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Wall Thickness Requirement

PoplarGreenthumb's picture

I am currently undergoing an extensive renovation and looking at thickening my existing 2x4 walls to retrieve the best R vaule yet to also determine the dew point. Can someone please be of assistance in this area..



There are a couple of (post #209776, reply #1 of 5)

There are a couple of different ways to do this; it's your doors and windows that will determine what you can do.

Why? Because the walls can't really be any thicker than the window casings- and changing that can be quite involved.

Many take the straightforward approach of slaving 2x2's to the existing studs, and moving the electrical out to match the new wall face. The challenge here is to also also trim back the finished floor; it's bad practice to simply build atop the existing floor.

The drawback to the 'simple' approach is that you end up with a greater contrast between the insulated wall bays and the solid wood of the studs. 

Another approach is to strip the siding off the house, add an inch or so of foam board, then replace the siding.This approach has the advantage of breaking the thermal 'bridge' made by the solid wood of the studs.

Simply adding new siding atop existing siding is a bad idea; you're too ;likely to create a cavity that will trap moisture and foster mold.

For all the work you're doing, I'd ask you to consider spray foam as your insulation. That's best done by a contractor. The end result is a totally sealed wall, a much stiffer wall, and greater soundproffing. Combine spray foam in the walls with foam panels on the outside, and you can easily achieve 10x your existing insulation value.

R value and dew point. (post #209776, reply #2 of 5)



If you have a target R value in mind for your new wall assembly, what is it?  At minimum, you need to comply with the building code.  Without knowing where you are located, we cannot know what that value is either.


What do you want to know about dew point?  Dew point is defined as the temperature at which moisture in the air condenses on a solid surface.  Such as the outside of your beer bottle.  It is important to note that barometric pressure and relative humidity are factors as well.

That said, if you're concerned about "where" moisture might condense in relation to your wall assembly, I suggest you start by reading this.

Even if you are not planning on installing exterior rigid foam, it is still worth the read.  The article also explains the relevance of climate zones, and provides a link so you can tell us your climate zone.


Without more information on your R value goal, location and the details of your renovation, we can only guess.  You said "extensive", but this is a subjective term.

Some very basic advice would be to pay attention to air sealing the wall(s) while you have the chance.  At, or around code minimum R values, air sealing will have a greater impact on comfort then the actual R value.




Keeping your expensive heat in the home - insulation. (post #209776, reply #3 of 5)

A Thermos Flask is a good example of the perfectly insulated room, an airtight water vapor tight room..... a space surrounded by a vacuum, the only heat loss being through the neck and stopper.

Unfortunately, no one makes a room sized vacuum flask, yet.

In designing a home, we need to consider that heat always moves to cold. The first law of thermodynamics. As we want our home to be warm and cheap to keep warm, we need to consider how our heat escapes........mainly by radiation, then conduction.

The worst thing you can do, is to place your heater against a wall, where half of all the heat radiated disappears through the wall, half the convected heat follows. The warmest place in the average room is between the heater and the wall.

A well insulated floor, say six inches of polystyrene on top of the existing floor, with pex low temperature under floor heating topped by oriented strand board flooring makes the most of your heat input, being quick to heat up and cool down, a typical floor runs at 75F and keeps the room at 72F. With this set up very little heat is lost.

Cover the room side of the walls and ceilings with 2 or 3 inch thick sheets of polystyrene covered and protected by drywall. This will keep the heat in the room and will avoid losses through the walls and ceilings by conduction. Keep in mind that heat doesn't just disappear through the wall, it heats the wall through 360 degrees, with heat going down as well as up.....heat always moves in a straight line from point of source in all directions. This is why it is important to completely isolate the heat inside the room from contact with any part of the frame.

If you have double pane windows, it is a good idea to fix a third sheet of glass or plastic over the inside to improve their insulation.

Finally, always have an airtight porch over every door that can let your warm air escape outside, a sealed porch will stop that sudden outward rush of warm air,  and equally unpleasent and expensive rush of cold air inwards.

In a perfect world with (post #209776, reply #4 of 5)

In a perfect world with unlimited time and budget I'm sure your suggestions would work very well. However, I'm sure the OP has neither of those, I know I don't. So, we're talking walls only, not floor, not ceiling. My understanding is that only 10% to 15%  of the heat loss in an already insulated  house goes through the walls so how much can the average homeowner afford to spend on upgrading wall insulation. Wouldn't air sealing the walls be a more effective use of money? What about closed cell foam which air seals and insulates?

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

The best thing you can do for (post #209776, reply #5 of 5)

The best thing you can do for the walls in the typical 2x4 house is to remove the interior trim and foam around the windows.  Can easily cut heat loss in half.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville