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Tyvek house wrap pros & cons
28 February 2004

Q. I have recently been told by a home sales representative that Tyvekģ house wrap when improperly installed can contribute to house mold. Is this true and do you recommend Tyvek or not? I was told that if the Tyvek was installed and not immediately covered or the sheathing was wet when the Tyvek was installed that it would actually contribute to mold growth. Any advice would be greatly appreciated.

A. Thatís a new one on me that Iíll put into the urban myth category. Improperly installed house wrap, which is what Tyvek is among a large field of synthetic sheet materials currently used in new construction, can lower the buildingís resistance to air-flow through the outside walls but Iíve never seen it contributory to a mold problem.

Tyvek by Dupont was the first of a series of large sheet, semi-permeable vapor and air retarders introduced into the housing market during the mid 1970s. Remember, this was the period when everyone in the homebuilding world was trying to build houses tighter so that they would be ever less energy consumptive to heat or cool.

Tyvek was one of those products that came out of research without a specific market and had to have a market developed for it. Other notable products with a research-borne past include such things as latex paint, Post-It Notes by 3M and Silly-Putty. Tyvek hit the building industry at just the right time and became established as a high-tech air/vapor retarder. It has other uses, too, that you'd recognize.

As I've mentioned, it's an air and vapor retarder, not a moisture barrier. It is permeable and it needs to be. Some of the most respected minds in modern building science used to think that the best application of Tyvek was for making Federal Express envelopes or throwaway coveralls that painters or hazardous material clean-up workers wear. I've seen study after study of the product's use on new homes and these studies tend to come down to who's doing them and what axe they're grinding. Some say it works great while others say it amounts to a barely measurable hill of beans. The truth is in the middle and installation attention and workmanship is key.

Also it depends what sort of siding is intended to go over it. Vinyl and aluminum siding, for example, do not require any wrap or sheathing paper by code. Whereas brick or cedar shingles require a moisture barrier-- something Tyvek isnít. Thereís lots of confusion about that. I think itís a good idea to place a house wrap such as Tyvek under sidings that donít require if for moisture but for vapor retardation. For me itís a construction quality issue. Building codes are minimum standards of conduct and many confuse them with quality control standards that they are not.

Tyvek really rises and shines in rehab work where old siding has been torn off a house that was originally sheathed the old fashioned way with horizontal or diagonal boards.

The relative tightness to air (and/or vapor) movement through the outer skin of a building is dependent upon a catalogue of building factors and the application of house wrap is but one. However any house wrap must be meticulously applied or itís next to useless. You can contact the supplier or manufacturer to get exact details of how it must be installed. If you pass a building site and see it flapping in the breeze, know thatís not right.

If the sheathing was wet when Tyvek was applied it will dry. Since the sheathing is vertical itís not likely to be soaked through and surface water will run down. Houses tend to get wrapped prior to insulation and drywall so those open walls, if wet, will air dry quickly.

Things like nailing schedules of exterior sheathing, the way windows and doors are set, how well the inter-wall insulation is installed, what goes on around outlet boxes, wiring and fan penetrations and diligence of the drywall installation all work in concert making the house tight and resistant to air movement through the walls.

By the way, inherently tight buildings need controlled venting. Loose house wrap creates a method of venting called "unscheduled venting" which is a fancy phrase for saying the walls leak air. Over the past few years new houses with s idings that didnít require any sort of barrier behind them started to develop a problem of carpet staining near the floor/wall intersection. Obviously it only showed up on light colored carpeting and homeowners were blaming the heating and air-conditioning systems for it but further research demonstrated that it was air leaking into the house under the walls from outside and depositing tiny amounts of dirt into the carpet. The syndrome was called Ēfiltration soilingĒ. A properly installed house warp would have prevented that. Iíll discuss what to do about it if you have it at your house in a future column.

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