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Green Basics

Structure: Exterior Walls

UPDATED on February 22, 2016Walls Must Be Durable, Well-Insulated, and Weather-Resistant

ABOUT WALLS

###Separating the outside from the inside

The exterior walls of a house have several functions. Not only do they define the shape of a house, they also support the floors, walls, and roof. Equally important is their role in separating the house’s interior from the outdoors, and to do this effectively they have to block the weather with systems that insulate, shed water, and repel moisture and air infiltration.

While it’s important to understand the different roles walls play, if we treat them and their functions separately, we miss great opportunities to improve material efficiency, operating efficiency, and overall building performance. Green building integrates them all.

Wood-frame walls have been the predominant choice for houses in the United States for more than three centuries, with masonry walls a distant second. But today’s alternative products and techniques — many which are more energy efficient and have lesser environmental effects — are certainly worth examining.

A lot of time and materials go into building a house’s walls, and with the exception of a timber frame, all that structure is covered up when the project is finished — out of sight and out of mind. Yet decisions about wall construction have consequences that last as long as the building does, including how much maintenance it will need, how energy efficient the envelope will be, and how difficult the structure will be to repair or modify.

ABOUT CHOOSING A TYPE OF WALL

An overview

For a broad view of the issues that affect residential wall design, read How to Design a Wall.

Green factors

Energy efficiency. Where will the insulation go? Will the R-value of the completed wall be high enough? How will the wall be sealed against air leaks? A focus on energy efficiency pays dividends over time.

Sustainability issues. Can the resources used to build the structure be produced on a sustainable basis?

Local building requirements. Areas prone to hurricanes or earthquakes, for example, may have specific rules to help structures withstand extreme natural events that affect specific areas of the country.

Durability and initial cost. Keep in mind that differences in initial cost may not seem quite as dramatic when weighed against the expected life span of the house.

Combined functions. Wall systems that combine structure with finish have an inherent material efficiency advantage and should be seriously considered.

MORE ABOUT WALLS

Add more foam for a better wall

The performance of almost any wall, in any climate, can be improved by adding a layer of exterior foam. If the wall already has exterior foam, it can be made greener by making the foam thicker. Remember, depending on the type of foam and thickness, foam-sheathed walls may need to dry only to the interior. For walls with more than three inches of any foam or with any thickness of foil-faced polyisocyanurate foam, never include interior polyethylene or other impermeable interior finishes.

Siding is the first line of defense

Walls are a house’s “skin,” and as such must protect the building from rain, wind, and sun. Siding is the first line of defense, but how siding is applied and the kind of water-resistive barrier (WRB) installed beneath it have a lot to do with how durable the walls will prove to be. The skin also includes doors and windows, important components of a home’s thermal envelope but also sources of damaging air and water leaks if not properly installed.

Interior walls define spaces and affect livability

Floors and interior walls don’t keep weather out, but they often do more than just define spaces. How you lay out partitions can affect airflow, solar heat gain, natural lighting, and even how efficiently pipes, wires and ducts are laid out. Structural demands may dictate where interior walls and floors go, which is why you should plan your mechanical systems and framing at the same time.

Doors can help or hinder

Exterior wood doors just under 2 inches thick don’t offer much in the way of insulation, just R-2 or less. When weatherstripping is of poor quality or worn out, the effects are magnified. Doors don’t represent a huge amount of wall area, but they can help nullify all the effort of insulating outside walls carefully. Insulated doors will help, along with high quality weather-stripping. Window area in doors, along with sidelights, should be kept on the small side or eliminated altogether.

Storm doors may seem like an antiquated idea, but they can be helpful in reducing energy losses while providing an extra weather barrier. They’re especially useful when the primary door is exposed to the elements and not protected by a roof overhang or porch.

4 Comments

  1. cZ3nbvJiUr | | #1

    advanced framing
    I have been reading many of your articles on advanced framing... 2x6 walls 24"oc. As I was planning the construction of our new cottage, I consulted with many subcontractors on advanced framing. Without exception, each one (insulation, framing crew, siding, trim) all advised me against going that route. They discussed issues like difficulty hanging cabinets on a wall 24"oc, wavey siding, the need to go to 5/8" drywall to avoid wavey interior looking walls, the lack of studs for wide window & door trim and even.... "the house will be too tight" ! How would respond to that ? I was surprised and disappointed.... I think I am going to go with 2x4 walls, 16"oc w/1" rigid foam sheathing.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Alok J.
    Alok J.,

    1. The purpose of thermal insulation is to slow the transfer of heat through building assemblies (floors, walls, and roofs).

    2. To learn more about the global warming contribution of XPS, see the following blog by Alex Wilson:
    "Avoiding the Global Warming Impact of Insulation."

  3. bsandersga | | #3

    response to irwin
    I am adding a large addition to my house and this is what I am doing; 2X4 studs 16" OC, OSB sheathing, 1" XPS, 3/8" air space, then Hardie panel. This meant that I could stay with 1/2" drywall, I would have another stud to screw the Hardie panels to, and my framer was already familiar with this kind of standard framing, AND 2/4s are cheaper in the long run I think. We'll see how it all works when I get finished later this fall. By the way, I am in Athens, Georgia, a mixed humid climate with 5 months of heating and 4 months of cooling.

  4. Daniel Morrison | | #4

    Irwin Karwick
    I posted your comment as a question in our Q&A forum, there may be more answers there. Here are some quick answers:
    1. Cabinets can be hung with cleats or solid blocking installed behind drywall. See [How to Install Blocking to Anchor Cabinets](http://www.finehomebuilding.com/item/4927/how-to-install-blocking-to-anchor-cabinets-solidly)
    2. Crowned and twisted studs cause wavy siding and drywall.
    3. Backing can be added for wide trim, or simply don't skip the trimmer studs in your advanced framing scheme. See [The Future of Framing is Here](http://www.finehomebuilding.com/how-to/articles/future-framing-walls.aspx?collection=72770)
    4. Stud spacing has nothing to do with how tight your house will be, BUT, houses can't be too tight, only under-ventilated. See Can Houses be “Too Insulated ” or “Too Tight"?

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