Need ideas how to seal gap between siding and foundation

[AlwaysBeenASaver]

I have a cement slab foundation and shiplap board siding, attached diagonally. There's a gap along the bottom edge of the siding where it meets the cement foundation, about 4" off the ground. I'd like to seal this to make the house more airtight and keep cold air out. First, is this the right thing to do? Second, how do I do this? I tried putting exterior caulk in the gap, but it's so close to the ground I couldn't get the caulk gun at a decent angle, the end of the caulk gun hits the ground before the tip is pointed up enough at the gap, and I made a mess in the spot I tried - caulk on the visible part of the cement. I'm open to suggestions, or just tell me if I shouldn't bother with this?

[Lulee]

My initial thought would be to use spray foam, possibly with caulking done on top of that.  But I suspect we'd both be wrong.  From various This Old House episodes seen over the years, I suspect that there is intentionally some air allowed between siding and the vapor barrier that should be underneath.  This would allow any moisture from weather penetration or that has escaped outwards from the house to condense on the back side of the siding to safely dry out before creating rot or allowing mold to grow.  I look forward to hearing from our more expert posters in the forum.

[AlwaysBeenASaver]

Now that you mention a vapor barer, I should mention, in case it's relevant, the house was built in 1975/76, and I'm guessing it was done somewhat low budget. Under the siding is thick black paper that comes in large rolls - not sure what it's called, but it looks very much like what I think roofers use under the shingles. I don't know if that would be considered a vapor barrier, but it's what's there, and installing a proper/modern vapor barrier isn't on the project list :-) The house most definitely isn't sealed or wrapped.

[bobechs]

Stuff cats in there.

Cats are free, and there's always more where they came from.

[Lulee]

As a cat lover, I can't go along with bobechs' suggestion.  But points for thinking outside the box, I suppose!😀

I believe you have tar paper under your siding based on your description.  Not a vapor barrier, as you say, but it prevents water that makes it past the siding from getting any further into the wall.  Theoretically, you could seal the tar paper to the foundation with foam spray/caulking/something similar.  Until someone with more siding and air sealing experience joins in, my thoughts would be around how to determine if this is really going to help much or are there other things you can do that would bring a greater return on your efforts.  If there aren't large gaps, perhaps focusing on attic insulation, air sealing around doors and windows, etc may more effective.

Later on if you replace the siding you can put on a vapor barrier, seal holes, add rigid insulation and at that point, perhaps seal around the cladding where it meets up to the foundation if you decide to as part of the project.

Sorry not to be of more help but I've never heard of anyone considering what you've suggested except to repair holes from rodent or insect intrusion and that was in a spot by spot effort, not around the entire perimeter.

[lthenderson]

You definitely don't want to seal that joint up. It is there intentionally to allow any moisture that gets behind the siding to leech out on top of the tar paper. If you sealed it up, you are essentially creating a water balloon between your siding and the tar paper looking for an exit into your house which can cause more damage. This also allows the house to breath and prevent mold from growing on the backside of your siding.

[Greg]

Without a little more detail I can't say for sure if the gap is intentional or just an artifact of the siding installation method.  If you want to seal it, use foam "backer rod" of a size that will just barely stuff in there.  Another option is metal step flashing.  The right profile of flashing could block off the gap and make sure any water is directed back out at the bottom.

[AlwaysBeenASaver]

Hmm, obviously I don't want to seal it up if it's supposed to be there to let moisture out. Not sure what types of details to provide, here's a pic from last summer where the cement coming up out of the dirt is the foundation and the light grey diagonal boards are the siding. The gap is where the two meet. The foundation goes up a few inches more beneath the siding, so the lowest set of nails holding the siding on are a few inches up from the bottom of the siding. Foam backer rod sounds like a good method to use if it should be sealed!

[BudgetSlasher]

I would be hesitant to seal the siding to the foundation. While my experience is more with vinyl siding, sealing the siding to the foundation would deny any moisture that developed behind the siding from driving rain between seams in the siding, condensation behind the siding, ect an exit at the bottom. Instead it may stay trapped behind the siding until it find another way out (gap between pieces of siding and the like) and the longer things stay wet the more likely rot becomes.

In addition (again talking from the vinyl experience) the siding is not really the air barrier (vinyl often has a drain hole in the bottoms of each "layer" to allow moisture to escape, so sealing it would do little to nothing for your air infiltration.

On the other hand what may help is sealing you sheathing (plywood/OSB) to the foundation. There should be no water behind the sheathing to worry about and the sheather/drainage plane act as the air barrier.

[paddedhat]

You definitely don't want to seal that joint up. It is there intentionally to allow any moisture that gets behind the siding to leech out on top of the tar paper. If you sealed it up, you are essentially creating a water balloon between your siding and the tar paper looking for an exit into your house which can cause more damage. This also allows the house to breath and prevent mold from growing on the backside of your siding.

This is correct. Do not seal this juncture, as you will be blocking an exit for any moisture that is trapped between the back of the siding and the tar paper. . This moisture IS behind the siding, and a result of everything from complex factors, like vapor pressure, to things as simple as water penetration at caulk joints and window and door trim. Diagonal siding is notorious for water issues, and foaming or caulking the bottom of the siding may not end well.

[AlwaysBeenASaver]

Thanks for the input everyone, I'm going to leave well enough alone and not seal up this gap, assuming as some of you stated that it's probably necessary to have the gap there for water/moisture to leave.