This Old House S47 E15: Round Ducts Win, Spray Foam Goes In, and Tom's Window Flashing Masterclass

The HVAC Upgrade That Isn't a Gut Job

Remember Richard's pleasant surprise in Episode 12? The existing HVAC system was in decent shape, which meant targeted upgrades instead of a six-figure replacement. Episode 15 delivers on that promise, and the approach is a masterclass in working smarter, not harder.

Richard and HVAC installer James Bouchard from Back Bay Mechanical evaluate ductwork options, and James makes a point worth remembering: round ducts outperform oval or rectangular ducts for airflow. The physics are simple — air moves most efficiently through a circular cross-section because there are no corners to create turbulence and resistance. Round ductwork delivers more CFM (cubic feet per minute) per dollar of energy spent pushing air through it.

In old houses where space is tight, rectangular ducts are common because they fit in shallow joist bays. But where you have the space, round is king. For this project, using round ducts where possible and upgrading connections without replacing the entire system is a budget-conscious decision that maximizes comfort without breaking the bank.

If this sounds familiar, it should: the Asheville episodes covered HVAC placement in Episode 2 and air sealing in Episode 5. The building science principles are the same — tight envelope, efficient distribution — but the application is completely different. Asheville was a flood-damaged crawl space vs. attic question. Needham is optimizing a 130-year-old system in a cold climate. Same physics, different context.

Closed-Cell Spray Foam on the Third Floor

Charlie removes the third-floor ceiling to make room for new insulation from Anderson Insulation, and he recommends closed-cell spray foam. His reasoning is sound: closed-cell provides full coverage, maximum R-value per inch (approximately R-6.5 per inch vs. fiberglass batts' R-3.5), and acts as both an air barrier and vapor retarder. It doesn't require gutting the walls — you can spray it in the ceiling cavity and get excellent performance.

The spray foam debate is a real one in the building science community:

For closed-cell: Highest R-value per inch, doubles as air and moisture barrier, structural rigidity, doesn't absorb water
Against closed-cell: Most expensive insulation option ($1.50-3.50/board foot installed), uses blowing agents with global warming potential, harder to inspect for defects, and if installed incorrectly, can trap moisture in unexpected ways
Alternatives: Open-cell spray foam (cheaper, lower R-value, permeable), dense-pack cellulose (excellent performer, lower cost, but requires different application), or rigid foam boards (good performance, labor-intensive to install)

For a third-floor ceiling in a Massachusetts Victorian where the attic air handler needs help, closed-cell spray foam is a strong choice. The R-value per inch matters when ceiling depth is limited, and the air-sealing properties reduce the HVAC system's workload — which ties directly back to the "keep the existing system" strategy.

Tom's Window Installation: The Flashing Lesson

Tom and Kevin install a new double-hung window in the dining room, and Tom does what Tom does: turns a seemingly routine task into a tutorial on why details matter. He emphasizes proper flashing of the window opening before setting the window — applying self-adhering membrane around the opening in a specific sequence (sill first, then jambs, then head) so that each piece overlaps the one below it in a shingle-like pattern.

This is the same principle as the chimney flashing in Episode 4: water runs downhill, and every joint should be lapped so water is directed out, never in. It sounds obvious. It is not universally practiced. Improper window flashing is one of the leading causes of water damage in residential construction, and it's entirely preventable with materials that cost about $30 per window.

Leveling an Old Floor

Inside, Kevin and Charlie address a sloped parlor floor — a common reality in old houses where foundations have settled unevenly over decades. They shim the joists and install a new, level subfloor, which is essential prep before any finish flooring goes down.

If you live in an old house, you know the feeling: put a marble on the floor and it rolls. Shimming joists is the standard fix — thin tapered pieces of wood placed on top of the joists to create a level plane. It's fussy, precise work that requires a long straightedge and patience. The alternative — self-leveling compound poured over the existing floor — works for minor variations but can't correct the inches of slope common in houses from the 1800s.

DIY Confidence Scale: Episode 15

HVAC duct evaluation/replacement: Hire a Professional. Ductwork sizing, static pressure calculations, and system balancing require HVAC expertise. A poorly designed duct system wastes energy and creates comfort problems no thermostat can fix.
Spray foam insulation: Hire a Professional. The chemicals are hazardous during application, the equipment is specialized, and improper application causes more problems than it solves.
Window installation with flashing: Advanced Intermediate. A competent DIYer can install a replacement window, but the flashing sequence is critical. Study Tom's technique before you start. Or better yet, watch this episode three times.
Floor leveling with shims: Intermediate. Time-consuming but straightforward with a laser level. Budget a full day for one room and you won't be disappointed.

Additional Resources

Next time: Charlie visits a millwork factory where custom trim is made to order (if you love watching raw lumber become beautiful finished pieces, this one's for you), Tom explains rain screens, composite decking goes in, and the kitchen layout starts to take shape. The Needham project is hitting its stride.