interior at the top of the masonry block.
Figure 7: Control joint over top of building paper (not recommended)
This problem can be corrected by using a weep screed for the control joint and ensuring that the building paper drainage plane is installed in a shingle lap fashion over the screed. In addition, the transition joint should also be protected by a strip of waterproof self adhered membrane. The membrane flashing will prevent moisture that builds up at the base of the stucco just above the screed from being driven to interior at this location.
Figure 8: Weep screed used as a control joint with building paper shingle lapped over top of the flange
