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Insulating existing exterior walls

I need to add insulation to exterior wall. Do I have to rip off the exisiting drywall and replace what's there? It's most likely blown in insulation and I don't want to deal with the mess. The room is very narrow, so I don't want to lose space Can I just put styrofoam insulation overtop, then plastic vapour barrier , then drywall?


Re: Insulating existing exterior walls

Can I just put styrofoam insulation overtop, then plastic vapor barrier , then drywall?

If your thinking about applying sheets of rigid foam over top of the existing walls then cover that with drywall .... here's my thoughts ... sure you can ... it is a good solution in some cases.

Just remember you will encroach into the room from the combined thickness of the foam and drywall.
It's a good idea to check with your local building authority to see if they will allow this. I would be surprised they wouldn't since the foam would be covered with drywall.... but you never know.

By applying those sheets of foam you accomplish two things :

  1. increasing insulation R value
  2. providing a continuous thermo break from the existing wall

You can fasten the sheets of foam by using mechanical fasteners ... screws or nails that have plastic washers ( or buttons ) or by using adhesive. If you decide to use an adhesive just be sure to use one that is designed to be used for foam otherwise the incorrect type will have a chemical reaction and actually melt the foam ... ruining it.
Once the foam sheets are in place I recommend to tape all the seams with a house wrap tape to completely seal them. This will stop any air movement which will prevent drafts and likely any condensation issues. This will also eliminate the need to apply the plastic sheet vapor barrier.

The best description that I've run across was from a insulation contractor .... a foam coffee cup we are familiar with.
If you have an ice cold liquid inside the cup exposed to warm ambient temperatures there will be no condensation forming on the outside of the cup. The opposite .... a hot liquid in the cup exposed to cold ambient temperatures will show no condensation.
There has been lots of ongoing discussions as to types of foam whether to use open or closed cell foam while I personally recommend a closed cell type. There is a link included to help describe the different types.

Then attach the drywall over top making sure to use long enough screws for getting into the studs.

When this is done you will have a thicker wall and likely require extending the receptacle boxes located at these walls.... hopefully there is enough wire length to allow for this. There is a link of an extender included for an example.
You will have to remove all window and door trims that will be on these walls and then reapply them after. You likely need to extend the window and door frame to accommodate the added thickness as well.

You may consider using spray foam in the existing box to seal for drafts and adding insulation. Just make sure to leave enough clearance for the receptacle and to check with your local building authority if this is acceptable.




Hopefully this helps.:)

Mark Barone
Re: Insulating existing exterior walls

My walls have no insulation and i hesitate to insulate them for fear moisture will be trapped in the wall and cause mold and rot the wood. Is there a way to insulate the walls and not have this moisture problem?

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