1920s Rehab - Remove or Keep Planking Under Drywall

13 Replies

Hey BP friends,

I'm rehabbing a 1920s house in San Antonio, Texas, that has shiplap planking covered by vinyl siding on the exterior and shiplap covered by drywall on the interior walls and ceiling.

My family and I are demoing the house and have already removed the interior drywall and trim, but... what about the planking?

Should I remove all the shiplap planking from all interior walls/ceilings and replace it with drywall?...Or would you recommend keeping the interior shiplap planking and putting new drywall over it?


A. The foundation is pier and beam

B. The house will need all new electrical, plumbing, HVAC, trim/casings

C. The house will need exterior wall insulation

D. Most of the old shiplap planking is splitting as we remove it, so it isn't even salvageable

E. Would the interior planking help at all with insulation since the house is so old??

My thoughts are rather than take all the planking off, we could remove the bottom few planks in order to facilitate all new electrical and plumbing replacement. After these trades are done, I can install new planking at the bottom and use blow-in insulation to fill the wall stud cavities. What do you think?

Thanks for all your advice!

@Kevin Castaneda

From the pictures, I cannot tell of the planks are shiplapped or tongue-and-grouve.  Here they would be the latter.

Those planks have little insulation value themselves (my reference chart indicates the R-value of wood ranges from 1.2 to about 2.5 per inch depending on the type of wood and how dry it is), but they will help hold blown insulation in the wall cavities and you will not have to apply screening or patch drywall (or even plug the holes you will drill in the planks).  On the ceilings and interior walls it will help with sound deadening (a little).   I wouldn't bother removing them unless they are rotten.   If you need to pop a few to run electrical or plumbing that's not a big deal- but you will need to cut the laps or tongues off both sides of the planks you remove to have the best chance of not splitting them.   

If the house has not been previously insulated - it wasn't when built - I would blow-in either dense packed cellulose or rock wool, then hang sheetrock.  In your climate, I would probably use faced polyisocyanurate on the the external side of the wall assembly.

Plan sounds reasonable ... the more insulation the better whether you live someplace hot or cold as it keeps the heat on the correct side of the wall.

Sounds like a lot of fuss for insulating. That said, everything, even still air, has an R value. I don’t know what your overall design is like but I suspect paying for blown in vs. batts probably isn’t worth it for more than a small area.... and if you can make that small area an interior wall that doesn’t need insulation anyway, even better. If it fits, expose the shiplap on one wall and make it a feature. But don’t do it without appropriate (~3x) return.

Just my thoughts.

termite consideration, wiring installation.... My thinking is if it is that old the home will sell better with modern materials and features. 

@Roy N. , Thanks for the detailed walk-through. Not sure what kind, but the house does have foam board installed on the exterior between the vinyl and exterior shiplap. 

Account Closed, @Rick Pozos

There’s gotta be a way to only pull a board or two for updating elec & use a holesaw (or pull another board at the top) to pop in an access for the insulation guy.. is this a flip or a long term hold? Ive never done blown in on exterior walls (though I do have one home from ~’46 that definitely has it) just because it settles and in the cold here it sucks, you get a ~20*C spread from top to bottom on a 9’ wall.

I mentioned using it as a feature mainly because if you’re trying to maintain some of the character of the time period you may be able to make it work but if for example you’re turning this into a modern designed / staged home exposed planks may not really fit in.

I always look for a way to turn an expense into a profit zone. Featuring the planks may do that, may not depending on application

@Kevin Castaneda

I would anticipate the "white foam board" under the siding to be EPS - which was one of the first rigid, petroleum-based, insulations.  It is also commonly used in thinner sheets (1/2 - 3/4") when over-siding as a means to even out the surface of the original siding - and to provide a claim of additional insulation.   EPS is only R2.5 - R3/inch.   It is almost a certainty that the EPS is not taped to provide you an adequate vapour retarder on the warm side of the wall assembly.   If you are planning to reside the exterior, I would dispose of the EPS and use 1.5 - 2" faced, polyiso (R10 - R12) on the exterior (with taped seams), then build-out your window boxes (if installing new windows, build/install bucks before you insulate and install your windows after insulating.


There will be no issues with the blown insulation and electrical wiring or pex supply lines whether you use cellulose or rock wool.  My preferred insulation is rock wool - whether batts or blown - as it is hydrophobic, unappealing to insects, naturally non-combustable and dense) and inert.


Account Closed

All blown-in will settle over time, however if you use dense packed (blown-in slightly wet) cellulose or rock wool and a good installer, your settling should be minimal.   It is probably not worth the extra labour to remove all the planking to install rock wool batts.

Account Closed, I wasn't planning on fully replacing the exterior vinyl as it looks in alright condition except for a few chips in a couple places that I might spot treat. With the wood exterior underneath the foam, I don't think the stud cavity will have moisture problems?

Originally posted by @Kevin Castaneda :

Account Closed, I wasn't planning on fully replacing the exterior vinyl as it looks in alright condition except for a few chips in a couple places that I might spot treat. With the wood exterior underneath the foam, I don't think the stud cavity will have moisture problems?


[Caveat:  My hands-on experience is in a cold environment where much of the year the conditioned space of a building it being heated.  As such, we apply vapour retarders on the inside of the wall assembly - warm side of the insulation.  Without a retarder, warm moist air would be driven out through the wall assembly and condensate on the external sheathing (on the external face if your are lucky and there is sufficient external insulation).]

Your climate is more-or-less the reverse of ours and you spend much of the year cooling the conditioned space.  Without any barrier, this will draw warm, {perhaps} moist, air in from the exterior of the house which will condensate on the internal side of the wall assembly.   The existing {EPS} foam on the outside of your house may provide some moisture control, but depending on its porosity and whether seams were taped - or a vapour retarder/barrier applied before the foam - it will only be partially (or, perhaps marginally) effective. 

If there is no retarder/barrier in-place, you will not prevent moisture from being drawn into the wall assembly, but at least it will be able to dry to both sides.   We have used this approach on older Victorian and Second Empire buildings - no vapour retarder/barrier installed and relying on a vapour retarding primer / wall paint to provided some degree of control.  We are also very deliberate to use rock wool insulation within the stud bays (and never fibreglass) to ensure moisture that does enter the assembly will not be held there by the insulation. 

Those boards going cross-wise really add to the stability of the house. Most of the 20s and 30s homes in San Antonio are going to be built that way. I would use 1/4" sheetrock over the shiplap. It is so easy to add wiring with the shiplap. All you have to do is cut a board with a recipricating saw or a jigsaw or remove a board and then put it back. If you take it out completely and leave it out, you would be making a big mistake.

@Seth Teel is the one to talk to locally. He has done the most historic home rehabs that I know.

I have seen some homes in Dignowity where they leave the shiplap exposed and stain it. Beautiful. You can also talk with people at the city development office more specifically at the historic design and review office. They are very knowledgeable.

I am just getting ready to work on my personal home that was purchased a few months ago. It is a 1935 build in the Jefferson Conservation District. The HDRC has helped me tremendously. So has Seth.

I agree with Rick Pozos. I discussed your questions with my partner (who guides all our rehabs) and he concurs that the horizontal boards help with stability as Rick mentioned. We also agree that you should just drywall over the existing planks. I would leave a wall or two exposed if you can - it really can be a beautiful feature, adding character to a room. I did one recently and whitewashed it. It was worth the extra work. Additionally we recommend entering from the top rather than the bottom of the walls for blowing insulation into the stud cavities. There’s no problem with wiring or piping by adding the insulation. Good luck to you!

@Kevin Castaneda   

We do a lot of historic and historic era renovations, all of which have the wood lap in the walls.  We typically leave as much in place as possible.  We remove any boards that are severely degraded by water or termite damage.  The remaining boards are cleaned up (nails & screws removed, cleaned, and address any protrusions).  As for insulation, there a few ways we tackle it:

  • Remove lap boards (some or all) install batt insulation, replace lap boards if desired.
  • Remove top lap board (or drill holes) and blow-in insulation
  • If you plan to re-do the siding on the exterior of the house, we have pulled the siding and installed batt insulation from the exterior of the home, leaving the lap boards up on the inside, then siding the exterior. 

The conventional wisdom is to leave the lap boards (if you can). It ads stability to the structure giving the hope a much sturdier feel.  From my experience (both in personal homes & with clients), access to the wall for plumbing or electrical maintenance is usually not an issue. We do full re-wires, and all new plumbing in our renovations, so I am sure that cuts down on maintenance issues. But even when we've had an issue where we needed wall access, it's nothing drywall saw and/or Sawzall can can't cover.  

Hope this helps.  If you have any questions, shoot me an email or a call. 

@Kevin Castaneda

For insulation...

You can also consider using spray foam insulation.  There are 2 types "open cell" and "closed cell".  Closed cell will give ~R-6 per inch but you will have to remove all the ship lap to install this system.  This will provide a vapor barrier.  If you choose to leave the ship lap you can then use the "open cell" which gives you about ~R-4 per inch and no vapor barrier.   I have used both on projects.  I have open cell in my rental.  Here is a link to some info. 


Not sure what code you are using but 2012 IECC requires r-13 in walls.  Not a fan of rock wool due to its low r-value.

@Jim Adrian

We use a fail amount of polyurethane foam, but have found it is not always the best solution for older properties (Edwardian or earlier).  While  Rock Wool has a lower R-value than closed cell foam, rock wool batts or dense packed will provide R3.8 - R4.3 per inch, exceeding the performance of open cell foam. 

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