We have a brick home built in the 50s. It has plaster walls with about a 2" gap between the plaster and brick with no insulation. I have read all the debates regarding insulating  behind the existing walls. So here is where I need help. Could I build 2x4 walls over the existing plaster and insulate them and then cover them with drywall? What would be the steps? Vapor barrier? There is none in the cavity now. The house is on a slab. I live in northern Indiana where it is cold and windy in the winter. The exterior walls are very cold to the touch. I have insulated the attic.
Thanks for any help you can give!

Posted By randy7652 on 29 Nov 2013 11:02 AM
We have a brick home built in the 50s. It has plaster walls with about a 2" gap between the plaster and brick with no insulation. I have read all the debates regarding insulating  behind the existing walls. So here is where I need help. Could I build 2x4 walls over the existing plaster and insulate them and then cover them with drywall? What would be the steps? Vapor barrier? There is none in the cavity now. The house is on a slab. I live in northern Indiana where it is cold and windy in the winter. The exterior walls are very cold to the touch. I have insulated the attic.
Thanks for any help you can give!

So, the lath is just 2" from the brick, and the brick is the structural wall?

How are the floor & ceiling joists attached?

How deep are the roof overhangs, and how many stories tall?

It's highly likely that you would be able to be pretty safe with an interior studwall approach, and no vapor barrier would be safer than 6-mil poly.  Since there is a 2" gap, the brick qualifies as "vented cladding", in building code terms, and if there is no plywood or plank sheathing, just plaster on lath supported by furring or something, the plaster & lath layer is highly permeable, allowing any interior wall assembly you cook up to dry into the cavity.  Most cavity walls are designed to convection-dry, with weep holes at every foot or so along the bottom course of bricks, with the top vented into a vented attic, or sometimes via vent-holes to the exterior. (If it vented into the attic, hopefully you didn't block that when you insulated the attic.  But if you did, it can be fixed in a couple of ways.  Filling the cavity is not a good idea in your climate, since it carries some rot risk at the joist attachments or , or a high risk of mold/rot in the wooden lath. Keeping it vented cures a lot of ills.

Since the interior studwall isn't structural you have some flexibility in how you build it.  To meet code min in your climate (US climate zone 5) you need a "whole wall" R of about R13-R14.  You could opt for 2.5" of foil-faced rigid polyiso glued to old plaster wall, secured at each stud with 1x furring through-screwed to the studs 16-24" o.c. on which you can hang blue-board or lath or other interior finish, only giving up ~3.25" of interior space.  If you're going for a true high-R performance there is a longer term rationale for as much as 4-4.5" of polyiso (two layers with seams staggered between layers, both sets of seams taped with FSK tape.)

Or, you could put up an inch of polyiso and install a 2x4 studwall 24" o.c.  tight to the iso, insulated with blown or damp-sprayed cellulose, or with UNFACED batts.  Since it's not structural, single top/bottom plates are fine, as is the wider spacing, which lowers the thermal bridging. With the inch of iso and R15 rock wool (fitted obsessively perfectly) you'd be at about R16-R17 whole-wall, well above the code min, but it eats up about 5" of interior space.

Detail the rigid foam as your primary air barrier- that means caulking/foaming the top and bottom seams, and any plumbing/electrical penetrations, and carefully taping the seams of the facers.  Since the foil facers are true vapor barriers, you don't need and absolutely don't WANT and interior side vapor barrier.  Air tightness on the gypsum is important, and standard latex paint is sufficiently vapor retardent to not end up with soggy insulation at the foam/fiber layer.  The ratio of foam-R to fiber R is important- anything less than R5 at the foam layer and you'd have to dial it back to R11s (or lower) for the cavity fill. Higher-density fiber is more air-retardent against convective loss of performance- anything below R13s would be a performance hit bigger than the labeled R might imply.  An inch of foil faced 1.5lb density poly iso is R6-6.5, which is good for cavity fill up to R15 in your climate without interior-side vapor retarders.

Brick walls with stubby roof overhangs are subject to signficant rain wetting, and brick will store quite a bit of that moisture, leading to sustained intense moisture drives when heated up in the sun.  The shorter your overhangs, the more important it is to maximize the air flow through that masonry cavity.  Foil faced iso would block those drives from getting into an interior studwall cavity, but you still have whatever wooden lath/furring etc in the cavity to keep dry, as well as the joist ends, which need to dry into the cavity, but become susceptible to those hours of steam-heated moisture drive. It's sometimes worth spraying a flash-inch of FrothPak or other closed cell foam on the joists where they penetrate the cavity as an air-seal/vapor retarder,  but in a well vented cavity and/or deep roof overhangs you can get away without taking those measures.

So much information, thanks! To answer some of your questions....It is a one story home on a slab.The overhangs are approx 2'. As I look next to a electrical box, I cannot see lath. All I can see is a board that resembles drywall but have been told they are plaster walls. Behind the board there is only about a 2" gap before the brick. When I had the insulation blown in the attic I think I recall the gentleman telling me he was going to stuff the openings, if he did (Iwill have to check) what is my next step?

Take a hole saw and drill a test core in a closet wall or something to figure out what all the layers are between the interior paint and the cavity.  The odds are it's fiberboard or gypsum sheathing.  If the finish side plaster is on gypsum-board on a a structural 2x4 wall with empty cavities and exterior gypsum or fiberboard sheathing it's safe to just insulate the 2x4 cavities.  If you go that route  1.8lbs density new-school fiberglass such as Optima/Spider/L77  or 3.2-3.5lbs dense-packed cellulose probably your best bet for getting a good retrofit.  This might be do-able from the attic without messing up the finish walls, or not- depends on the framing construction.  The cavities below window framing etc would have to be dense-packed by drilling from the interior.  If it's a gypsum-sheathed both sides 2x4 wall with a brick veneer, you would end up with about an R10 whole-wall R after factoring in the thermal bridging, with is below current code min, but a lot less work than building out more on the interior, and a heluva lot better than what you have going now.

If the insulation contractor blocked the top of the cavity in the attic in a way that isn't easily reversed, you can core 1.5- 2"  vent holes to the exterior every 3', and pop in some screened vent caps, designed for soffit venting, or drill a 1/4-1/2" hole in the vertical mortar every third brick.  Hopefully the weep holes at the bottom of the wall haven't been filled in by re-pointing, but if yes, drilling holes in the vertical mortar on the bottom course works.

Having 2' roof overhangs all around on a 1-store bodes well for being able to dodge severe issues related to exterior moisture drives.  In an air-conditioned building the riskier points would be east or south facing walls where dew-wetted brick can see a daily blast, but as long as you don't have foil or vinyl wallpaper or some other severe vapor retarder on the interior side, the moisture will likely pass harmlessly through the wall rather than adsorb or condense creating  mold conditions.  In Canada where interior polyethylene vapor barriers is the norm, these summertime moisture drives in brick buildings can be pretty severe in air conditioned buildings.  The purpose of the poly is to keep interior moisture from building up in cold walls during the winter, but it causes other issues in summer in buildings with claddings such as stone or brick than can retain a lot of moisture.  US climate zone 5 doesn't have nearly the wintertime moisture issues as north of the border, and if the building can be designed to work without vapor barriers (usually can, with a 2" vent space to work with), it's better/safer to avoid that trap.

I have the same kindof house only on a crawl, what a POS not sure what they were thinking back then, nor did I know much at the time I bought it nor did I have other options, the brick from the outside looks like the normal ornamental brick, it is actually more like a cinderblock only 3 inch tall there is some sort of a furring strip on the inside giving not even 2 inch gap, maybe 1, and drywall is installed to it, in some places I noticed some foil type insulation I think, the old dryer vent gave it away, and yes don't lick the outer wall tongue may get stuck.

I've considered the double wall at some point but the dam thing is small already I gave up on it for other reasons too but if you were to try something here is an option I came across recently, from the other side of the pond http://www.youtube.com/watch?v=_OloldpTF-g

not sure what they were thinking back then

The same thing that they are thinking now? Build it minimum and folks are sure to buy?

the video appears to show foamglass being installed. you can do that; works great, but there are less expensive alternatives. Go to http://www.greenbuildingadvisor.com/ and search for "exterior insulation blogs", settle down for several hours, read them and read the comments. You can install cellulose, mineral wood (Roxul) (as a spray, batts or sheets) spray foam, sheet foam or foam glass. Each has +'s and -'s; they all work.