Like an enormous gift, almost all new construction and renovation these days are wrapped in (typically) white, shiny-ish paper.  Usually, this paper has writing and little dots all over it.  This stuff is House Wrap.

House Wrap, or more correctly, “Wind Barrier,” is typically a fibre-reinforced cloth about as thick as a bedsheet.  It is usually attached using “cap-head” nails (nails with plastic washers factory-attached to sit between the metal nail head and the House Wrap to prevent tearing of the latter by the former), and all seams between different sheets of Wrap are closed special tape.  The most common brand is Tyvek, made by DuPont, and indeed many builders refer to all House Wrap as “Tyvek,” regardless of the brand.  Outside of DuPont’s product, many other building material companies produce House Wrap, with some lumber yards and big box stores having their own brand. 

Before I get into more detail about House Wrap, I just want to mention that there is one other increasingly common form of Wind Barrier, patented by Huber Building Products, called the Zip Wall system.  It’s part of their line of high-end Oriented Strand Board (OSB), along with Zip Roof and Advantech subfloor.  The Zip Wall system replaces typical wall sheathing (the plywood or similar material attached to the outside of exterior wall framing) with proprietary OSB that is coated in a sort of water-resistant barrier.  If you see an in-process project with brownish-red panels around the exterior walls, it’s probably this.  Instead of adding House Wrap on top of these panels, you just cover the seams (in a particular way) with their proprietary Zip tape.  I’ll post something in the future about the pros and cons of this system relative to traditional House Wrap, but for now, I’ll get back to the more common Wind Barrier type.

House Wrap is installed by unrolling it onto all exterior walls, before siding, such that the sheets are oriented horizontally, always starting from the bottom of the walls.  While these Wraps come in all different widths, 9ft is a common width for single-story buildings, eliminating the need for multiple “courses” (rows) of the Wrap.  Where multiple rows are necessary, each row overlaps the row below it by a few inches, with its bottom seam taped to the row below using the special tape (not the same as Zip tape, and less expensive).  The mentioned cap-head nails are then sporadically driven in (sometimes staples are used instead), as these only serve to prevent the Wrap from blowing off the house until the siding is installed.  Windows and doors are typically first covered up by the wrap, but are later cut out in a particular way that minimizes the chance of water leakage into them down the road.

OK, so what does House Wrap do?  Well, as the “wind barrier,” it covers seams between the sheathing panels (again, the plywood or OSB panels to the outside of exterior wall framing) as a second line of defense before the air fights with your insulation in a battle over whether or not your heat (or AC) stays inside your home.  The first defense is the siding that will be installed after the Wrap, but different siding types and installations have different levels of efficacy when it comes to keeping out air.  Siding is also not a perfect defense against water, in particular when that water is driven up into seams between siding panels.  (This is why we recommend and can include large panel sizing as a great option for Homebuilt’s kits!)  When properly installed, House Wrap is an extremely effective barrier against such “driving rain”.

Equally important is what House Wrap does not do.  House Wrap is specifically not a "Vapor Barrier", meaning that airborne moisture is allowed to pass through it.  This is very important (and why you can’t just wrap your house in plastic!), because in many climates it is critical for airborne moisture to be able to move from the inside of the wall cavity (where the insulation is) to the exterior, preventing mold from growing in your walls.  (More on this in a future post.)

Finally, how do you pick House Wrap?  Well, the most well-known brands are of course among the most expensive.  In my opinion, it’s a bit of a difference between “works well” and “works great”.  While nearly all common brands have solved this problem, builders used to complain about workability issues with certain House Wraps.  They didn’t flatten easily off the rolls, they were too rigid to wrap around corners, the tore easily at nails, or the bond between the House Wrap and their House Wrap Tape wasn’t very strong.  Now the decision is much more about which you trust for long-term performance.  House Wrap needs to allow water to “whisk” off of it, which may work well initially but could dull in performance over time.  It also needs to do something almost magical – prevent water from getting in but allow water vapor to get out.  This all happens at the micro level, of course, but which brand do you trust has the experience and testing to guarantee this performance over time, and which’s little micro holes will get plugged up with micro crap and start acting as a covert vapor barrier?  Which material has been tested and shown that it will not degrade when exposed over time to penetrations by nails of all different metal and coating compositions.  Will your heavy fiber cement siding start to tear the House Wrap at these penetrations?  Will your oily cedar siding clog up those micro pores?  Will the sharp edges of vinyl siding cut into the House Wrap causing a tear that will grow over time?  In my opinion, none of the current products on the market have themselves been field tested and verified over a long enough time period to give a definitive answer on performance.  So, it becomes a matter of what manufacturer do you trust:  The one who makes other top building materials?  The one that specializes in more general high-tech materials and chemicals?  The one branded by your favorite lumber yard or big box store, which has an established reputation to uphold in doing so?

Also, consider available sizing and Tapes when selecting a House Wrap.  If you have 9 foot high exterior walls, and only one of your available brands sells 9’ wide rolls, buying that one will make your life much easier.  If you can only find a certain brand of House Wrap Tape, you might want to stick with that brand’s actual House Wrap, if for no other reason because who knows what’s hidden in the fine print of their warranty information.

Finally, look at the stated features of your options.  Does one cost more because it is doubly thick, and do you expect your House Wrap to take an abnormal amount of beating from you, your siding, or sitting out in the elements for a while before the siding is installed?  Does it have markings on it that will help you to line-up and quickly install your siding?  Most importantly, consider the efficacy of your particular siding and its installation as a first line of defense against water and air leakage.  If it’s a bunch of little pieces with many seams, or a heavy, abrasive, or oily material, or you think that the installation details or execution might be more of a water and air deterrent than a barricade, go for the top end House Wrap.  It doesn’t cost much more, and you’ll have decades of slightly lower energy bills and no mold removal to make up the difference.

Thank you for reading,

Dennis Michaud

Co-founder, Homebuilt

 


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