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  1. #1
    Join Date
    Aug 2011
    Location
    Los Angeles, Ca
    Posts
    3

    Default Insulating exterior walls

    I'm repairing and painting my house so I thought it would be a good time to insulate the the walls on the old part.

    The original house was built in 1938 and has 9" dutch lap or v-rustic siding. There is no sheathing just tar paper and the siding.

    I was thinking of doing dense pack cellulose and I have some questions.

    I can't find a blower to rent that will do dense pack are the Home Depot style good enough?

    How do I patch the holes so they don't show? The companies I've called that are reasonably priced don't patch.

    If I or they drill through the siding how do I best patch the holes?

    Or should I remove a row of siding at the top and bottom of the wall staple the tar paper and put up some osb in place of the siding while they blow it in and then replace the siding?

  2. #2
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Insulating exterior walls

    The insulation can be blown from the inside.
    In some cases it can be advantageous to easily patch 2 inch holes in interior walls than risking water issues on exterior patches.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #3
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,731

    Default Re: Insulating exterior walls

    I would encourage you to nor insulate the walls, it is not cost effective. Los Angeles is a pretty mild climate so any insulation will take a long time to pay back, possibly never achieving a payback.

    If you spray cellulose into the walls, then you need to add a vapor barrier on the interior side of the wall. If you don't, moisture from the interior air will penetrate the wall and as it migrates through the insulation, it gets progressively colder and condenses on the insulation and the studs. Over time, this will rot the studs and reduce the insulation value to something as low or lower than you have right now.

    Since the old part of the house is uninsulated, it probably has a lath and plaster interior wall surface. A lot of people are reluctant to give this up and replace it with sheetrock, but that is what you should do if you want to insulate the wall properly. This also greatly increases your costs.

    Now for the payback. Your wall currently has an R value of around 3. Over 10% of your wall is studs. The studs have a value of about R-4.5, the lath and plaster R-0.2, the air space R-0.75 and the exterior sheathing about R-1.25. So 10% or more of your wall is R-6, the rest is R-2.2. The heat loss through your wall insulation is typically only about 20% of your total heat loss.

    Now if you blow cellulose in the wall cavity, you are raising the cavity to a total of R-13. And that is only when the insulation is absolutely dry. Now another factor comes into play, lateral heat movement. When the wall is uninsulated, this is not a factor as heat is always looking for the path of least resistance, which is through the wall cavity.

    Now some of the heat will travel laterally along the wall to the studs where it finds a path of lower resistance. It then travels through the stud to the outside sheathing where it travels both laterally and radiates outward. If you used a infrared camera at night in the winter on an insulated wall, you can see the studs clearly.

    The amount of heat loss through the studs requires calculus to figure it out, but a rough estimate is something a little over 50% higher than it was in an uninsulated wall. That makes your insulated wall about an R-9 or 10, when it is dry. A small amount of humidity in the insulation from water vapor and it will be half that.

    Rip out the lath and plaster, insulate and add a vapor barrier and you will protect the from moisture damage and keep the insulation at a good R-9 or 10.

    Now take your total winter heating cost and your total summer cooling costs, multiply that by 0.2. from that, your potential savings are maybe 75%. So if your total heating and cooling costs run $500, you could save maybe $75 per year. The interest from a loan, or interest lost from taking from savings could be greater than this.

    Remember, on the heating and cooling costs, that is not your total utility bill. In your climate, the heating and cooling bill is probably a lot lower percentage of your total utility bill, most of your bill is heating hot water, cooking, lights, entertainment etc.

  4. #4
    Join Date
    Aug 2011
    Location
    Los Angeles, Ca
    Posts
    3

    Default Re: Insulating exterior walls

    Thanks for the feedback.

    I already knew insulating the walls wasn't the best use of insulation. It was a since I'm painting the exterior this is the time to do it.

    The cost was actually not bad. After local utility rebates it is $.90 per square foot. So to do the uninsulated part of the house is about $600. So about a 9-10 year return based on your numbers which are close enough. Half the house is an addition that has bat insulation in the walls.

    I can't do it from the inside because drilling all those holes through the plaster is a major mess and the house is inhabited.

    Because of the hassle with patching and a low return I'm not going to bother. I'll spend the time and money on air infiltration and attic insulation that was on the list anyway.

    Thanks again

  5. #5
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,731

    Default Re: Insulating exterior walls

    Good plan, I think you will get a better ROI, especially on the infiltration part.

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