Home>Discussions>INTERIORS>Molding & Carpentry>Plaster and Lath - remove or not to remove?
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Jess1113
Plaster and Lath - remove or not to remove?

My late 1920s Craftsman house in Washington is in need of additional insulation. Sometime in the 60/70s the homeowners sprayed in the Formaldehyde insulation from outside but the house is not retaining heat. I'm wondering if I need to take out the walls in order to remove the old insulation and replace with better with the added benefit of improving the knob and tube electric at the same time or do I leave it in place and try to find other places the heat is escaping. Where I've seen it, (bathroom fan placement) the insulation seems to be in good shape filling the space of about 4 inches. The plaster/lath walls are also in really good shape without pulling away from the lath anywhere I've seen, just some cracks in the larger walls.
Any ideas would be great, I know I need to update the electrical so that is going to be pretty expensive without ruining the walls, I'm sure.
Thanks, Jessica

Re: Plaster and Lath - remove or not to remove?

Since some insulation work has already been done, I wonder if it might be a good idea to have a "thermal" survey done (if there is someone in your area).

This will clearly show where all that expense, sorry I mean heat, is going!:rolleyes:

Be a shame if the new insulation is blocked by old stuff in the way (assuming blown stuff again).

I have a friendly fireman who has access to a rescue thermal imaging camera. Amazing what it shows!

Just a thought....

Timothy Miller
Re: Plaster and Lath - remove or not to remove?

Howdy, first what is the attic insulated to? and what type of insulation? How are the windows? Storms? Is the floor insulated? I'd bet, as with allot of old homes, the attic does not have enough in it. An the attic access door likely not properly insulated and air sealed. If not enough attic insulation consider cellulose as it retards air movement and has fire retardant- be aware if the wiring is old( knob and tube do not install insulation over it as it can result in a fire). So consider easy fixs- check the seals of the doors & trim & windows and caulk as needed/ or weatherstrip. Caulk around any ceiling light fixtures to stop air movement. Insulate the homes first floor to r-25 if acessable & attic to R-49 . If you have a stick of incense walk around the windows with it and see if the smoke shows air movement- poor mans air infiltration test. HVAC been checked? If forced air have it checked to see if it is working properly. Before you open up the plaster walls.

keith3267
Re: Plaster and Lath - remove or not to remove?

The short answer is yes, but the question is, do you want to? There is something about a lath and plaster wall, it feels so solid and you just don't see them anymore. By comparison a sheetrock wall seems kinda cheap.

But, from an energy standpoint, lath and plaster conducts heat about 2 to 3 times better than sheetrock. This is important because it is short circuiting the foam insulation. about 10% of your wall is the framing (2x4 studs). The R value of these studs is essentially about R-4. That would only have a small affect on your heat loss if it weren't for the lath and plaster. The plaster is absorbing heat from your interior and conducting it laterally to the studs, so more heat is being channeled out through the studs than would be without the lath and plaster. Sheetrock is also a good conductor of heat, but not as good, so less heat is conducted to the studs.

Removing the lath and plaster will expose the wall so that you can do the rewiring you want to do. It will also expose any voids in the insulation so you can take care of that. Now if you really want to increase the thermal efficiency of your wall, you can add some foam board panels under the sheetrock. This will serve to break the short circuits. It doesn't have to be very thick to be effective. 3/4" is plenty, even a 1/2" would do a lot.

But now for the bad news, removing the lath and plaster might not be very easy. Some of the foams used back then were very sticky and they will want to hold onto the lath like gorilla glue. You may do a lot of damage to the foam when trying to get the lath off. Cleaning out the rest of the foam will not be easy either. But filling the voids created by removing the lath should not be too hard. You will just want to make sure that the new foam is compatible.

keith3267
Re: Plaster and Lath - remove or not to remove?

Tim makes some good points here. In a typical 1 story home of your era with uninsulated walls, only about 20% of the total heat loss is through the walls. Windows and doors account for another 20%, infiltration 20%, floors 10% and the ceiling 30%. In a two story house, the walls account for about 30% and the ceiling and floors a little less than in a one story house.

These are just rough, but typical estimates, you would have to have an energy audit to get more accurate numbers. If you are going to spend a lot of money on energy efficiency, you should have an audit done so you know where to get the best bang for the buck. A thermal imaging camera is a real good tool too. Sometimes a lot of heat escapes through a fairly small hole. It would be a shame to spend a lot of money and miss that hole.

keith3267
Re: Plaster and Lath - remove or not to remove?

One more thing about the thermal imaging camera, don't assume a cold spot means good insulation. It can mean that cold air is penetrating that point and cooling off the surrounding surfaces. Check any cold spots from the inside as well. In fact, the camera should be used all over the interior as well as the exterior.

ordjen
Re: Plaster and Lath - remove or not to remove?

A good place to start is by contacting your energy utility companies. Many have free or low cost services which will come out and give you an energy audit with improvement suggestions with typical payback periods.

Re: Plaster and Lath - remove or not to remove?

Keith is sooo right about small holes!

We missed insulating a boxing around an internal soil and vent pipe and boy did it pull cold air right down and across the horizontal boxing, into the vanity unit, filling it with cold air.

When we opened the vanity door, all the cold air would 'fall out' onto your bare tootsies in the morning lol!

officer_krupt
Re: Plaster and Lath - remove or not to remove?

when ever thinking of demolition work on a house that old always consider the chance that lead based paint may have been used.......You could always go over the plaster with drywall.If your house is above grade make sure the floors are insulated...

Mastercarpentry
Re: Plaster and Lath - remove or not to remove?
ordjen wrote:

A good place to start is by contacting your energy utility companies. Many have free or low cost services which will come out and give you an energy audit with improvement suggestions with typical payback periods.

Ordjen, thanks for saying that. I had plum forgotten about this. Most utilities offer this service even though it sounds conter-productive to their sales. It simply costs them more to upgrade their system than it does to continue using what is already there for as long as it will last- thus in saving you money they save even more. A true win-win!

Phil

craigmg
Re: Plaster and Lath - remove or not to remove?

+1 on the utilities. Mine had a discounted survey for about $130. They did a blower door test, thermal camera and provided many recommendations. Surprisingly our 1892 house was very tight. I was thinking I should be aiming at drafts through doors and pipes. Turns out our biggest problems are small missed bits of caulking around the window trim that is one the walls.

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