For the transition from hardwood flooring to tile, I've seen several posts recommending: 1) an expansion gap of 1/2 the width of a tile joint; 2) that's then filled to within 1/4" of the top of the gap with foam caulk backing; 3) that's then filled to the top of the gap with sanded caulk color-matched to the hardwood; and 4) that's then covered with whatever finish is used on the hardwood.

To my surprise, those posts recommended the use of sanded caulk without any inquiry about, nor any information furnished about, whether the grout used in the tile was sanded or unsanded. As it is my understanding that sanded grout is typically used when a tile joint exceeds 1/4", and because it seems highly unlikely that an expansion gap that's 1/2 the width of a tile joint is likely to exceed 1/4", the recommendation to use sanded caulk in that hardwood-to-tile transition would seem to be for some reason other than to match the material used in the tile joints. So, why do some installers seem to recommend the use of sanded caulk in the hardwood-to-tile expansion gap without knowing whether sanded grout was used in the tile joints?

A related issue is the recommendation I've seen to cover the caulk with the floor finish. Many floor finishes, when cured, are quite brittle, whereas the primary reason for using caulk is so that the material can expand and contract as necessary with changes in temperature and relative humidity. Shouldn't I expect the relatively brittle floor finish sitting over relatively flexible caulk to crack over time as the caulk expands and contracts? If so, wouldn't it be better to install the caulk without a layer of floor finish on it?

You don't have to use any particular product - the goal at the flush tile to hardwood transition is to hide the necessary expansion gap that the hardwood needs for proper expansion and contraction. If it was laminate, you'd typically install a t-mold over the gap and be done with it; with hardwood you typically don't want to use a molding or reducer unless you have to due to material height differences.

I've used all kinds of fillers, caulk, silicone, putty in these gaps; it really depends on how much gap you need to leave and how much traffic over the area you are expecting. A laundry closet, for example, doesn't see as much traffic as an ensuite would. In a low traffic areas, just use a color matched putty and you're good. For high traffic, you can try a color matched silicone, since that has enough flex to allow for some expansion. IDEALLY, you want to monitor your humidity all year round and keep it stable to avoid expansion altogether.

The only grout that I have seen being used for floor tile is sanded. One reason I recommend sealing the sanded caulking that comes close to the tile grout color is that the sanded caulk usually dries out and seems to be another color. The other reason is that the sanded caulking gets dirty and looks gray if it doesn't have a coat of finish on it. Finish that can expand and contract with the wood will work on caulking and never flake off when it is applied to the caulking, in my experience.

Thanks for your comments, folks! Based on your advice, we’ll be going with caulk that matches the hardwood flooring color and we’ll cover it with floor finish. Our ceramic tile joints are less than ¼” so our tile grout is unsanded. As a result, we’ll be using unsanded caulk in the hardwood-to-tile expansion joint. (Pete A: I gather from tile experts that in addition to their recommendation to use unsanded grout in tile joints less than ¼”, they also recommend avoiding the use of sanded grout if the tile has a gloss sheen rather than a satin or matte sheen as the sand may tend to dull a gloss sheen as an installer wipes up the excess grout during installation.)

I vote for non-sanded It's more flexible IMO without the sand.

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