DIY PCM Encapsulation for Secondary Glazing — Containers, Sealing, Failure Modes

The translucent-PCM glazing article established that the realistic retrofit for an existing window is a sealed PCM panel added as secondary glazing behind the sash — and that containment, not the PCM, is the hard part. This reference is the how-to layer: which container, which encapsulation scale, how to seal it, and the failure modes to design around. It is compiled from PCM-plastics compatibility tests, salt-hydrate stabilization chemistry, thermal-cycling durability data, a full-scale fill-and-seal glazing prototype, and DIY/maker practice.

Scope note: this is materials/fabrication guidance synthesized from the literature and maker sources — not a validated build. No fully documented, measured amateur translucent-PCM window build was found; that is a genuine gap this project could fill.

Step 0 — pick the PCM first (it decides everything downstream)

Two DIY-viable chemistries, and the choice cascades into container, sealant, and failure modes:

Comfort-band paraffin (Rubitherm RT21HC–RT25HC, ~21–25 °C) — cheap, non-corrosive, no supercooling, but thermotropic: clear when molten, milky-white when frozen (which doubles as solar shading). Favor RT22-type: 700-cycle data shows solidification-side latent-heat loss of only ~5% for RT22HC vs 23% for RT25HC. Paraffin swells polyolefins, so container/sealant choice matters.
Salt hydrate (CaCl₂·6H₂O ~26–30 °C, or Na₂SO₄·10H₂O “Glauber’s salt”) — translucent in both phases (better for a see-through panel), higher volumetric latent heat, but needs additive stabilization and corrodes metals. This is the chemistry GlassX uses.

Goia’s full-scale prototype (a 15 mm glazed cavity simply filled with RT35 paraffin, measured over 6 months) is the credible proof that “fill a sealed glazed cavity” works — just pick a comfort-range grade, not RT35.

Container — the translucent envelope

Option	Clarity	Paraffin	Salt hydrate	DIY verdict
Twin-wall polycarbonate	translucent (clear/opal)	resists paraffin oil	resists sat. CaCl₂/Na₂SO₄	Best all-round — ready-made channels, greenhouse-cheap, seal the ends yourself
Acrylic / PMMA box	clearest	OK if clean wax	OK	Best optics, but crazes under stress + solvent-cut PCM — keep stress-free
Glass IGU cavity	max clarity, inert	✓	✓	Not hobbyist-fabricable (needs sealed-unit making)
PVC-U	not clear	<0.1% mass change	<0.1%	Most inert but opaque — only for hidden trays
LDPE/PP film pouch	translucent	swells 6–7%	mild (≤1.8%)	Cheap/easy seal, but paraffin permeates thin film — use for salt hydrate, not paraffin
PET bottle/tube	clear	oil-attacked (C)	ok	Zero-cost tube arrays; watch oily paraffin grades

Headline pairing: twin-wall polycarbonate is the DIY default — it resists both paraffin oil and saturated CaCl₂/Na₂SO₄. The one caveat: additives/plasticizers in solvent-cut paraffin oils can migrate into PC and craze it under stress, so use a clean wax-grade PCM. Acrylic wins on clarity but only if the panel is kept free of assembly stress (no over-tight clamps), because daily melt-expansion cycling adds stress that triggers crazing with any trace solvent.

Encapsulation scale — free-liquid vs shape-stabilized

The core tension: staying translucent vs not leaking.

Free-liquid macro-fill (pour liquid PCM into a sealed channel/box) — easiest DIY, best clarity, but leak-prone and, for salt hydrates, prone to phase separation. This is Goia’s and the maker-forum approach: seal both ends of multiwall polycarbonate, fill each channel ~90%.
Shape-stabilized / gelled — PCM held in a matrix so it never flows even when melted. Here the chemistry splits sharply on clarity:
- CMC-gelled salt hydrate = the DIY translucent sweet spot. Glauber’s salt thickened with ~11 wt% low-MW carboxymethyl cellulose forms a 3D gel that pins salt+water together — 130 J/g retained, zero supercooling, stays translucent, no free liquid to leak. Recipe: heat stoichiometric water to 50 °C, stir in CMC at 800 rpm 30 min, add salts sequentially, cool, refrigerate overnight.
- Silica-/graphite-stabilized paraffin = disqualified. Fumed silica (~9 wt%) stops leaks but composites go fully opaque and lose 25–50% of latent heat (a non-melting pore-wall layer stores nothing). Fine for a hidden thermal-mass panel, useless for a window.
Buy pre-made macro-encapsulated pouches/panels (e.g. HDPE FlatICE) — no free-liquid handling, but HDPE is translucent-to-opaque, not clear.

Rule of thumb: for a see-through leak-free panel, gel a salt hydrate (CMC). For a self-dimming panel where the milky solid state is acceptable, free-liquid paraffin in sealed polycarbonate is simplest.

Sealing

Standard clear acetoxy RTV silicone swells and de-bonds in molten paraffin. Use a mechanically clamped gasket or an oil-resistant / fluorosilicone RTV instead. (Salt hydrates don’t attack silicone, so ordinary silicone is fine on a salt-hydrate panel — the swelling problem is specific to paraffin.)
Rigid PC/acrylic: solvent or ultrasonic welding gives a monolithic seam; heat/impulse sealing works for PE-film pouches (but the film absorbs paraffin).
Always leave an expansion void (see below) — never solvent-weld a 100%-full cell.
Vapor barrier, not just a lid: salt hydrates lose bound water through permeable coatings, dropping capacity — the seal must block water-vapor permeation, not only liquid. Target a ≥5,000-cycle (~14 yr) durability bar.

Failure modes to design around

Failure	Numbers	Mitigation
Leakage (the dominant one)	up to 57% capacity loss via PCM escaping the coating	Sealed rigid cell, vapor-tight seams; must be leak-proof at peak summer temp (liquid phase)
Volume expansion	paraffin ~10–15%, salt hydrates ~10% on melting	Leave ~15–35% headspace / flexible face; never fill 100%
Supercooling (salt hydrates)	borax 4 wt% cuts CaCl₂·6H₂O supercooling 14 °C → 4 °C; SrCl₂·6H₂O → ~2 °C	Add a nucleator; don’t overheat >40 °C (kills nucleation centers → permanent supercooling)
Phase segregation (salt hydrates)	anhydrous salt settles each cycle, progressively killing capacity	Thickener — sodium polyacrylate or the CMC gel above
Corrosion (salt hydrates)	stainless 316/304 safe (0–1 mg/cm²·yr); aluminium corrodes, copper caution	Keep salt hydrate off bare metal — plastic/glass walls, no metal fins
UV yellowing	sealed paraffin showed no yellowing at 700 cycles; but a clear IGU passes ~50–70% of UVA	Seal out air/O₂ + add a UV-filtering/UV-stabilized outer layer for a sun-facing panel
Water-vapor loss (salt hydrates)	bound-water loss drops energy density	True vapor-barrier seal (above)
Condensation/fogging	trapped moisture in the sealed air gap	Dry fill, desiccant in the headspace, vapor-tight seal

The standard salt-hydrate additive stack: melting-point adjuster (KCl) + nucleator (borax ~4 wt%) + thickener (sodium polyacrylate or CMC). All cheap, DIY-grade.

Recommended DIY build paths

Always-clear panel → CMC-gelled Glauber’s salt in a twin-wall polycarbonate sheet, ends clamped with gasketed caps, ~20% headspace, borax nucleator, non-metal throughout. Translucent both phases, leak-resistant, ~130 J/g.
Self-dimming panel → free-liquid RT22HC paraffin in sealed twin-wall polycarbonate, channels filled ~90%, oil-resistant sealant or clamped gasket, UV-filter film on the sun side. Clear when cool, milky when hot (passive shading); simplest fabrication.
Avoid: silica/graphite-stabilized paraffin (opaque), thin LDPE/PP film with paraffin (swells/permeates), and any bare aluminium/steel/copper touching a salt hydrate.