Siding over lath and plaster

4 months ago

Hello, we are planning on installing new siding on our 140 year old home. However, we have an issue under the current siding.

The current siding was placed over 1/8” insulating foam board. The foam board accordions out into a 4 foot wide by 20 foot long sections.

Whoever did the foam and siding did not tape the seams and there is no house wrap.

It was also placed directly over lath and plaster (yes, it is lath and 5/8” think horse hair plaster on the exterior, not stucco). When the current siding was nailed into place, it caused a lot of the plaster to break. There are large 5 to 10 square foot sections of plaster missing in multiple locations.

The missing plaster, combined with thin foam board with untapped seams causes our home to be very drafty.

Should I remove the lath and plaster and install regular OSB sheathing and house wrap with 1” foam on top for insulation, or leave the lath and plaster and install sheathing over top of it?

If I go with the second option, it will cause the wall to be further out and now I have to trim the windows/doors etc. out so they aren’t sunken in behind the siding.

I need to know what my smartest option is so it is done the correct way.

Featured Answer

Lorraine Leroux

4 months ago

Once you remove the lath and plaster, do you have open cavities to put in some Roxul Rockwool insulation? If yes I would do that. Then the layering would be new Roxul insulation between studs if possible, OSB sheathing, House Wrap, (seams taped), Rigid Foam Board (taped), Furring Strips, Siding. Depending on your climate of course as I am in Canada. The house wrap over the OSB is that it creates an air channel for moisture to evaporate since the rigid foam is a moisture barrier. As far as windows and doors being "sunken in" as you say that is pretty normal for doors and windows to be trimmed out and closer to the interior than to the exterior but saying that YES they will need to be flashed properly and aluminum would be a great option bent and formed on site. A lot more work however the payback in home heating and cooling savings will be well worth it. If you have any grants that will help with this I would certainly consider applying to help with the costs.

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Comments (8)

millworkman
4 months ago
" Should I remove the lath and plaster and install regular OSB sheathing and house wrap with 1” foam on top for insulation, or leave the lath and plaster and install sheathing over top of it? "

I would definitely do this, it will enable you to have a better surface for the new siding and you can properly air seal. The better way however is the insulation board FIRST, directly on the sheathing then the housewrap over the insulation board. The way you describe will trap moisture.
Isaac Noble thanked millworkman
Lyn Nielson
4 months ago
agree with @millworkman. Cleaning up any old and uneven base before you install siding.

Are you installing vinyl siding or wood/cement siding?

The finished product is only as good as the prep. Start from scratch and have peace of mind when you are finished.
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3onthetree
4 months ago
Removing it will allow you to insulate the walls and air seal. Also easily deal with any electrical you so choose.
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Patricia Colwell Consulting
4 months ago
I would say yes since it i damaged and you need insualtion and wrapbefore new siding
Isaac Noble
Original Author
4 months ago
@3onthetree good point about the electrical! Thank you!
Isaac Noble
Original Author
4 months ago
@Lorraine Leroux There is nothing behind the lath. Just studs. I was worried about it looking silly but you make a good point!
Isaac Noble
Original Author
4 months ago
@millworkman Thank you! I have read conflicting information as to which should come first. OSB, wrap, foam board, or OSB, foam board, wrap? I don’t want water. I’ve been thinking I need to remove the lath and plaster but needed validation. From the comments here, it seems like a no brainer now!