R-value measures one thing: how well a material resists heat moving through it by conduction. That is genuinely useful information, and it is also about a third of the story. It says nothing about air leaking through your attic, nothing about moisture, and nothing about whether the stuff was installed well enough to perform the way the label says it will.
That gap is why a homeowner can get two quotes, pick the bigger number, and end up with the colder house. This page explains what the number means, what it hides, how many inches of each material it takes to get there, and why we look at air sealing before we look at R-value.
The one idea to take away
A leaky R-38 attic underperforms a well-sealed R-30 attic. Air moving through insulation carries heat and moisture straight past it, and no R-value on the bag stops that. Seal first, then insulate. That order is not us being cheap with your money, it is the order the physics works in.
What R-value actually measures
R-value is thermal resistance. It is a lab measurement of how much a material slows conductive heat flow, taken under steady, controlled conditions: flat sample, no air movement, no moisture, no gaps, uniform temperature on each face. Higher number, more resistance. It is also additive in a straightforward way. Two inches of a material rated R-6 per inch gives you R-12 through that layer.
Because the test is standardised, R-value is honest for the one job it does. It lets you compare materials against each other on conduction alone. The trouble starts when people treat that single lab figure as a prediction of how a real assembly on a real house in a real Seattle February will behave.
What R-value does not measure
- Air leakage. The biggest one. Fiberglass batts are air permeable, so warm indoor air moves through them and out of the house. The batt still tests at its rated R-value. Your house still loses the heat.
- Moisture. Wet insulation loses much of its resistance, and in a Puget Sound crawl space or a vented attic, wet is a normal condition rather than an accident. R-value assumes dry.
- Radiant transfer. The lab test isolates conduction. Real attics also move heat by radiation, which is why a radiant barrier can help in some assemblies and why R-value alone will not tell you.
- Installed quality. Compressed, gapped, or wet material does not deliver its label. A batt stuffed around wiring, cut short at the ends, or squashed behind a pipe can lose a serious fraction of its rated performance.
- Thermal bridging. R-value is a property of the material, not of the wall. The studs, rafters, and joists running through that insulation are a separate and much weaker path, and they are ignored by the number on the bag.
None of that makes R-value useless. It makes it one input. If you only ever remember one thing from this page, remember that the insulation is only as good as the air barrier in front of it, which is why we quote air sealing before we quote depth.
R-value per inch of the materials you will actually be quoted
Here are the materials a Seattle homeowner is likely to see on a bid sheet. The spray foam figures are the ones we work to. The rest are widely published typical ranges, not precise values, because they vary by product, density, and manufacturer.
| Material | Typical R per inch | Notes |
|---|---|---|
| Closed-cell spray foam | About R-6 to R-7 | Air seals and, at sufficient thickness, acts as a vapor retarder |
| Open-cell spray foam | About R-3.6 to R-4 | Air seals, stays vapor open, absorbs bulk water |
| Fiberglass batts | Typically about R-3.1 to R-3.8 | High density products are quoted higher. Air permeable |
| Blown fiberglass (loose fill) | Typically about R-2.2 to R-2.7 | Needs real depth. Can settle and can be disturbed by air movement |
| Blown cellulose | Typically about R-3.2 to R-3.8 | Denser, settles over time, absorbs and holds water |
| Rigid foam board, EPS | Typically about R-3.6 to R-4.2 | The white beaded board |
| Rigid foam board, XPS | Typically about R-4.5 to R-5 | Published values commonly drift down over the life of the board |
| Rigid foam board, polyiso | Typically about R-5.5 to R-6.5 | Highest per inch of the boards, but performs worse as it gets colder |
Treat every one of those ranges as a starting point. Installed quality moves real performance a lot more than the difference between two products at the top of the same range. A cellulose job blown to full depth with the attic properly sealed will beat a foam job sprayed thin and gapped, every time.
Material R-value versus whole-assembly R-value
This is where the quoted number and the real number diverge most, and almost nobody explains it to homeowners.
When a bid says R-21 walls, that is the cavity insulation. It is the number for the fluffy part between the framing. Your wall is not made only of the fluffy part. Roughly a fifth to a quarter of the surface area of a typical framed wall is wood: studs, plates, headers, corners, and the extra framing around every window and door. Wood is a much better conductor of heat than insulation, so every one of those members is a shortcut through your wall. That shortcut is called thermal bridging.
Softwood framing runs somewhere around R-1.2 per inch, so a 3.5 inch stud is roughly R-4 while the cavity beside it is R-13 to R-15. Average those two paths together across the whole wall and the assembly performs meaningfully below its cavity rating. The same thing happens at rafters in a cathedral ceiling and at joists in a floor over a crawl space. On a cold morning you can sometimes read the stud spacing off a wall with a thermal camera, because the studs are visibly colder.
What to do about it: continuous insulation outside the framing (rigid board under siding, for example), or spray foam applied so it covers the framing faces rather than only filling between them, or simply accepting the derate and knowing your real number. What not to do is compare a cavity R-value on one bid against a whole-assembly R-value on another and think you are comparing like with like. Ask which one you are being quoted.
What depth actually means in practice
R-value per inch stops being an abstraction the moment you ask how many inches will fit. Here is roughly what it takes to reach three common targets, using a mid range value for each material and rounding. Real jobs vary by product.
| Material | To reach R-21 | To reach R-30 | To reach R-49 |
|---|---|---|---|
| Closed-cell spray foam | About 3.5 in | About 4.5 in | About 7.5 in |
| Open-cell spray foam | About 5.5 in | About 8 in | About 13 in |
| Fiberglass batts | About 6.5 in | About 9 in | About 15 in |
| Blown cellulose | About 6 in | About 8.5 in | About 14 in |
| Blown fiberglass | About 8.5 in | About 12 in | About 19.5 in |
Now put that against the cavities in an actual Seattle house. A 2×4 wall gives you 3.5 inches of depth. A 2×6 wall gives you 5.5 inches. A 2×8 rafter in a tight cathedral ceiling gives you 7.25 inches, and if the roof is vented you have to give some of that back as an air channel above the insulation.
So in a 2×4 wall, closed-cell gets you to roughly R-22 to R-24 and there is no version of fiberglass or open-cell that will do the same, because the inches simply are not there. That constraint drives a lot of our work. It is the reason closed-cell is worth its premium in a shallow cavity and the reason it is often a waste of money in an open attic where depth is free. If you have unlimited depth, buy the cheaper R. If you have 3.5 inches, buy the denser R.
Attics are the opposite situation. Nothing is stopping you from putting 14 inches of blown material over a well-sealed attic floor, and that is frequently the smartest money in the whole house. Read spray foam versus fiberglass batts in an attic for how we decide, and how much spray foam insulation do I need for the depth math on a specific job.
Code and recommended levels in climate zone 4C
Seattle and the Puget Sound sit in climate zone 4C, marine. Mild, damp, few extreme days, a long grey heating season. Recommended attic, wall, floor, and crawl space levels are set by climate zone, so what is appropriate here is not what is appropriate in Minnesota or Phoenix.
We are deliberately not going to print a table of Washington State Energy Code R-value requirements on this page and tell you it is current. Energy code gets amended, jurisdictions adopt amendments on their own schedule, and the requirement that applies to your project depends on the assembly and on the scope of work. A full gut remodel, an addition, and a homeowner voluntarily topping up an existing attic are three different regulatory situations, and only some of them trigger a code minimum at all.
If you need to know the current requirement for your project, confirm it with your local building department before you sign anything. We confirm it per job as part of the estimate, in writing, for the specific assembly we are proposing to work on. If someone quotes you a code number off the top of their head, ask them where it came from.
One more thing worth saying about this climate specifically. Zone 4C has a mild winter and a dry summer, which means the payback on raw R-value here is slower than it is in a cold climate, while the payback on stopping air movement and controlling moisture is very good. That is not a sales position, it is the reason our advice looks different from advice written for the Midwest.
How we actually arrive at a number for your house
Find where the air is going
Attic hatch, top plates, chases, can lights, rim joists. This usually costs less than adding depth and does more.
Measure the cavity
What is the actual depth, what is already in it, and is it wet, settled, or chewed up.
Pick material by constraint
Shallow or damp assembly gets closed-cell. Open attic with free depth gets the cheaper R per dollar.
Write down the real number
Which R-value you are being quoted, cavity or assembly, at what depth, in what product.
The R-value is one input, not the answer. The right spec comes from looking at the actual house: the depth you have, the air leaks you have, the moisture you have, and what you are actually trying to fix. We will tell you when a few hundred dollars of air sealing solves your cold bedroom and you do not need us to spray anything.
Common questions about R-value
Is a higher R-value always better?
No. R-value only measures resistance to conductive heat flow, so a higher number is better only if conduction is your actual problem and only if the material is installed well enough to deliver it. A leaky attic insulated to R-38 will lose more heat than a well-sealed attic at R-30, because air moving through insulation carries heat straight past it. There is also a point of diminishing returns: going from R-0 to R-20 is a large improvement, and going from R-40 to R-60 is a small one for a similar amount of money.
What R-value do I need for an attic in Seattle?
It depends on the assembly, on what is already up there, and on whether your project triggers an energy code requirement. Seattle is in climate zone 4C marine, and recommended levels vary by zone and by the part of the house. Rather than quote you a number we cannot stand behind, we confirm the current requirement with the local building department for the specific job and put it in the estimate. What we can say generally is that in an accessible vented attic, air sealing first and then adding depth is usually the best value in the house.
Why does my R-30 batt insulation not feel like it is working?
Three usual suspects. One, air is moving through or around it, which the R-value never accounted for. Two, it is compressed, gapped, or cut short around wiring and framing, so the installed performance is well below the label. Three, thermal bridging through the joists is dragging the whole assembly down. A batt lying loosely over joists with daylight visible at the edges is not an R-30 assembly, whatever the packaging says.
What is the difference between cavity R-value and whole-assembly R-value?
Cavity R-value is the rating of the insulation between the framing. Whole-assembly R-value averages in the framing itself, the sheathing, the drywall, and the air films, which is closer to how the wall actually performs. Because wood conducts heat far better than insulation, the assembly number is meaningfully lower than the cavity number on a typical framed wall. When you compare two bids, check which number each one is quoting, because they are not the same thing.
Does spray foam have a higher R-value than fiberglass?
Per inch, closed-cell clearly does, at about R-6 to R-7 against roughly R-3.1 to R-3.8 for batts. Open-cell at about R-3.6 to R-4 per inch is in the same neighbourhood as batts on conduction alone. The real advantage of both foams is that they also air seal, which fiberglass does not, and that matters more in most houses than the per inch difference. Where foam wins decisively is a shallow cavity where you cannot fit enough of anything else.
Does insulation lose R-value over time?
Some does. Loose fill can settle and lose depth, which loses R-value proportionally. Materials that get wet lose performance while they are wet and may not fully recover. Some rigid boards drift down from their initial rating as the blowing agent ages. Foam that has cured properly and stays dry holds its value well. If your insulation is decades old, wet, flattened, or rodent damaged, the honest first step is looking at what is actually up there rather than adding more on top of it.