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Thread: Insulatiing an exterior wall - will this work?

  1. #1
    DIY Member shluffer's Avatar
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    Default Insulatiing an exterior wall - will this work?

    I was searching the internet for different ways to insualte my sunroom, and I ran into an interesting idea. The gist of it is to insulate the cavity between the 2x4s with R-13, and install rigid foam board to close up the walls. He then sealed the seams with tape, added furring strips on top, and secured the sheetrock to the furring strips.

    The upside of this is that for an older home like mine, it should do a good job of sealing the exterior wall. It should also be good for soundproofing. On the other hand, an moisture that makes it into the wall will only have one side to dry out intead of two.

    Anyone have any insite as to if this is a good idea? If it is, I have a couple more questions. First, do I need the furring strips, or can I secure the sheetrock to the studs through the rigid board? Second, it seems to me that a moisture barrier is pointless once I have the rigid foam. Should I use unfaced insulation? Lastly, I noticed that there is R-15 insulation avaialble at 3.5 inches thick. It is about three times the price of R-13. Is the higher R value worth the cost?

    Thanks in advance for the help.

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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by shluffer View Post
    Is the higher R value worth the cost?
    How much do you pay per BTU or kwh? How long will you live there?

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    Homeowner geniescience's Avatar
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    They make house wrap sheeting too. They make Great Stuff expanding foam too.


    Making it airtight on one side only is normal procedure in climates that depend on it to be comfortable indoors during the coldest half of the year. Whether you will do it right depends a lot on your ability to understand the factors. For example, do you know when the vapor barrier should be closest to the outermost layer and when it should be closest to the innermost layer? If not, post information about the climate where you live. Or your location.

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    DIY Senior Member dlarrivee's Avatar
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    Let me make a few things clear:

    You only want ONE vapor retarder in this design, kraft faced batts are a BAD idea.

    Foam does NOT make a good sound proofing material, you'd need to DECOUPLE the surfaces for that.

    I've never seen it done on the inside before, how would you push your electrical boxes out to the finished drywall surface?

    The good thing about this design would be the thermal break you would get, ie. the studs would not be touching both exterior sheathing and interior drywall.

    Lets hear what climate you live in and I'll tell you if it's a good idea or not...

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    DIY Member shluffer's Avatar
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    I'll try to reply to everything at once. If more info is needed, let me know.

    I'm in CT so electricity and oil are rather expensive. We haven't yet decided how we are going to heat the room, so I can't answer how much it costs. Prior to this project, the room was allways cold and we used kerosine. We are likley to either install a vent free gas heater, or possibly install a heatpump for the house (converting over from oil). I'm not sure which way we are going yet.

    I also can't answer how long we are going to be here. It will either be another year, or another five. Its unlikley to be in between.

    Its allso worth mentioning that this room has three exterior walls, and no second floor above it. So it is exposed on four sides (two are interior - the floor and one wall).

    I can live without the soundproofing. It was just gravy.

    Electrical boxes would have to be cut out of the foam. I would concider either putting the outlets in the floor (I have to check code on this). they sell inserts for outlets to insulate them, or at least seal them better. Those would be added if I put them in the wall.
    Last edited by shluffer; 11-02-2010 at 04:58 PM.

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    DIY Senior Member dlarrivee's Avatar
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    I didn't ask if you were going to cut them out of foam, I asked how you expected to get them out far enough when you add X inches of foam in front of the stud. Boxes are anchored to the studs, not the foam...

    I would worry more about what sort of insulation is at the top of this structure rather than the walls...

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    DIY Member shluffer's Avatar
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    I misunderstood the question. I was planning on either using deeper boxes (I can get them from an inch to 4 inches deep) or using flush mount boxes, not the boxes that mount on top of the sheetrock, rather the ones with tabs that hold it against the wall.

    for the ceiling I was going to go with R 38 or R30 depending on if I drop the ceiling. Currently, I only have 10 inches of space so I'm stuck with R30. I would like to use the same method and cover with rigid insulation. I'm concerned about overheating the roofing material. I would like to leave some vent space between the roof and the insulation, but I haven't yet figured out how to do it without using lower than R30 or dropping the ceiling a couple of inches. The lighiting is also going to create issues. My wife would like me to install cans. I haven't found a good way to insulate around them yet. I would like to go with ceiling fans. They use less space in the ceinling and are therefore easier to insulate around. Another option would be using a radiant barrier against the roof and using a lower R value insulation underneath it (I won't have enough room for R30). Once again I'm worried that a radiant barrier would overheat the roof in the summer. Any suggestions?

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    Homeowner Thatguy's Avatar
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    Asphalt roof life does depend on having adequate attic ventilation, but I'm not sure that whatever you do in the way of addt'l insulation would pay for itself in fuel savings within 5 years.

    Possibly 1/2" of air space below the roof would provide adequate natural convection. The Engineering Toolbox site may have some methods for figuring this out.
    http://www.google.com/cse?cx=partner...+air&sa=Search

    For dark roofs sunlight may put up to 1 kw per sq. meter into your roof.
    Last edited by Thatguy; 11-03-2010 at 02:26 PM.

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    DIY Senior Member dlarrivee's Avatar
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    Just use the staple-on styrofoam soffit baffles for ventilation...

    What do you expect to insulate the roof with other than foam, fibre?

    Is this going to be a cathedral ceiling, you said you only have 10" to work with?

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    DIY Member shluffer's Avatar
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    First of all, I would like to thank everyone for there help with this. I appreciate your patiance in pulling the needed information out of me.

    The roof over this room is flat rolled aspholt. The rafters holding up the roof are 2X10s. The lathe (the ceiling is plaster) is connected directly to the rafters. I am in the process of removing what is left of the plaster. I plan on affixing my sheetrock to the rafters preserving the original ceiling height in the room. So I have 10 inches to work with. My first thaught was to use R30 fiberglass rolles, then possibly a layer of the rigid styrofoam. I'm still not sure if the styrofoam will be worth the cost. The R30 is 9.5 inches thick whcih will leave some space between the roof and the insulation. I'm open to any suggestions on this. I'm trying to learn more about this sothat I can properly balance cost and efficiency.

  11. #11
    Homeowner Thatguy's Avatar
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    Whatever air space you have under the roof, 10 to 60 air changes per hour is probably enough to adequately cool the roof.

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