Helpful? 0

How best to insulate framed walls inside stone walls?

I have an old stone farmhouse with approximately 18"thick sandstone walls in good repair. Inside the stone walls there is a layer of 1" thick boards horizontally, then 2x4" studs, then another layer of boards, then a layer of plaster and then paneling. I am renovating the upstairs first, and have the top fire blocking off of the wall cavities. I can blow insulation down from the top everywhere except below the first floor windows, so I want to do this while I have the upstairs walls open.

What, if anything should I put in the first floor walls? I have the ability to blow cellulose and fibreglass. I'll re-do the downstairs walls someday, but hopefully not for a long time. what should I look out for as far as vapor, moisture, etc? the basement is fairly dry, but still a stone basement with all that that includes.

Thanks for the help. I've learned a lot here.

Asked by Karl Schwingle
Posted Sun, 02/09/2014 - 20:54
Edited Mon, 02/10/2014 - 07:13


4 Answers

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Helpful? 1

There are two potential problems here:

1. Stone walls do a good job of conducting heat, so you can't insulate a stone building in a partial way. You need to insulate 100% of the walls to prevent thermal bridging. If you just insulate half the rooms, the insulation won't do much.

2. Thick stone walls are sometimes damp, due to driving rain or vapor flow from the interior during the winter. That's why you probably don't want to have any fluffy insulation (cellulose or blown-in fiberglass) touching the interior surface of your stone walls.

The best way to proceed would be to open up all of your walls and install closed-cell spray foam on the interior side of the stone walls.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/10/2014 - 07:17

Helpful? 0

I kind of figured that was the answer i was going to get.
1: I would be insulating all the walls, the below the window areas I can get into from below.
2: we have a small spray foam rig and an insulation blower, so should I spray an inch on the outer layer of boards and then dense pack inside of that? should I do more than one inch? could I use one of the slow rise pour-in products?

Answered by Karl Schwingle
Posted Tue, 02/11/2014 - 07:58

Helpful? 0

You probably want to install enough closed-cell spray foam to keep the inside surface of the cured foam above the dew point during the winter. The colder your climate, and the thicker the layer of dense-packed cellulose that you intend to install on the interior side of the foam, the higher the R-value of the spray foam layer has to be.

For more information on these issues, see these two articles:

Calculating the Minimum Thickness of Rigid Foam Sheathing

Are Dew-Point Calculations Really Necessary?

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/11/2014 - 08:16

Helpful? 0

So we stripped the walls, and have it back to the studs. with full 4" studs, should I leave a 1" airspace between the boards and foam and then add 2.5" of foam (open or closed cell?) which will leave about a 1/2" airspace between the drywall and foam (and allow a little bit of margin of error for smooth drywall installation? then to vapor barrior (4-6 mil) or not?

or should i foam right against the boards (I'm leaing towards sheets ripped down with the edges 2- part foamed) with 3.5" foam leaving that 1/2" airspace between the drywall and foam. again, vapor barrier?

I can get EPS cheaply enough that it's an attractive option, as long as the vapor permeability is OK. I really would like to not have to 2-part the whole surface.


Answered by Karl Schwingle
Posted Tue, 02/18/2014 - 12:52

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