What is a blower door test and is one worth insisting on?

A blower door depressurizes the home and measures how much air leaks through the envelope. Yes, insist on one, and insist on an intermediate test while the walls are still open. A test at final gives you a number. A test at rough-in gives you a chance to fix what the number is telling you.

Is spray foam better than fiberglass?

Spray foam air seals and insulates in one step, which is its real advantage. Fiberglass or dense-packed cellulose installed carefully behind a properly detailed air barrier performs comparably and costs less. The determining factor is installation quality far more than the material.

What is thermal bridging?

Heat bypassing insulation by traveling through framing members, which conduct far better than the insulation between them. In a conventionally framed wall, studs and plates can account for a meaningful share of the wall area, which is why continuous exterior insulation matters.

Does a tight home have air quality problems?

Only if it is built tight without being ventilated. A tight envelope plus mechanical ventilation gives you filtered air on purpose. A leaky envelope gives you unfiltered air through the wall cavity and the crawlspace, which is worse, just less visible.

Building Envelope Basics for Colorado Homes

If you spend an extra twenty thousand dollars on a home, spend it on the shell.

This is the most consistently ignored advice in residential construction, because the shell is invisible. Nobody walks into a home and admires the air sealing. Buyers look at the kitchen, the equipment brochure, and the smart thermostat, and builders respond to what buyers look at.

Meanwhile the envelope is deciding how the home feels for the next fifty years.

The load comes first

Mechanical equipment does not create comfort. It conditions whatever load the envelope hands it.

A leaky, poorly insulated home with an expensive furnace has an expensive furnace running constantly against a load that should not exist. It will be uncomfortable anyway, because comfort is not just air temperature. It is also radiant loss to cold surfaces and the sensation of drafts, and neither is fixed by making the air warmer.

The same home built tight and well insulated has a smaller load, which means smaller equipment, which means lower first cost on the mechanical side offsetting part of the envelope investment. This is why the two decisions should be made together and are usually made separately.

The three that matter

Air sealing

Air leakage is the largest single factor and the cheapest to address, because it is mostly labor and attention rather than material.

Air moves through a home continuously, carrying heat out in winter, humidity in during summer, and dust and pollen year-round. It leaks at penetrations: top plates, rim joists, electrical boxes, plumbing penetrations, can lights, and the attic hatch.

The critical point is that all of this is accessible during framing and inaccessible afterward. An air sealing detail missed at rough-in is a permanent attribute of the home.

Insist on a blower door test at rough-in, not only at final. The intermediate test finds the leaks while there is still time to seal them. A final test just tells you what you built.

Continuous insulation

Insulation between studs is interrupted by the studs. Wood conducts heat considerably better than insulation, so every framing member is a small thermal bypass, and there are a lot of them once you count plates, headers, corners, and intersections.

Continuous exterior insulation, a layer of rigid foam or mineral wool outside the sheathing, interrupts those bridges. It also moves the sheathing closer to indoor temperature, which reduces condensation risk inside the assembly.

Colorado’s climate zones run from 5 to 7 across the populated parts of the state, which is genuinely cold. The Front Range is mostly zone 5, the foothills and higher valleys run colder. Continuous insulation earns its cost across all of them.

Windows

Windows are the weakest part of any wall assembly, and at Colorado’s altitude and sun intensity the specification matters more than in most places.

Two numbers govern. U-factor measures heat transfer, and lower is better. Solar heat gain coefficient measures how much solar energy passes through, and the right value depends on orientation.

That orientation point is where most specifications go wrong. Buyers and builders order one window package for the whole home. South-facing glass in Colorado can deliver useful winter heat if it is specified to admit gain and if there is overhang to shade it in summer. West-facing glass mostly delivers unwanted afternoon heat in August and should be specified to reject it. Same home, different answer per elevation.

Where Colorado differs

Altitude. Windows are manufactured with a gas fill between panes, and units built at sea level and shipped to 5,280 feet or higher experience a pressure differential. Manufacturers address this with capillary tubes or by building units for altitude. Confirm the units are specified for your elevation.

Sun intensity. Ultraviolet exposure at altitude is higher, which affects exterior finishes, sealants, and roofing material life. Cheap sealant fails faster here than the label suggests.

Diurnal swing. Colorado routinely swings a wide range between day and night temperature. Thermal mass and a good envelope both damp that swing, and a poor envelope tracks it, which is a large part of why some homes feel uncomfortable regardless of thermostat setting.

Dry air. Low humidity means the wetting risk inside wall assemblies is lower than in humid climates, which gives you more latitude on assembly design. It also means winter indoor humidity can get uncomfortably low in a very tight home, which is a solvable problem with humidification and a reason to think about it during design.

Ventilation is not optional

Building tight without ventilating is the one genuine mistake in this direction.

A tight home needs mechanical ventilation, ideally a heat recovery ventilator, which exhausts stale air and brings in fresh air while recovering most of the heat from the outgoing stream. In Colorado’s dry winters an energy recovery ventilator, which also transfers some moisture, is sometimes the better choice.

The framing to understand: a tight home with ventilation gives you filtered, controlled air. A leaky home gives you uncontrolled air through the wall cavity, the crawlspace, and the garage. The second is not healthier. It just feels less deliberate.

This matters more each year as wildfire smoke becomes a routine part of Colorado summers. A tight envelope with good filtration is the difference between a home you can breathe in during a smoke event and one you cannot. Our writing on wellness in the built environment covers that in more depth.

What to ask your builder

What blower door result are you targeting, and will you test at rough-in?
Is there continuous exterior insulation, and how much?
What are the U-factor and SHGC for each elevation, and are the units specified for our altitude?
What ventilation system is included, and is it an HRV or an ERV?
Who is responsible for air sealing, and is it a scoped trade or an assumed part of framing?

That last question is the diagnostic one. On projects where air sealing belongs to nobody in particular, it does not get done well. On projects where it is a named scope with a named responsible party, it does.

For what the code requires as a floor, see our writing on energy efficiency, and for how these choices age, see designing homes for the 2030s.

The envelope decides everything