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Opinion on adding foam board insulation on the interior over existing drywall.

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30K views 30 replies 16 participants last post by  Donohue Const  
C
#1 ·
Greetings all.

I'm doing a small remodel on a 100 year old home to use as rental property. There is zero insulation in the walls. The walls have the original lath and plaster with 1/2" drywall over that. I really don't want to tear all that out and open the walls. I'm not a big fan of blowing in insulation from the outside. For one, it's too expensive for this project and I know there can be issues with putting insulation in the walls of old houses because there's no vapor barrier behind the plaster. I know there are many opinions on the subject. I've opened the walls of old houses before to find the insulation wet and moldy due to the lack of a moisture barrier on the inside. I thought perhaps I could install 1" foil faced foam board over the existing walls, fur it out and install 3/8" drywall over that. I know the R-value would be minimal, but it's better than zero R-value and should help at least some to slow down the heat transfer. My concern is, will this also create a moisture problem. Will moisture be trapped between the foam board and the new sheetrock? Any and all opinions would be appreciated. Thanks.
 
C
#7 ·
My concern with blowing in insulation whether it be from the inside or the outside is the issue of creating a moisture problem since there's no vapor barrier behind the plaster. I have seen evidence of this problem when I've opened up walls in older homes that had insulation blown in. I realize there are opposing opinions on this.
 
C
#6 · (Edited)
"You need to convince me that you are really a contractor, then I will offer my advice."

Well, I'm not exactly sure how I can convince you, but I've been a contractor for 30 years. I was a sub-contractor for a home builder early on in my career for 8 years before I went on my own. I do small residential remodels (kitchens, baths, etc.) but mostly manage rental property, doing the maintenance, repairs and turn overs. I own a few rental properties myself and just picked up this house I'm working on now.
 
C
#12 ·
Hey there. You're actually in my town. Yeah, I realize I'll have to extend some outlets and it would only affect one window which I just replaced, so it's not trimmed out yet. And actually, it's not all the exterior walls. The house has had much newer additions added to two sides of it, so those walls became interior walls. I also thought by adding furring strips over the foam it would create an area of dead air space between the sheetrock and foam that would add additional R-value to it.
 
M
#14 ·
Opinion On Adding Foam Board Insulation On The Interior Over Existing Drywall.

Yank the interior finish and insulate it right. Like mentioned it is air leaks and not so much vapor barrier that cause issues. You can do a pretty good job of air sealing and insulating if you gut the interior and do it right. I wouldn't even consider what you are thinking. Complete waste of time.
 
C
#17 ·
No, that's not me. My company is CU General Contracting. I've been in business in Champaign/Urbana for about 30 years. Most of my work now is on rental property. Repairs, rehabs, etc. I realize what I'm purposing to do is far from the optimal solution, but I just hate tearing out old plaster walls, especially when they've also been covered with drywall. And when you get old and have achy joints, you get lazy : )
 
#20 ·
I'm assuming this is balloon framed.

Step 1: get rid of any water leaks / moisture sources

Step 2: air seal. Keep in mind, some of the old plaster interior and exterior walls will be open clear into the attic

Before step 3, make sure you don't have knob and tube in the walls. Also make sure you know how ceilings are handled - you can have some major difficulties blowing into ceilings.

Step 4 : blow the walls OR put the foamboard on the outside OR trench all the ceilings and put the foam board continous on the inside walls (except for the subfloor, etc.

If all you need is a vapor retarder, use a vapor retarder paint on the exterior walls - tear out just to put a 6 mil plastic sheet in place is a little overkill, IMO.

There is no cheap, easy, fast solution. Putting the foam board on the inside pretty much keeps you from blowing the walls later.
 
F
#21 ·
I would go in the attic and block of the top of every interior and exterior wall. I would block it off with wood and then foam any air gaps. I would go to the basement and block of and foam any gaps down there. If you can get the walls to stop connecting to the attic it will instantly feel warmer. This of course is only a start but it is a rental.
 
L
#26 ·
Gotta go with TNT on this one... spray foam injection all the way. To me this whole deal is about like asking if you want to be shot or stabbed... imho spray foam injection is being stabbed the least to me.
 
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#27 · (Edited)
these houses are never nice to us. gutting is obviously the best option. bandaids and quick fixes lead to high energy bills/unhappy renters. one of my clients usually consults me before buying houses, but two houses back he didnt. he asked me to bandaid it up, and the can of worms kept growing. which is a common occurrence. so in the meantime.... foam injection is what i would do as well.

unless someday the house would need re-wiring??
 
#28 ·
The problem with injection foam is will it last - highly unlikely as I still hear cases of pulling away, etc... The next is if you haven't solved your flashing errors you are going to run into water & rot issues.

As for "if wiring needs replaced" - if that is a concern, you shouldn't be installing it in the first place especially if it is K&T or aluminum. I would say the older style romex type but as most houses here use conduit that isn't as big of a concern.

As it is a rental - air seal & fire block are going to be your biggest savers with least risk. Next look at the attic for insulation - but if there is any K&T don't insulate unless you rewire first --- Airsealing & insulation articles
 
C
#29 ·
I agree. And injection foam is a crap shoot as to whether the cavity is evenly and fully filled or not since you can't see it. No K&T thank god. Was rewired at some point. And what an easy rewiring job that was with unobstructed walls. Will do as suggested and just seal off leaks for now and add more insulation in the attic. This will be the first month that I've really had the heat on and it's been well below zero many times, so I'm a little scared to see the heat bill.
 
9
#30 ·
Back to the original question: Doing foam OVER plaster would be no better/worse than any other method when it comes to moisture, IMO - depending on how sound the plaster is.

I have never seen it done tho.

Man, that's got to be a LOT of work too - especially if you give a damn what the casing/mop boards look like afterwards.
:no::no:
 
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