Interesting you should ask this question. I am just completing a stick frame project, in coordination with an engineer, where we did just as you wondered. The 1" rigid insulation was placed on the interior (ceiling and walls) with sheetrock placed on top.

There are a few considerations: all electrical boxes (switches, outlets, etc.) need to be positioned out an inch to account for the extra material. We chose to tape the rigid at every seam and used it as the vapour barrier. This is where the engineer comes in. There was a great deal of resistance on the part of the builder to insulate and vapour barrier in this fashion. The building inspectors were on board and passed the strategy no problem. In the end this approach does reduce interior space, but the floor plan was adjusted at the design phase to accommodate that loss.

The theory is that the interior rigid reduces, to an absolute minimum, the issue of thermal bridging across the studs.


Here is a question I have after doing this project.  If I was committed to stick framing and went with spray foam insulation to boost the air barrier/insulation value of the cavity ... does the spray foam go around the interior side of the studs? Or, would this idea of rigid insulation on the interior still be useful to reduce thermal bridging?

Every idea seems to create more questions.
Thanks goodness there is a forum like this where I can ask them.

Matthew

Rigid on the interior walls:

I forgot to mention that one of the biggest issues we had was with the plumbers. They struggled with the idea of how the wall system would work with their routing/placement of supply lines and drains. In the end, we had several situations where they needed to come back and adjust the position -- resulting in metal plates being used to ensure future fasteners (screws/nails) don't penetrate these water lines.

If we do it again we'll have a better understanding of how clear we'll need to describe this to the sub-trades.

M

I tried to do what you described, with no success.

1/2" XPS over the studs for a thermal break then 1/2" gyp board. The problem seems to be the surface of the XPS - it's so slippery I was never able to get the gyp to stay in place on the walls. Even with screws every 8" the gyp would gradually slip downward pulling the heads through the paper. It just kept slipping downward until I yanked it off.

EPS might work on walls, either one would probably work fine on a horizontal ceiling.

Maybe glue is needed in cases where the wallboard is free to slide downward (ie, nothing under the bottom edge).

One concern that I would have with installing EPS on the inside of the house in cold climates is that by doing so the location of the dewpoint would be moving toward the interior of the wall.  I would prefer to have the EPS on the exterior although I have done it either way in the warm climes of the South.

We added 1 inch on the interior of the pool room on the exterior walls. No plumbing in those walls and only interior windows, but the electrical was a problem because with normal nail up electrical boxes you couldn't move them that far. They ended up screwing them in, in each location to get them out far enough. The windows weren't a problem, because they were in 2x4 interior walls. I really like the result and wish I had done it on more exterior walls now.

You can get the thermal break you're after by putting it on the interior, but it makes the air-sealing of the interior both more important (because the studs and sheathing are now colder, with more hours/season at condensing temps for interior air), and more difficult, due to myriad micro-detailing it takes to make it truly air-tight.

When you have a choice, it's better to put the foam on the exterior of the studs & sheathing, reducing the number of condensing hours on the susceptible wood.

Posted By Dana1 on 24 Nov 2010 03:17 PM
You can get the thermal break you're after by putting it on the interior, but it makes the air-sealing of the interior both more important (because the studs and sheathing are now colder, with more hours/season at condensing temps for interior air), and more difficult, due to myriad micro-detailing it takes to make it truly air-tight.

When you have a choice, it's better to put the foam on the exterior of the studs & sheathing, reducing the number of condensing hours on the susceptible wood.

Agreed - stop the cold where it starts not after it gets in! Also all the electric boxes break the foam.