Jim writes from Nashville, Tennessee: "I am renovating a 100 year old house. The original siding is lap board that was covered with asphalt shingles. The shingles were attached to the house with nails that are no more than 3 to 4 inches apart. I am removing the shingles and nails and will patch and paint the lap board.

There is no sheathing or insulation of any kind under the lap board. I am removing the interior walls and will put insulation between the studs prior to installing the drywall interior walls. I am considering placing roof felt between the studs against the interior side of the lap board before installing the wall insulation. My reason for doing this is to provide a last line of defense against water penetration through the lap board wall. (With thousands of nail holes I will probably miss filling a few.) I hope this would keep the insulation sufficiently dry.

What do you think and if this sounds like a reasonable solution what felt weight would you recommend?

I know that roof felt is not a vapor barrier, but would it give me some protection? Given the age of the lap board I do not want to risk damaging it by removing it to put sheathing under it. I intend to leave the lap board siding."

Answer: If you take the siding off you will need to re-sheath the walls for structural strength. I don't think the felt will keep water out; it will probably only allow water to run down to the bottom of the cavity and saturate the bottom wall plate. While sidings will shed water, most are not intended to keep water out entirely and that is why a weather-resistant barrier has to be installed prior to the exterior finish. I don't see how that can be successfully done from the interior of the wall cavity and therefore my recommendation is to remove the siding and sheath the wall, install either taped foam board or housewrap and then reside.