How Does Wood Swell and Shrink With Humidity?

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How does wood swell and shrink with humidity? The change happens inside the cell wall, at a scale far smaller than a board. Two research articles on spruce describe the mechanism in detail. One study reports that the “swelling/shrinkage strains are highly anisotropic” in the cell wall. The other studied “the moisture-induced changes in cellulose microfibril bundles of softwood secondary cell walls”. This article explains what each study measured, what it found, and where the evidence stops.

Quick answer

• Water moves between the fine fibers of the cell wall. During drying, the study describes water leaving “the space between the microfibrils”.
• The change is directional. One study found the strains are “highly anisotropic in the transverse plane of the cell wall”.
• Fiber packing changes dominate at higher moisture levels: “changes in the packing of microfibrils dominate at moisture contents above 10–15%”.
• Across a whole sample, one study measured “the sample width decreased by 7% between MCs 29% and 4%”.

How we chose

Both sources are primary research articles rather than retail pages or buying guides. The first study used “synchrotron radiation-based phase contrast X-ray tomographic nanoscopy” and focused on “the S2 layer of the latewood cell walls of Norway spruce softwood”. The second study took a different route: “By combining X-ray scattering experiments with molecular simulations” it compared models against measurements. Each source is cited only for the claims that a verbatim quote supports, and the sections below keep the two studies separate so readers can see which one says what.

The cell wall is the moving part

The first study names the layer it examined. It describes the “hygroscopic swelling and shrinkage of the central and the thickest secondary cell wall layer of wood (named S2)”. The authors describe that layer as “a natural fibre-reinforced nano-composite polymer and is strongly reactive to water”. That wording places the response to humidity in the wall material itself, not in the surface of the board.

To isolate the layer, the study cut “micropillars with a cross section of few micrometres” from it. The paper explains the reason for removing the rest: “The thin neighbouring cell wall layers are removed to prevent hindering or restraining of moisture-induced deformation during swelling or shrinkage”. In other words, the neighboring layers would otherwise limit how much the sample could move, so the researchers took them away before measuring.

The study also places the layer in the tissue it came from. The micropillars were “fabricated from the S2 layer of the latewood cell walls of Norway spruce softwood”. Latewood is the part of a growth ring the study examined, and the results describe that material rather than wood in general.

Water between the fibers

The second study gives the basic motion in one sentence: “the distance between microfibrils increases and the microfibril bundles swell with increasing moisture content and vice versa while drying”. The phrase “vice versa” means the reverse happens as the wood dries, so the spacing shrinks again.

The fibers involved are very small. The paper states that “microfibrils with a diameter of 2–3 nm are a central component of the secondary cell wall structure”. Because the scale is so fine, the study works with scattering measurements and simulations rather than with a visual inspection of the board. The experiments used “never-dried spruce wood (Picea abies) in controlled moisture conditions”, so the findings describe that spruce under set conditions.

During drying, the study describes water leaving the gaps between fibers: the microfibril spacing decreases “as water withdraws from the space between the microfibrils”. The authors checked their model against the measurements. In their words, “The molecular models successfully reproduced the experimentally observed linear swelling of the microfibril bundles above a few per cent MC”. Earlier simulation work had already described stages of this process: “Simulation studies addressing plant cell wall nanostructure have identified different stages in the moisture-induced swelling of wood cell walls”.

Why the change is not equal in every direction

The first study reports that the expansion depends on direction. It states that “the swelling/shrinkage strains are highly anisotropic in the transverse plane of the cell wall, larger in the normal than in the direction parallel to the cell wall’s thickness”. Anisotropic here means the amount of change depends on which way the material is measured.

The study proposes a possible cause for this pattern. According to the paper, “This ultrastructural anisotropy may be due to the concentric lamellation of the cellulose microfibrils as the role of the cellulose microfibril angle in the transverse swelling anisotropy is negligible”. The wording “may be due to” marks this as a proposed explanation, not a settled result.

The same paper connects this structure to the material’s stability. It concludes that “The hierarchical configuration in wood optimally increases its dimensional stability in response to a humid environment with higher scales of complexity”. The study presents the layered arrangement as a way the wood manages moisture changes, rather than as a flaw.

Moisture level matters

The second study separates two kinds of change by moisture level. Its finding is that “changes in the packing of microfibrils dominate at moisture contents above 10–15%, whereas deformations in cellulose crystallites take place closer to the dry state”. In practical terms, the packing of the fibers is the main factor across the higher moisture range, while the crystalline part of the cellulose responds mostly near the dry end.

The study also says that the spacing between fibers accounts for much of the size change. “This (de)swelling of the microfibril bundles is responsible for a large part of the dimensional changes of the cell wall”. The paper adds that “Fibrillar aggregation is a significant source of drying-related changes in the interior of the microfibrils”, which points to the way fibers cluster together as they dry.

How small changes add up across a board

The change in the cell wall material is not the same as the change in a whole piece of wood. The first study notes that “The volumetric swelling of the cell wall material is found to be substantially larger than the one of wood tissues within the growth ring and wood samples made of several growth rings”. Wood tissue, growth rings and full samples all restrain the cell wall, so the overall movement is smaller than the wall-level movement.

The second study gives a sample-scale number. “At the macroscopic scale, the sample width decreased by 7% between MCs 29% and 4%”. The paper uses “The moisture content (MC)” as its abbreviation. The study compares this with the nanoscale result: the sample change is “a slightly smaller change than the shrinkage of 10% in the microfibril bundles detected by SAXS over the same MC range”. The SAXS abbreviation refers to the scattering method, since the paper uses “small-angle X-ray and neutron scattering (SAXS, SANS)”.

The gap between the two numbers has an explanation in the paper: “This difference is explained by other cell wall layers and different parts of the wood tissue restraining the macroscale swelling behavior”. The study does not measure how much of the restraint each layer provides, so the gap is described rather than quantified.

What we could not verify

This section lists what the two studies do not establish for everyday wood use. The passages used in this article describe a research setting, and several questions a reader might bring to the topic fall outside it.

What the sources do not say

The studies describe spruce under controlled conditions, and the quoted passages do not give a target moisture level for any board used in a house, a deck or a garden. They also do not give a rate of change for a board that moves between wet and dry weather. The second study says plainly that “the exact mechanism behind the observed changes is still unclear”, which means the nanoscale picture is still being refined.

The first study works on “micropillars” cut from one layer, and the passages used here do not show how that result carries over to a whole board with paint, stain or sealer on its surface. The quoted material also does not address lumber storage, kiln drying or any treatment. For a different question about garden beds, readers can compare the soil side in compost for raised garden beds and DIY raised bed soil mix, which do not address wood movement.

Frequently asked questions

Why does wood change size with humidity?

The second study ties the change to fiber spacing: “the distance between microfibrils increases and the microfibril bundles swell with increasing moisture content”. Drying reverses the process, with water leaving “the space between the microfibrils”.

Does wood change size equally in every direction?

No. The first study states that “the swelling/shrinkage strains are highly anisotropic in the transverse plane of the cell wall”, and that they are “larger in the normal than in the direction parallel to the cell wall’s thickness”.

Which moisture range matters most for fiber packing?

According to the second study, “changes in the packing of microfibrils dominate at moisture contents above 10–15%”. Below that range, the study says deformations in cellulose crystallites “take place closer to the dry state”.

How much did a sample shrink in the second study?

The study reports that “the sample width decreased by 7% between MCs 29% and 4%”. This is one sample in one study, so it should not be read as a general figure for all boards.

Do these studies explain how to store or treat lumber?

No. The quoted passages cover cell wall structure and microfibril bundles in spruce, not stored lumber, finishes or treatment. Readers who need guidance on those topics will need other sources.

Sources

• Hygroscopic swelling and shrinkage of latewood cell wall micropillars reveal ultrastructural anisotropy. PubMed Central, https://pmc.ncbi.nlm.nih.gov/articles/PMC4006252/ (HTTP 200, opened 2026-10-28)
• Nanoscale Mechanism of Moisture-Induced Swelling in Wood Microfibril Bundles. PubMed Central, https://pmc.ncbi.nlm.nih.gov/articles/PMC9284609/ (HTTP 200, opened 2026-10-28)