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Finishing Interior Tongue and Groove. Need to backprime?

Tanner C | Posted in General Questions on

Hey folks, I did some searching but struggled to find many answers to my question:

I’ll be installing loads of tongue and groove in my interior, both on the walls as well as on a vented cathedral ceiling. Boards are 6″ pine as well as some cedar.

While I meticulously stained all 6 sides of my exterior siding boards (installed with a rainscreen) I think I like the idea of installing the interior boards dry and then brushing or spraying them all afterwards with a coat or two of water based poly. Does anyone have concerns with this method? Is cupping a real issue if only one side is treated on an interior wall? Is it actually easier as I assume?

My walls are 2×4 with well-taped exterior sheathing and will be insulated with mineral wool. I was planning on putting the boards right up over the studs. Rafters are also 2×4 and I plan to “cut and cobble” 3.5″ polyiso for the rafter bays (leaving a 1″ air gap below the roof deck) tape the seams, and maybe additionally apply an interior membrane or taped 1/4″ ply for air-seal redundancy. Additional opinions regarding the potential stupidity of any of these assemblies is welcome! I’m in climate zone 4, coastal northern California. 


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  1. Expert Member
    Michael Maines | | #1

    Tanner, interior trim is rarely finished on all sides. If you have wide swings in interior relative humidity (or temperature, which directly affects relative humidity) you could get some cupping. If your relative humidity stays reasonably consistent, you should not have cupping.

    That's assuming the trim is close to its ambient moisture content when installed. What that moisture content is will vary. In northern CA, I think if your trim is in the 8% to 14% range you are probably in good shape, but others will know better for your location.

  2. PBP1 | | #2

    I think there may "technically" be a difference between cupping and buckling. Cupping is for an individual board, which will do so regardless of its neighbors while buckling is from the board itself and its neighbors. If installed at too high of RH, then the boards will be "big" and if gaps are loose, you could get open gaps at low RH. To the contrary, if installed at too low of RH with tight gaps, the boards will buckle when RH increases. There are humidity meters for wood, mistakes are costly to repair (I know). And, do the math, a 1/8 inch increase in board width for 24 boards is 1/8 * 24 = 3 inches.

  3. Tanner C | | #3

    Thanks! This installation is for a larger tiny house heated by a wood stove, so I do actually expect some temperature and humidity swings. Nothing too crazy given the coastal climate here, but if I have it at 70F with the stove on and then leave for a trip, it could conceivably be in the 30's/40's with high RH when I return. Perhaps taking some extra time to treat all sides would make sense in my particular situation...

    Based on my measurements last winter of wood stored in an open barn, wintertime equilibrium moisture content is in the vicinity of 15% here. This fall has been fairly dry so I bet the boards are still a bit dryer than that.

  4. Walter Ahlgrim | | #4

    Why not, if time and money are no object. It is just 33% more labor and materials for the hope that it may last longer.

    It seems unlikely any pro would get paid to paint the hidden side.


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