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Wall insulation and air infiltration

GBA Editor | Posted in Energy Efficiency and Durability on


I am doing some remodeling and I want to improve my energy efficiency at the same time. I will be replacing some windows in my 1950s house in Austin, TX, and I would like to improve the wall insulation and reduce the air infiltration as well.

From inside to out, my wall construction is: 1/2 inch drywall, 2×4 framing with about 1 inch of paper backed fiberglass insulation, felt paper, and vertical rough cedar siding. There is no plywood, OSB or other sheathing behind the vertical cedar. One wall of the house has limestone on the lower half of the wall with the cedar on the top half.

How would you recommend to reduce the air infiltration and insulate this wall construction?

I have read of many combinations of spray foam, sheets of foam insulation, plywood/OSB, and other types of insulation but it is not clear to me what is best for my situation/location.

Thank you for your responses.


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  1. Riversong | | #1

    Are you replacing the siding, the doors and windows, the interior drywall? In other words, is this an interior renovation or an exterior renovation? How is the vertical siding attached - on strapping or on blocking between the studs?

  2. GBA Editor
    Martin Holladay | | #2

    There is no single answer to your very general question.

    You may be interested in in checking out the current issue (the July/August 2010 issue) of Home Energy magazine. On page 44, there is a short article titled "Retrofit of a Drainage Plane," with recommended details for insulating walls without sheathing.

    Julie Groth installed strips of Homeslicker in each stud bay, from the interior, against the siding, and then a layer of 1/2-inch rigid foam. Once these were in place, the stud bays were filled with spray foam.

    Of course there are disadvantages to this technique, but it is an intriguing solution to the problem of maintaining an air space behind the siding.

  3. Tom P | | #3

    Hello Robert and Martin,

    I am renovating my house a room at a time. I plan to replace most windows and some doors. I want to keep or reuse the siding since it is in excellent condition. The siding is attached to blocking between the studs. It can be easily removed.

    The house style is now called mid century modern. It is a single story house which was thoughtfully designed with true south orientation, 4 foot eave overhangs, terrazo floors on a concrete slab, limestone and cedar exterior. My family is interested in interior renovation, really just an update as the house has a good floor plan. The bedrooms are on the west end of the house and that is where I will be working first.

    Depending on the wall or room I can do the work from the inside or outside. The south wall has limestone on the bottom half and cedar on the top half so for that wall I will probably remove the drywall, insulate and air seal from the inside. The windows are in the cedar siding part of the wall above the limestone. I will need to remove some siding to replace the windows so I can remove it all without extraordinary effort if it is best to add sheathing, etc.

    The west wall is a gable end wall with all cedar siding. I think this wall needs improvement for air and moisture infiltration so I think it would be best to work from the outside and minimize the interior disruption. It is the warmest wall of the house as it gets the late afternoon sun. The electric meter is on this wall for a slight complication.

    The north wall is all cedar siding and under the 4 foot overhang. It could also be sealed and insulated from the outside to minimize interior mess.

    Part of the east wall is solid limestone and the rest is cedar outside with panelling inside. The windows are in the cedar sided walls. The panelling/cedar wall could be insulated and sealed from inside or outside.

    I hope my description wasn't too long and that it helps you understand my house. I am looking for a recommendation on how to seal and insulate one wall from the inside but maybe all the rest from the outside. I don't want to greatly increase the wall thickness but I will consider all advice.

    Thank you for your replies and your advice.


  4. Riversong | | #4

    Is the vertical cedar siding shiplap or board and batten or ???? Is there polyethylene vapor barrier under the existing drywall?

  5. Tom P | | #5

    The siding is board and batten. The boards are about 10 inches wide and the battens are about 8 inches wide. No polyethylene vapor barrier is present.

    Thanks again,

  6. Riversong | | #6

    With no interior vapor barrier, you have many options.

    Remove siding and add either structural sheathing or rigid XPS foam board, WRB, strapping and re-side.

    Or flash-&-batt from the interior. Or install strips of felt or other weather barrier in each stud bay, insulate and drywall, perhaps adding cross-hatched framing for thicker insulation and thermal breaks or adding an interior layer of XPS before drywall.

    The most weather-resistant option would be to remove siding and add sheathing and WRB from the outside. You may have to flash over the existing limestone veneer to maintain continuity of WRB.

  7. Tom P | | #7

    Thank you for your replies. I hope you had a nice 4th of July.

    I will remove the siding, use spray foam insulation then add structural siding, strapping and reside with the board and batten cedar, revising the flashing as necessary. I still have a few more questions.

    How thick or thin does the siding and strapping need to be?

    Do you recommend sheathing like ZIP system as it includes WRB?

    Do I need to use felt paper instead of WRB because the siding is cedar?


  8. GBA Editor
    Martin Holladay | | #8

    Q. How thick does the siding need to be?

    A. If you use rough-cut board and batten siding, the boards are usually 7/8 in. or 1 in. thick. The battens are usually the same thickness as the boards.

    Q. How thick does the strapping need to be?

    A. A minimum of 3/8 in. Most people use 3/4 in. thick strapping.

    Q. Do you recommend sheathing like ZIP system as it includes WRB?

    A. Your choice. Either plywood or Zip System sheathing is preferable to ordinary OSB.,

    Q. Do I need to use felt paper instead of WRB because the siding is cedar?

    A. Not if you are installing strapping (vertical furring strips). Use either product.

  9. Tom P | | #9

    I appreciate all the replies. Houses are routinely torn down in my neighborhood to build new ones and I'm trying to update mine and reuse as much as possible instead. I have another sheathing question and a few more about board and batten siding.

    My siding is 1 inch rough cedar as you said. The question I meant to ask was how thick does the sheathing need to be?

    Sorry for so many strapping questions. My house doesn't have strapping now. The siding is nailed to blocking between the studs.

    I think the strapping should be horizontal since my siding is vertical. Is this correct?

    Should I fill in the spaces between the boards at the bottom strapping to keep pests out? My battens are about 8 inches wide and the gaps between the boards are about 3 inches.

    What material is best for to use for the strapping?


  10. Perry525 | | #10

    Tom, you really need to go back to basics.
    The biggest killer of wood is damp.
    You have the rain attacking from the outside and water vapour from the inside.

    The inside needs to be totally sealed, air tight, to prevent the water vapour that you and your family create moving through the holes and cracks in the wall surface and settling on the colder framing and creating mould and wood rot.

    At the moment, the water vapour is able to travel right through the wall, in or out as it seeks the coldest area.

    If you are sealing the outside, to stop the suction effect of the wind, pulling warm air into your home during the summer and pulling warm air out during the winter, you must use a modern high tech membrane, that stops the rain coming in and allows the water vapour to escape.

    A water proof/water vapour proof plastic sheet under the drywall on the inside, turning each room into an air tight box and a rain proof/water proof membrane under the siding etc, that will allow all water vapour that gets into the walls, ceilings and floors to escape.

    Then think about insulation. The best form of insulation is closed cell polyurethane foam, sprayed between the rafters, joists and framing, the next available is closed cell polystyrene sheet, carefully fitted to avoid gapes, holes and cracks.

    Insulation works best when its on the inside, under the drywall. In this position, it prevents the waste of heat, that would otherwise go towards heating the framing and the spaces between the framing.

    Very often placing the plastic sheet, then an inch or two of polystyrene followed by drywall, over the framing, tightly fitted, no gaps, will stop the water vapour and your heat going out.

    With most homes, most heat is lost through holes in the walls, ceilings and floors.

    Where the insulation is placed on the outside, it prevents the water vapour from escaping, it then condenses onto the nearest cold surface the wood framing.

    The long term success of external closed cell insulation is totally dependent on the installation of the inner water vapour proof plastic sheet.

  11. GBA Editor
    Martin Holladay | | #11

    I assume you are writing from Britain. Your advice is completely inappropriate for Tom, who lives in Austin, Texas.

    If he followed your advice and installed an interior layer of polyethylene, his walls could be quickly destroyed by summertime condensation. (In Texas, inward solar vapor drive can easily lead to condensation against cold interior poly.).

    Since your building science knowledge does not cover warm climates, Perry, you should be careful about offering advice to your trans-Atlantic cousins.

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