House Wrap

House wrap is a synthetic moisture-resistant barrier that goes over your exterior sheathing, under the siding. Housewrap is synthetic sheeting wrapped around a house to protect against moisture infiltration. Like building paper, it is a water-resistive barrier, and like all WRBs, its primary function is resisting the infiltration of water into a building assembly.

Here’s the part people get wrong. It’s not insulation. Housewraps are not a form of insulation. If moisture from either direction builds up within stud or cavity walls, mold and rot can set in and fiberglass or cellulose insulation will lose its R-value because of the heat-conducting moisture. So the wrap protects the insulation. It doesn’t replace it. The insulation stays in the wall cavity where it belongs.

The other thing it does — and this matters more in a Montana winter than almost anywhere — is breathe. Housewraps provide a vapor-permeable layer that resists liquid water from the outside, while also allowing water vapor to escape the assembly, enabling the wall to breathe — for example, if water vapor is driven from the interior to the exterior during a heating season, the housewrap will allow the vapor to escape. Long heating season, warm moist interior air pushing outward — that vapor has to get out, or it condenses in the wall and rots your framing.

One more job worth understanding. The wrap acts as a backup drainage plane. Primary water management comes from water-shedding characteristics like overhangs and exterior claddings. Secondary, redundant water management uses a weather-resistive barrier to protect the sheathing and framing from moisture damage. A weather-resistive barrier is a drainage plane. It allows water that has penetrated past the siding to drain away from the wall system. Your siding is the first line. The wrap is the backstop for whatever gets past it — and something always does.

Choosing the Right Type

The market’s loaded with options now. Housewraps are available in dozens of varieties. Most are nonwoven or woven plastic fabrics made from either polyethylene or polypropylene. Some have micro-perforations to let water vapor pass through, and others are designed to let water vapor diffuse through the fabric itself.

For durability and moisture handling, lean toward nonwoven, non-perforated products. Tyvek HomeWrap is the classic example — durable polymer that won’t rust or rot, with superior tear resistance compared to other leading non-woven, nonperforated wraps, and breathable with a water vapor transmission rating of 5 perm minimum. That breathability number is what lets your wall dry to the outside.

The fastest-growing category is drainable wrap, and the reason is simple physics. Drainable housewraps incorporate both a water-resistive layer and a drainage gap, greatly facilitating water drainage from behind the exterior cladding. Unlike typical housewraps, which provide only a water-resistive layer and can trap water behind cladding, drainable housewraps provide a continuous drainage gap that allows water to quickly escape. How much better? Compared to standard housewraps, the newest drainable housewraps — especially those providing a minimum one millimeter gap — can be as much as 100 times more effective at removing bulk water from the wall.

If you’re running fiber-cement or another rigid cladding, the drainable type isn’t optional thinking. A textured, three-dimensional wrap keeps water from getting trapped between a hard cladding and the sheathing. Tyvek’s drainable product comes in a 10-foot by 125-foot roll built for exactly this.

Spec	Detail
Material	Nonwoven or woven polyethylene/polypropylene
Vapor rating (Tyvek HomeWrap)	5 perm minimum
Drainage efficiency (Tyvek, ASTM E2273)	Greater than 90%
UV exposure window (Tyvek)	Up to 120 days
Code standard	ASTM E2556 Type I or Type II
Common roll widths	3 ft, 5 ft, 9 ft, 10 ft
Common roll lengths	100 ft, 150 ft, 165 ft, 200 ft

For code acceptance, look for ASTM E2556 compliance. Building codes typically require a water-resistive barrier in all exterior walls. The barrier shall comply with ASTM E2556, Type I or Type II, or a No. 15 felt complying with ASTM D226, Type I.

Installation Is What Actually Keeps Water Out

This is the whole ballgame. Installation quality matters more than which wrap you buy. A premium roll installed sloppy will leak. A basic roll lapped and sealed right will keep a wall dry for decades.

Install it shingle-style — every layer overlaps the one below so water runs down and over, never behind. Materials higher on a wall should overlap materials lower on the wall — consider the path a drop of water would take running downward. Laps must be of sufficient length to prevent water entry; 4 to 12 inches of overlap is typically recommended. Material should also extend around corners by 6 to 36 inches.

The membrane alone does not stop air. While housewrap functions as a water-resistive barrier, it does not automatically serve as a continuous air barrier unless seams, fasteners, and penetrations are properly sealed. Air leakage control depends on installation detailing rather than the membrane material alone. On Montana’s windy, open sites that air-sealing is money — sealed seams cut the air leakage that drives up heating bills all winter.

Seal seams with the right tape. Not whatever’s in the truck. Overlap and seal all seams, and seal penetrations. Tape is usually used to cover seams, while sealant is used where tape may not provide sufficient adhesion, such as sealing to wood or concrete. Use manufacturer-approved tapes and sealants, not generic tape such as duct tape. Tyvek tape, the various flashing tapes, Zip System seam tape — match the tape to the wrap. The stock here includes Tyvek tape in 2-inch by 165-foot rolls and flashing tapes ranging from 2-inch up to 12-inch widths.

Flashing integration around windows and doors is where most walls fail. Each layer has to direct water over the one below. The accepted approach for openings is the modified “I”-cut: the modified I-cut wrap creates three flaps that wrap around the window opening. Self-adhered flashing like Vycor Plus or Tyvek Flex Wrap handles the sill and jambs — Vycor Plus comes in 4, 6, and 9-inch widths, and the Flex Wrap conforms to the rough opening.

Don’t forget the bottom. To maintain a continuous air retarder around the building envelope, housewrap should cover the seams between framing members, such as between the bottom plate and foundation. Seal that bottom edge down or wind drives water and air right up under it.

Fasteners matter too. Wide-crown staples, nails with a large head, or nails with a large plastic washer are recommended for wood frame construction. Distance between fasteners is specified by the manufacturer or codes, but 12 to 18 inches is typical. On a windy site, cap nails earn their keep — they hold the wrap against wind loading before the siding goes on.

A note on roll width and crew size. Housewrap is available in 3-foot rolls, but rolls are usually 9 feet wide and require two people for installation. Plan the labor. A 9-foot roll fights one person on a windy day.

Don’t Leave It Exposed Too Long

House wrap is a construction-phase product, not a finish. Housewrap can only be exposed for a few months, as it is not a weather-resistant layer, and weather elements including sunlight and temperature can damage it. Tyvek’s window is generous — it withstands up to four months, 120 days, of UV exposure — but the clock is running. Given Montana’s short building season, plan to get cladding on well inside that window. If the wrap weathers and degrades before the siding goes up, you’ve defeated the whole purpose.

Frequently Asked Questions

Does house wrap add any insulation value? No. House wrap adds no R-value. Housewraps are not a form of insulation. Its job is moisture and air management. The insulation stays in the wall cavity, and the wrap protects it — because wet fiberglass or cellulose loses its R-value when moisture conducts heat through it.

What’s the difference between standard and drainable house wrap? Standard wrap is a flat water-resistive layer that can let water pool behind the cladding. Drainable wrap adds a gap. Drainable housewraps provide a continuous drainage gap that allows water to quickly escape from the wall system. With a minimum one millimeter gap, they can be as much as 100 times more effective at removing bulk water than standard wraps. For rigid claddings and wet exposures, drainable is the smarter pick.

Can regular tape seal the seams? No. Use manufacturer-approved tapes and sealants, not generic tape such as duct tape. Generic tape lets go, and a seam that opens up is a path for water and air. Match the seam tape to the wrap brand.

Does the wrap by itself stop air leaks? Only if you seal it properly. Housewrap does not automatically serve as a continuous air barrier unless seams, fasteners, and penetrations are properly sealed. Air leakage control depends on installation detailing rather than the membrane material alone. Tape the seams, seal the penetrations, seal the bottom edge — then it works as an air barrier.

How long can house wrap sit exposed before siding goes on? Depends on the product, but it’s months, not years. Housewrap must hold up to wind and resist UV for 30 to 90 days. Tyvek HomeWrap withstands up to 120 days of UV exposure. Sunlight and temperature degrade it, so schedule your cladding before the window closes.

What goes around windows and doors? Flashing, integrated with the wrap so water always sheds downward and outward. The modified I-cut wrap creates three flaps wrapped around the window opening. Self-adhered butyl or membrane flashing at the sill and jambs seals the rough opening against the wind-driven rain and snow that gets forced into openings.