To have a well-sealed and energy-efficient house it is very important to use the right kind of material between the timbers. We found a product called "duct liner" that is used to glue inside large sheet metal ducts to reduce the heat and cooling loss. This product is made of fiberglass and is 1/2 inch thick, with a backing on one side that serves to hold it together. The rolls we purchased were 4 feet wide and 100 feet long. The first step was to cut the roll into 8 ft lengths, the same length as the timbers. We then used a guide to cut these pieces into 3 inch strips. This corresponds to the flat side of a full length timber.
By using a staple gun to tack these strips into place, they won't slide when the timbers are nailed together with the 6 inch pole barn nails. When the nails are driven, with a large hammer or small mall, to pull the timbers down tight, you will have a solid bond. What you will find as you get started is that landscape timbers vary somewhat and are not uniform like a board that has been planed to make a perfect piece of lumber. The duct liner material is spongy which will compensate for the variations in the timbers and also when they expand or contract due to changing weather conditions. We also cut small pads from the duct liner to place at the end of the timbers as they went together and at the corners. Use a good silicone caulk to hold the pads in place and this will also serve as additional insulation to cover all the cracks.