If you read Andy Engel’s post “Key Considerations When Insulating Old Walls” and Martin Holladay’s “Insulating Walls Without Sheathing,” you will find some overlap that makes this detail a good one to know. It addresses both water management and insulation in old walls without sheathing. The assembly’s success is owed to an air gap between the back of the old siding and the new insulation that allows the wall to dry. It functions much like a rainscreen.
Old houses, special considerations
Often, when people are looking to upgrade their home’s energy performance, the first thing they want to do is add insulation to the walls. For those living in older homes, that can come with unique challenges—including the absence of sheathing, which they usually discover after ripping out the lath and plaster. If removing the siding to retrofit the walls from the outside in is not in the budget, the situation needs to be addressed from the interior. So, how do you add insulation to stud bays that abut the backside of siding? Which materials make the most sense? How will moisture be prevented from accumulating?
All siding leaks, so plan for it
The key here is to always assume that water will make its way into the wall. Whatever the means by which it is enters—whether it’s migrating from wet basement walls and condensing on the backside of cold siding or it’s bulk water pouring off the roof during a weather event—this detail allows it to drain down and out while providing drying potential.
This is where the rainscreen analogy comes in.
For bulk water management and drying potential, it’s critical to leave an air space between the back of the siding and the insulation. In this detail, that gap…
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