# At Sixty-Five Percent, Your PVAc Goes Soft

Hewn Shop Notes · https://hewn.build/shop-notes/at-sixty-five-percent-your-pvac-goes-soft

AI-drafted, not yet checked: Drafted by Chip, the writing system Hewn runs on Claude, from Hewn's craft corpus. Jonathan Salomons has not read this note yet, and no outside reader has checked it. Read it as a shop note: check a number before you cut to it.

Cite as: "At Sixty-Five Percent, Your PVAc Goes Soft." Hewn Shop Notes, hewn.build/shop-notes/at-sixty-five-percent-your-pvac-goes-soft. Drafted 2026-06-16; AI-drafted, not yet checked.

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It's July. The humidity gauge on the wall reads 68%. You have a case piece to glue up — cherry, dovetailed corners, six drawers — and you reach for the yellow glue because it's on the shelf, the joints are already cut, and Titebond works. You've been using it for years.

Three years later the customer calls. The drawers are loose. Not broken. Just loose.

That's not a bad glue joint. That's a material property you didn't know about.

PVAc is not a solid adhesive above 65% relative humidity. It is a rubber.

PVAc — polyvinyl acetate, what's in Titebond Original and Titebond II — has a glass transition temperature: the temperature below which it is rigid and glassy, and above which it flows. In dry conditions, that transition sits around 40°C. Well above room temperature. The adhesive is glassy, rigid, does what it's supposed to do.

Moisture changes this. Water molecules enter the polymer film and disrupt the interactions between chains — the same chemistry that keeps the material rigid. The Fox equation, which polymer chemists use to predict this transition in polymer blends, puts the plasticized glass transition of standard PVAc at about 17 to 24°C when the film has absorbed 3 to 5% water. That's what PVAc absorbs at 65% relative humidity. Room temperature is 23°C.

At 65% RH in a warm shop, your glue is at or above its glass transition. The material is in the rubbery state.

Rubbery does not mean broken. The joint does not pop. What it does is creep: slow, incremental flow under sustained or repeated load. The dovetail that slid crisply in March is slightly loose by September. By year three it's notably loose. The piece isn't failing — it's walking. And the only thing that changed is the season.

Hide glue does not do this. Its structural matrix is a protein gel, not a thermoplastic polymer. The moisture-dependent transition for hide glue sits around 75% RH, well above the range of any climate-controlled interior. At the humidity levels common in residential furniture — 30 to 70% RH — hide glue stays structurally glassy. It does not creep. A fair number of the antique pieces that are still tight were glued with hide glue. That's part of the reason.

If hide glue isn't in your workflow, the correct PVAc for humid climates is not "waterproof" PVAc. "Waterproof" on the label usually means Type II water resistance — it means the adhesive passed the D2 wet shear test, not that it resists creep under sustained humidity. What you need is D3 NMA-crosslinked PVAc or D4 EPI (emulsion polymer isocyanate). NMA crosslinking forms covalent bonds between polymer chains — a permanent network that can't flow even when the adhesive is in the rubbery state. EPI does the same thing differently. Both hold under humid load.

A standard D2 or D3 VAE product does not.

How to know which you have: look for "NMA crosslinked" or "EPI" in the product datasheet. Most manufacturers don't put it on the container. If you can't find the chemistry in the documentation, treat it as unknown and specify accordingly.

If you're building case furniture in coastal British Columbia, the Pacific Northwest, Ontario in summer, or any kitchen, Titebond II is the wrong glue. Not because it's a bad product. Because it's a rubber in your shop.