Replies

1. 
   KDESIGN | Nov 09, 2011 05:47pm | #1

   Some thoughts

   How are the walls finished on the interior?  If you add 2" of foam board on the exterior and seal the joints, that would be a vapor barrier.  But with only 2" of foam, the inner surface may or may not remain warmer than the dew point.  Adding a vapor barrier film at the inner surface of the film won't help because it will still be at the same temperature as without an added vapor barrier. 

   But the larger issue is whether 2" of foam is enough.  That masonry wall will probably always be considerably colder than the interior air temperature, so there will always be a radiant heat loss to the cooler walls.  At the normal comfortable air temperature of around 72 degrees, the room will feel colder because of that radiat loss.  The cold walls will feel like cold coming at you, but actually what you feel will be heat being drawn away from you. 

   So the only remedy is to maintain a higher air temperature to compensate for the radiant loss.  Generally, this kind of imbalance adds up to a difficult and uncomfortable situation.  I think you would be better off adding insulation to the interior of the walls rather than the exterior, if that is possible.  Maybe it would more practical to add more insulation to the interior than to the exterior. 

   In terms of heat loss, it won't make any difference whether the insulation is on the exterior or the interior for a given isulation thickness.  However, with the insulation on the exterior, the masonry wall will be a big heat sink in the interior.  So if you turn the heat down at night, it will take a longer time to warm up the walls in the morning.  So the thermal mass will work against you.  I don't see any situation where it could work to your benefit.   

1. 
   skrestore | Nov 12, 2011 10:59pm | #4

   Thanks. I guess I wasn't entirely clear. I was thinking of insulating the interior wall of the house but only the ones that are exterior walls. There are a couple os additions so putting insulation on the interir wals there wouldn't make sense. Essetially I would be studding out wall on the inside of the house. I do not thin the thermal mass counts for much. However, if the wall never warm  above the dew point since they would now be insulated from the warmth inside I was wondering if there might be condensation thus the question of the vapor barrier. I don't want to decrease the room sizes be 8" so why I was contemplating 2" with foam board so an R of ~14.

1. 
   KDESIGN | Nov 13, 2011 03:15pm | #5

   Skrestore,

   In that case, I would just fir out the walls to the inside by adding 2 X 2s and foam board.  I don't know what kind of finish is on the interior right now, but 2 X 2s would have to be attached to the surface.  Whereas, if you went to 2 X 4 or maby just 2 X 3, you could secure the studs by thier top and bottom plates.  Sometimes stone walls are not perfectly true or plumb, so attaching studs directly to them and getting them all in one plumb plane can involve a lot of shimming. 

   If you can tightly fit the foam or seal it at the edges, it would be an adequate vapor barrier.  But I would fit the foam in snug without any adhesive or sealant, and just use a film vapor barrier over the whole wall under the sheet rock.  If you use polyiso foam board with foil on both sides, and if you leave a small gap between the foil and the original wall, and between the foil and the sheetrock, you will gain the effect of two radiation barriers in addtion to the conduction / convection barrier of the foam itself.  You also get the benefit of two air spaces to add some R-value.

   To get the highest possible insulation value into the small cavity, I would even consider adding a layer of foil directly to the old masonry wall by some type of adhesive.  Then you have a third radiation barrier.   

   What the foil will do is this:  Starting from the interior side, the first layer of foil will reflect thermal wave radiation back to the sheetrock.  The second layer will be unable to emit thermal radiation to the masonry wall.  And the third layer will reflect thermal radiation back toward the room.  But there has to be a gap on at least one side of the foil to get the radiation barrier performance. 

2. 
   DanMorrison | Nov 09, 2011 06:12pm | #2

   It depends...

   skrestore,

   Including your location will help folks give accurate answers. Adding foam to the outside will put the thermal mass inside the thermal envelope, where it will do a lot of good. Insulating inside the walls will preserve the old stone farmhouse look, but it will place the thermal mass outside the conditioned space of your house, so it will compete with your insulation in a sense.

   For a good overview of insulating old masonry buildings, listen to Dr. Joe's podcast over at Green Building Advisor: http://www.greenbuildingadvisor.com/blogs/dept/building-science/insulation-retrofits-old-masonry-buildings-building-science-podcast

   Also over at GBA is a case study of a deep energy retrofit which had exterior spray foam, furring strips, and new siding: http://www.greenbuildingadvisor.com/homes/deep-energy-makeover-one-step-time

   Do you have any photos of the house?

   Dan

1. 
   KDESIGN | Nov 09, 2011 07:00pm | #3

   Thermal mass

   Having the thermal mass of the masonry walls inside of the insulation envelope is a good thing if you can store excess heat in the mass and get it back when you need it.  A classic example would be to have sun shine through a window during the daytime, and heat up a thermal mass that gives the heat back to the room during the nighttime when the sun is not shining.

   Another way and reason to store excess heat in a thermal mass would be to store heat from a wood stove in order to even out its heat output.  But with a convential heating system, I don't see any point in having the walls act as a thermal storage mass.  Why not just produce and distribute the heat you need when you need it?  Why store it?

   In any case, the thermal mass of the masronry walls is likely to contain less heat than the rest of the interior if it is in a winter heating climate and only insulated to R-10 or 12 on the exterior such as would be the case with 2" of foam.   So it would be a thermal heat sink creating a constant source of discomfort.