After the framing was finished the next thing to tackle was the exterior foam board and windows. The foam board, although not extremely thick, does help to prevent thermal bridging and provides a modest but noticeable increase in the R-value of the walls. Wood has an R-value of about 1 per inch of thickness. With 2×4″ walls this would give us an R-value of only 3.5 where the studs were. The 1/2″ foam board has an R-value of 3 so where it overlaps the studs the R-value is brought up to 6.5 which is almost double. This makes a big difference in the winter by reducing cold spots along the walls.
By spray foaming and taping the seams of the foam board it is now able perform double duty as both insulation and vapor barrier. At this point in the build waterproofing the house was important for us because we didn’t want to have to deal with another flooding incident.
The first step to the foam board installation was to measure and run a chalk line along the studs to ensure the foam boards were installed evenly along the house. The house is quite long so I had break up the chalk line into two or three different segments along the sides.
The chalk line can be seen in the picture below.
Since I wasn’t able to nail the boards on the trailer itself I used construction adhesive instead. Make sure that the adhesive you buy is compatible with foam board.
To secure the foam to the walls I used 1 ¾” roofing nails. There are special nails made for foam but they were expensive and wouldn’t have laid as flat to the foam anyway. I was careful to nail close to the edges of the foam board so that when I taped the seams I would also cover the nails. Nails that were in the middle of the board I also covered with tape to prevent leaking. Note that the fresh spray foam along the edges of the board I am nailing in the picture below has expended. I didn’t worry about wiping it away or trying to flatten it immediately after applying it to the seams. The spray foam tends to expand for awhile after application. This wasn’t an issue on the interior of the house since it was going to be covered with insulation anyway. On the exterior I applied the tuck tape before the spray foam cured and when I pressed the tape down it flattened the foam out.
Watch the video for a quick walk through of the foam board installed on our tiny house and another look at the metal strapping used to reinforce the framing.
Terms & Definitions:
Thermal Bridging: A thermal bridge, also called a cold bridge or heat bridge, is an area of an object (frequently a building) which has a significantly higher heat transfer than the surrounding materials resulting in an overall reduction in thermal insulation of the object or building.
R-value: the capacity of an insulating material to resist heat flow. The higher the R-value, the greater the insulating power.
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