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I just purchased a 1925 beautiful home that is stucco on the outside and old plaster on the inside. I have some carpentry experience and am realizing the incredible loss f heat to the exterior walls. There is ample space in each of the rooms in the house so I am contemplating ways to insulate. It seems blow in is too dangerous because of moisture issues.

I am considering building new 2x4 insulated walls within the existing exterior walls.

What do you think?

I wouldn’t. You can create moisture and mold problems, much less drastically change the size and character of the interior of your home.

Adding an insulated wall on the interior of your home will change the “perm” rating of your exterior wall. Perm rating is important because it controls the flow of VAPOR through the wall.

Depending on where you live, vapor will travel from the inside out (northern climates) or from the outside in (southern climates) or both ways (middle climates).

When vapor travels through the wall, it changes from a vapor to a liquid. If this liquid gets trapped in the wall, it will create rot and mold.

Here is an article that explains it better:
https://americas.siga.swiss/blog/building-science/do-you-need-know-what-perm-rating-is

All materials have a perm rating...even a coat of paint. Here’s an article that lists some materials and explains it better: https://www.buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials

Adding additional materials could trap moisture within the two walls and create rot and mold.

If you’re trying to save heat and reduce your heating costs, I’d concentrate on 1) adding insulation in your attic, 2) sealing drafts around windows and doors, etc., and then 3) insulate your exterior walls.

1) Heat rises, so concentrating on the ceiling makes sense. (The Department of Energy has a website that explains this.)

2) Drafts will lower the room temperature dramatically and keep your furnace working.

3) Yes, due to the age of your home, I doubt if they have any insulation in your exterior walls. Adding insulation and getting thermal windows (or storm windows) will help...but not as much as 1) and 2). (Besides, a new 2x4 wall is not that thick so you won’t be able to add much insulation.)

While this plan might offer great energy benefits, there are a number of drawbacks:

• Obviously you eat a lot of floor space (8" or more in both directions). In a 10' by 12' room you lose almost 8% of your area. In small rooms like entries and bathrooms this may be prohibitive.


• All window and door openings will need to be extended, meaning double-deep jambs and casing replacement.


• All electrical boxes will need to be moved out or extended.


• All cabinetry and shelving will need to be heavily modified or replaced, including any integral plumbing.


• All freestanding plumbing fixtures would need to be relocated, such as toilets and tubs.


• All base trim will need to be removed and reinstalled or replaced.

Houses built in that period (in Germany) by good chance have a single void in their outer wall insulating better than those same size walls built some decades earlier. Filling this void with a modern insulating material might improve insulation by more than 100%.

An insulation added to your walls from inside the rooms might cause big problems shifting the dew point to the inner side of the wall letting mold grow. You could solve this problem using CaSiO3 as insulation material. It takes thrice the space you'd need for usual modern insulation material, making your rooms smaller and shouldn't be sealed with wallpapers but painted with mineral colors only.

Whenever you insulate a part of your nice old house other parts will do the liquid exchange. So don't do to much for it might rouin the house. Your house has been built with a certain concept of heating. Trying to change that can be problematic. Not changing it by good chance is best for you and your house.

(I had to deal with an ANNO 1900 house)

God bless!