Bonus Video: Exterior-Insulation Retrofit — How to Choose Retrofit Details and Materials

If your budget is tight, you might consider substituting housewrap and synthetic roofing underlayment for the usual peel-and-stick membrane

with David Joyce

An exterior foam retrofit is an expensive undertaking. It’s sometimes hard to justify on the basis of energy bills alone. But when you need to replace the roofing or siding, you have a golden opportunity to roll an energy upgrade into the project. The extra cost of insulation is pretty minimal compared to the cost of new siding for the entire house.

Like a standard re-roof job, it’s important to protect the house from weather during construction. This is important partly to protect the homeowner’s belongings, but also to keep water out of the new roof and wall assemblies. When you add a lot of insulation to the outside of a house, you change the ability of the roof and walls to dry out if they get wet. For this reason, David and his crew seal up the house as they go, and then open up parts of it later—when it’s time to install new windows, for example.

Synergy Construction has done more than 25 exterior insulation jobs in the past few years. Each job is different, and so they’re always refining their processes. On this particular job, the budget was an issue, so Synergy worked with the homeowners to find places to cut costs.

The main cost-cutting measure on this job was at the layer between the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and the foam, where housewrap and roofing underlayment were installed. The best way to get a tight air seal is to strip the house of roofing and siding, and then completely wrap the house with peel-and-stick roofing membrane. But because peel-and-stick is expensive, Synergy substituted a synthetic roofing underlayment and housewrap in place of the peel-and-stick, choosing instead to concentrate on taping the seams and weaving the corners of the various layers.

The results were very good from an energy perspective, but at the cost of added time and labor. The best system for your house depends on your budget.

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