“Light and warmth shape how we live long before we notice the walls.”
Walk into a room on a winter morning and you can feel the insulation before you see it. The air either holds its warmth quietly, or it leaks away at the edges, leaving a faint draft around your ankles and a chill near the windows. The surfaces tell you a story. Drywall that feels cold to the touch, floors that pull the heat out of your feet, a ceiling where you sense the attic above. This is what we are really talking about when we compare spray foam and fiberglass. Not just R-values on a chart, but how your space holds light, temperature, and sound.
Energy efficiency sounds technical, but in a house it feels very human. It is how the air sits in a living room at 9 p.m., how the bedroom settles at night, how your HVAC system hums or stays quiet. Good insulation makes a room feel stable. Edges fade away. You are less aware of the outside shift from sun to shade, from wind to stillness. The envelope of the building does its job so well that you stop thinking about it.
Design is subjective, but I see insulation as a hidden layer of architecture. It is the soft volume between structure and finish that lets the rest of your choices actually work. Clean lines and large windows mean very little if your walls leak heat. A minimalist space with poor insulation always feels slightly off: too warm upstairs, too cold downstairs, a faint noise from the street that should have been filtered out. The geometry might be beautiful, yet the comfort is uneven.
With spray foam and fiberglass, we are not choosing between good and bad. We are choosing between two ways of shaping the unseen thickness of the wall. Spray foam wants to seal, to cling to every edge, to turn cavities into a continuous shell. Fiberglass wants to breathe a bit more, to fill voids with a light, fibrous mass that slows heat and sound but follows a more traditional rhythm of framing and air. I tend to prefer the clean continuity of foam in certain areas, though fiberglass works too, especially where the budget pushes back or the building must dry in both directions.
For the first 300 words or so, I do not want you thinking about product labels. Think about mood. Imagine sitting in a quiet room with low winter sun coming through a window. You touch the wall near the sill. It feels neutral. Not warm, not cold. The glass is where the temperature shifts, as it should, but the surrounding frame feels steady. Your feet are on a floor that does not draft from below. The mechanical system is not racing to catch up every time a cloud moves. That evenness, that stable envelope of comfort, is what insulation types are trying to create.
Now we can open the wall and talk about how to get there.
“A building is only as calm as its envelope.”
How Insulation Shapes the Feel of a Room
A room does not just hold furniture. It holds gradients. The air near the floor, the center of the space, and the ceiling all sit at slightly different temperatures. Good insulation flattens those gradients so the room feels consistent. That is where spray foam and fiberglass split.
Spray foam, especially closed cell, tends to create a tight, controlled environment. It expands into every gap, grips studs and sheathing, and forms a continuous layer that blocks air movement. When done well, you feel less draft, surfaces stay closer to room temperature, and sound from outside gets muted more than with a loose-fill approach. It can make a lightweight house feel more solid.
Fiberglass, whether batt or blown-in, behaves differently. It is not an air barrier by itself. It slows heat transfer by trapping air inside fine fibers, but it still wants a companion layer to control air movement. The feel of a fiberglass-insulated room depends heavily on how carefully it is installed and how good the air sealing is at joints, penetrations, and around windows and doors. When detailed well, it can be very comfortable. When rushed, you get cold corners, warm stripes on a thermal camera, and that familiar “drafty but not exactly” sensation.
From an energy efficiency perspective, spray foam often reaches performance targets with thinner thickness, while fiberglass usually needs more depth and more attention to detail. Yet the structure of your house, local climate, budget, and moisture strategy all matter as much as the insulation label.
Spray Foam vs. Fiberglass: The Architectural View
“Form follows function, but performance shapes form.”
When I think about insulation types, I think about how they change the assembly of a wall or roof. Every material brings thickness, weight, permeability, and a specific way of meeting other layers.
What Spray Foam Really Does Inside the Wall
Spray foam is more than just “high R-value.” It changes the behavior of the entire assembly.
There are two main forms used in houses:
– Open-cell spray foam
– Closed-cell spray foam
They look similar during installation, but they act very differently.
Open-cell foam is softer, like a dense sponge. It has lower R per inch and is vapor permeable. It is good at sealing air, good at filling odd shapes, and useful in certain interior applications where drying is still needed.
Closed-cell foam is rigid and dense. It delivers higher R per inch and is highly resistant to moisture and air. It adds structural stiffness to the assembly, almost like a thin structural layer inside the cavity. It can shrink slightly if not installed well, which is why the applicator’s skill matters.
From a design perspective, spray foam simplifies some details and complicates others. It can let you keep the framing thinner while still hitting energy targets, which is helpful when you want cleaner window reveals or less bulky wall thickness in a tight floor plan. It can also make the building much less forgiving of mistakes in vapor control, since closed-cell foam blocks drying in one direction. You are trading a more controlled interior climate for the need to be more precise with building science.
What Fiberglass Brings To The Assembly
Fiberglass is familiar, and that familiarity often works in its favor. Most builders know how to install it, and inspectors know how to evaluate it. The material itself is simple: glass fibers spun into batts or blown into cavities. It is non-combustible, does not shrink, and lets moisture vapor pass while still slowing heat.
Fiberglass is honest. It does not air seal for you. It does not solve bad window flashing or leaky electrical penetrations. It needs a proper air barrier, usually sheathing outside and drywall inside with good taping and gasket details. When paired with solid air sealing, it performs much better than many people expect. When thrown into cavities around wires and pipes without care, it creates gaps and compression that cut performance.
From a design angle, fiberglass works well when the wall thickness is not a constraint, or when you are comfortable making the wall a bit deeper. It keeps the assembly more vapor open, which can help with drying in mixed climates when used correctly with other layers.
Energy Efficiency: R-Value, Air Sealing, and Real-World Performance
On paper, insulation looks simple: R-values line up in a neat column, you pick the higher one, and call it a day. In an actual building, the story is more about continuity, air movement, and moisture.
R-Value: How Spray Foam and Fiberglass Compare
Let us talk rough numbers at typical residential densities. Local codes and manufacturers will give exact details, but this gives you a sense of scale.
| Insulation Type | Approx. R-value per inch | Typical Wall Cavity Use | Key Effect on Energy Use |
|---|---|---|---|
| Open-cell spray foam | R-3.5 to R-3.8 | 2×4 or 2×6 walls, rooflines | Good R-value + strong air seal |
| Closed-cell spray foam | R-6 to R-7 | Thin assemblies, rim joists, roofs | High R in limited space + vapor control |
| Fiberglass batts | R-3.0 to R-3.7 | Standard stud cavities, interior walls | Good R if installed perfectly + air barrier required |
| Blown-in fiberglass | R-3.7 to R-4.3 | Dense-packed walls, attic floors | Better cavity fill + fewer gaps + air barrier required |
Notice how close open-cell foam and fiberglass are on pure R per inch. The real gap opens when air sealing enters the picture.
Air Tightness: The Hidden Half Of Energy Efficiency
Heat leaves your home in two main ways:
1. Conduction: heat moving through materials
2. Air leakage: warm air escaping and cold air entering
Spray foam hits both. Fiberglass mainly addresses conduction and needs help with air.
Closed-cell foam is an air barrier and a vapor retarder at typical thickness. When sprayed across sheathing, studs, and penetrations, it creates a nearly continuous shell that sharply reduces uncontrolled air movement. This lowers heating and cooling loads and can let HVAC systems be smaller and run more steadily.
Open-cell foam is still an air barrier at code-level thickness. It slows air enough that drafts are drastically cut, which makes the interior feel more stable. It remains more vapor open, which changes your moisture strategy, but in terms of air it is very good.
Fiberglass, by itself, is not an air barrier. Air moves through it fairly easily unless you back it up with sealed sheathing outside and well-detailed drywall or membranes inside. Many real-world houses lose a lot of energy through tiny gaps at top plates, electrical boxes, rim joists, and window frames, even if the cavities look full of fiberglass.
From a designer point of view, this means: if you choose fiberglass, you must commit to proper air sealing as a separate scope. Spray foam, especially closed cell, bundles those layers into one material in many areas.
“Continuous insulation is not just thickness; it is the absence of weak points.”
Moisture, Drying, and Durability
Energy efficiency is not only about numbers on your utility bill. A wall that traps water and cannot dry will not perform for long, no matter how efficient it looks in year one. This is where the character of spray foam and fiberglass really matters.
Spray Foam and Moisture
Closed-cell spray foam is highly resistant to liquid water and has very low vapor permeability. It can act as a secondary water barrier and often as a vapor retarder. This can be a strength or a liability.
Strength:
– In flood-prone areas or at rim joists, closed-cell foam resists moisture intrusion.
– In roofs, it can reduce risk of condensation by keeping the sheathing warmer, if placed correctly.
– It adds a stiff layer that makes the assembly feel solid and reduces the chance of hidden air leaks that drive moisture into cold layers.
Liability:
– If water gets behind the foam from a leak, it can be slow to dry because the foam blocks vapor movement.
– Wrong placement in the assembly can trap moisture where you do not want it, especially in mixed or cold climates without proper exterior insulation or venting.
Open-cell foam lets vapor move more freely but still limits air, which often carries most moisture. In some roof assemblies, this can work well if you allow for drying and follow local codes and building science guidance carefully.
Skill matters here. Spray foam must be mixed and applied correctly. Too much thickness in one pass can cause off-ratio foam or long curing times. Gaps and voids are hard to fix once closed up.
Fiberglass and Moisture
Fiberglass does not absorb water in the same way organic materials do, but it can hold moisture within its fibers. Its big advantage is that it does not usually block vapor by itself. With the right membranes and cladding, the wall can dry inward or outward, depending on climate and design.
Strength:
– More forgiving in assemblies that need to dry in both directions.
– If a small leak occurs and is fixed quickly, the cavity can dry out if air and vapor control layers are correct.
– Easier to repair locally. You can open a section, replace the fiberglass, and close it again.
Liability:
– If air leaks carry moist indoor air into cold cavities, fiberglass does not stop that flow. Moisture can condense on cold surfaces and feed mold on nearby paper facings or wood.
– Wet fiberglass slumps and loses R-value until it dries and is reset, if it recovers at all.
For energy efficiency over 20 to 30 years, the best insulation is the one that stays dry and continuous. In many cases, that means pairing fiberglass with serious air sealing and thoughtful vapor control, or using spray foam in the most vulnerable areas and combining it with other materials elsewhere.
Cost, Complexity, and Where Each Material Belongs
From a pure design perspective, I like to match materials to their strengths rather than force one type across the entire house. The envelope becomes a set of zones.
Where Spray Foam Often Makes Sense
These are areas where foam’s performance usually justifies its cost and complexity:
– Rim joists and band joists
– Cathedral ceilings and complex rooflines
– Conditioned attics where ducts run
– Small mechanical rooms or tight spaces where depth is limited
– Areas with extreme wind or exposure where air leaks are common
Here, the ability of spray foam to both insulate and air seal in one layer translates into real energy savings and comfort. A tight rim joist alone can make the first floor feel far more stable in winter.
Where Fiberglass Often Works Well
Fiberglass shines where cavities are accessible, details are simple, and you are willing to pay attention to air sealing separately:
– Standard 2×6 walls with exterior sheathing and a good WRB
– Attic floors where you can build up depth
– Interior partitions for sound control
– Renovations where walls are open briefly and the budget is under pressure
Dense-pack fiberglass in particular can fill irregular spaces better than batts and provides decent R-value with fewer voids, as long as an air barrier is present.
Cost & Performance Trade-Offs
If we compare broad tendencies:
– Spray foam: higher material cost, higher skill requirement, stronger performance per inch, and bundled air sealing.
– Fiberglass: lower material cost, more forgiving sourcing, performance heavily tied to installation quality and separate air sealing.
Energy efficiency is not just peak R-value. It is about annual heating and cooling loads, which depend heavily on air leakage. A moderately high R wall with outstanding air sealing often beats a very high R wall with sloppy details.
Materials, Texture, and the Hidden Layer of Design
Architecturally, I do not stop at performance graphs. I think about how materials age inside the wall, even if the homeowner never sees them. The wall has a character, and it starts with that hidden layer of insulation.
| Property | Spray Foam (Closed Cell) | Spray Foam (Open Cell) | Fiberglass (Batts / Blown) |
|---|---|---|---|
| Density & feel | Rigid, adds stiffness, almost plastic-like | Soft, spongy, fills shapes | Light, fibrous, compressible |
| Air sealing | Excellent air barrier | Very good air barrier at proper thickness | Poor alone, needs separate air barrier |
| Vapor control | Vapor retarder at code thickness | Vapor open, allows drying | Vapor open, depends on facing & membranes |
| R-value per inch | High (R-6 to R-7) | Moderate (around R-3.5) | Moderate (R-3 to R-4.3) |
| Installation sensitivity | Very sensitive to crew skill, mixing, thickness | Sensitive to mixing and curing | Sensitive to gaps, compression, cutting |
| Repair & changes later | Hard to modify without removal | Hard to modify but easier to cut than closed cell | Easy to remove and replace in sections |
| Environmental load | Higher, depends on blowing agents | Moderate, often more benign blowing agents | Moderate, high recycled content possible |
From a minimalist design point of view, I like to see the wall as a single idea. If the exterior is thin, crisp, and flat, and the interior lines are lean, the insulation needs to carry its weight in a small amount of space. Closed-cell spray foam often wins here. Where I have more depth to work with and want a forgiving, vapor-open assembly, dense fiberglass with continuous exterior insulation and proper air control feels more balanced.
“Good envelopes are quiet. They let you forget the weather without denying it exists.”
Energy Efficiency Room by Room
Energy efficiency is experienced locally. Each space in a house feels the envelope in a slightly different way, and the choice between spray foam and fiberglass shifts with it.
Living Room and Social Spaces
These are the rooms where people linger. Large windows capture light, sometimes floor to ceiling, and the walls carry quite a bit of your heating and cooling load. In many designs, the living room has a higher ceiling, a vaulted area, or an open stair that connects floors.
For high ceilings and open volumes, spray foam in the roof or upper walls can keep stratification under control. Heated air will always rise, but with strong insulation and air sealing at the upper boundary, the temperature difference between the floor and ceiling shrinks. You feel less “heat stuck up there” that never comes down.
With fiberglass in these areas, I lean on dense-pack or high-density batts with very careful air sealing, plus attention to duct layout so conditioned air mixes evenly. When done well, the comfort can be excellent. The difference is how tolerant the system is of small mistakes. Foam handles them better.
Bedrooms and Quiet Zones
Bedrooms are where small shifts in comfort become noticeable. A 2-degree drop near a corner, a faint draft across the floor, or outdoor noise bleeding in can all affect how the room feels.
Either material can work well here. If sound control is high priority, you can combine fiberglass in stud cavities with resilient channels or double layers of drywall. Spray foam provides decent sound damping, especially open cell, but it is not tuned to acoustic performance the way a full sound-control assembly can be.
From an energy standpoint, air sealing at window frames, top plates, and outlet boxes matters more than the label on the cavity fill. Spray foam gives you a head start by default. Fiberglass demands more discipline, but the result can be very similar in a standard bedroom.
Attics, Roofs, and Mechanical Spaces
This is where spray foam can change the entire energy profile of a house. Turning a vented attic into a conditioned, foam-insulated volume can bring ducts into the thermal envelope, reduce losses, and stabilize temperature swings.
A typical vented attic with fiberglass on the floor can be very efficient if you:
– Seal every ceiling penetration
– Provide generous insulation depth
– Protect it from wind washing at the eaves
But many attics never get that level of attention. Spray foam applied to the underside of the roof deck creates a more controlled layer. The downside is cost and the need to think carefully about moisture and ventilation strategies.
For mechanical rooms, especially ones framed into tight corners or slab-on-grade basements, closed-cell foam at rim joists and foundation transitions often gives a cleaner, more reliable seal than fiberglass.
How To Think About Energy Efficiency When You Choose
We can pull this together into a simple mental model.
1. Picture the envelope as a continuous shell, not a set of walls and a roof.
2. Decide where you need absolute control over air and where you can accept a bit more tolerance.
3. Match spray foam to the most exposed or complex transitions and rooflines.
4. Use fiberglass in more regular walls and wide attic floors if the crew will commit to proper air sealing.
Energy efficiency is not about perfection. It is about stacking small advantages while avoiding serious flaws. A moderate R wall with clean details at corners, rims, and penetrations beats an extremely high R wall with gaps every few feet.
I tend to prefer using closed-cell foam surgically: rim joists, select roof areas, tricky interfaces between old and new construction. Then I let fiberglass and exterior continuous insulation do most of the quiet work in large runs of wall. This keeps assemblies more breathable where helpful and controls cost, while still hitting strong energy performance.
In a pure spray foam house, the interiors can feel very still, almost insulated in a literal sense from the outside. That calm is appealing, but I still like to maintain a clear drying path and a logical moisture strategy. Foam should never become an excuse to ignore flashing, drainage, or ventilation.
In a pure fiberglass house, comfort comes from rigor. Every joint must be sealed, every gap filled, every batt cut neatly around wires rather than jammed behind them. When that discipline exists, the interior can feel just as stable and quiet, though you might need a bit more thickness to reach the same heating and cooling loads.
There is no single correct choice between spray foam and fiberglass for energy efficiency. There is only the right assembly for your climate, your structure, and your tolerance for risk and cost. Once you see insulation as part of the architecture, not just a line item, the decision starts to feel less like a commodity choice and more like shaping the invisible wall that your daily life leans on.