Below-grade insulation upgrade for a house without concrete
Hello GBA,
My home is a converted carriage house from the late 19th century in eastern Massachusetts (zone 5). The house has no basement, nor a slab. As far as I can tell, beneath the subfloor we have pressure treated joists resting directly on the ground. I don’t believe there is any insulation, air, or vapor barrier underneath the floor, and I honestly don’t know what’s carrying the load of the building. There is no concrete or any obvious foundation elements. The building was converted to living space about 20 years ago, so all the interior finishes are relatively modern.We get a lot of bugs in the house, and in winter the floor is very cold.
In a few weeks we plan to have a wrap around patio installed. Before the pavers go in, the landscaping contractor will excavate about 18” on three sides of the house to install a drainable base. I figured that this would be a good opportunity to do some upgrades. I was thinking of pulling off the existing PT lumber water table, placing some mineral wool sheets as below grade insulation and putting on a new PVC water table. Additionally, I’d like to channel any rain water away from the exposed soil beneath our floor. The landscaping contractor recommended securing a rubber sheet (pool liner) to the water table below grade to divert the water into the yard.
This is an unusual house and I am looking for some feedback.
1. Does my plan to install mineral wool below grade make sense here?
2. Can anything be done to encapsulate the soil that do ant require jacking up the house?
3. Are there any options to improve the subfloor insulation?
4. Is there anything else I need to think about with the pavers going up directly to the wood house structure?
Thanks in advance for you advice,
Pavel
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Replies
Pavel,
1. If your carriage house was built in the 19th century, the floor joists aren't pressure-treated -- unless, of course, the entire floor system was replaced sometime in the last 50 years. Pressure-treated lumber didn't exist in the 19th century.
2. I hope that your landscaping contractor has consulted an engineer, to make sure that digging an 18-inch deep trench around the perimeter of your building doesn't undermine the footings -- or, in this case undermine the soil on which the floor joists are bearing.
3. Assuming that your landscaping contractor digs these trenches without causing a structural collapse, it would be a good opportunity to install vertical rigid foam at the perimeter of the building, from the bottom of the new trench up to the top of the floor joists. The above-grade sections of the vertical rigid foam should be protected by a material that prevents physical abuse, and you'll need to install horizontal Z-flashing at the joint between the top of the rigid foam and the lowest course of siding.