Should You Install a Housewrap Under Siding?

Housewrap stirs up a lot of confusion. Does it prevent moisture? Inhibit mold? Help prevent drafts? Actually, it does all three. Depending on how it’s hung, housewrap can either act as a moisture or wind barrier. So, should you install housewrap under siding? Read on to find out.

When used to slow winds, its job is to seal gaps and leaks in the OSB panels or plywood sheathing that makes up your home’s exterior walls. That keeps drafts from reaching your insulation and prevents heated and cooled air from seeping out through the walls. Although your siding is your home’s first defense against wind, many types have small gaps and cracks at the joints where siding panels line up. And that leads to air infiltration and seepage — the kinds of things that drive up energy costs.

Moisture barrier housewraps do the same thing, but for rain, snow, and other outside moisture. They’re usually made with a permeable material that allows water vapor to travel through them — but keeps out water droplets. This helps them dry out quickly, and prevents problems from mold that can happen when water sits on the surface.

If you’re thinking about installing housewrap during a siding project, here’s what you need to know.

How to Tell if Your Home Needs a Housewrap

Generally, a home’s construction method dictates the type of housewrap (if any) a building needs. A professional builder or contractor should always make the final call on housewrap — using it incorrectly can affect your home’s ventilation or cause mold to build up.

Additionally, where you live and your home’s climate will factor into housewrap, too. For instance, your contractor may install a vapor barrier on just one side of your home if you live in a cold climate. Housewraps and vapor barriers are usually addressed in local building codes (in fact, many places require some kind of weather-resistant barrier), so a professional contractor can also make sure that your home is up to code.

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Housewrap and Your Siding

To be clear, almost any kind of siding you can install provides an opportunity for water infiltration, but some types are particularly susceptible. In particular, homes sided with wood can usually benefit from housewrap barriers, since they have many seams where the boards overlap. Siding that’s assembled from large panels — such as vinyl or aluminum cladding — also makes a good candidate, too, since water can enter through cracks where the pieces are joined.

On the other hand, brick, stucco, and other masonry-based sidings can be used with housewrap — but your builder will need to use the right type. A micro-porous housewrap installed behind masonry can drive moisture into your home’s wall cavities.

Installing Housewrap With Rigid Foam Insulation

One of the biggest questions homeowners have about housewrap is whether it can be installed together with foam insulation. The answer, happily, is yes. In fact, the two products combined make your home super-insulated from outside wind and weather.

If you choose to install housewrap with rigid foam, generally, it should go under the insulation, not around it. However, you can rely on the advice of your contractor to make your final decision, since not every professional agrees on this order. Some builders think that homes with “picture frame” window construction — where windows are framed with strapping lumber before windows are installed — should have the rigid foam go on first, with the housewrap over that. Both ways effectively prevent air leaks and infiltration.

How Housewrap Affects Ventilation

While homes are built with ventilation systems, some builders believe that the majority of a home’s ventilation comes from air leaks, rather than diffusion. That means, in theory, you could potentially cause your home to be “too tight” if your existing ventilation system isn’t working properly. Some construction professionals think that homes need to “breathe” — and that sealing off cracks and gaps too well can prevent this process.

But many professionals that specialize in green building argue that there’s no such thing as a home that’s over-sealed — only one that’s poorly ventilated. Odors can be a real tip-off here, if cooking smells and other odors persist for hours after the food has been eaten, it may mean you have poor air exchange and need to take a second look at your home’s ventilation.

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Housewrap and Moisture and Mold

Improperly installed housewraps can actually increase moisture or contribute to mold growth. That’s why it’s so important to use a contractor who knows their way around weather-resistant barriers when you install a housewrap. Using the wrong materials for your siding, not taping off seams in the housewrap, or installing a moisture barrier with the strips hung horizontally — these can all wind up threatening your sheathing and may potentially cause leaks.

In the end, the decision to install housewrap under your siding is a personal one that you should make after discussing it with your contractor. However, if you’re interested, a residing project offers the perfect time to examine your home’s weather barrier and exterior insulation — and improve it, if need be. That way, your home will be more appealing — from the outside in.