Harvia WHM25 Chimney Vapor Barrier

The Harvia WHM25 vapor barrier seals the ceiling penetration where a Harvia steel chimney passes out of the sauna room. It closes the gap in the sauna’s vapor barrier layer around the chimney lead-through, preventing warm, moist sauna air from migrating into the insulation and roof structure, protecting the building and reducing heat loss.

Features

• Durable seal that stops moisture infiltration around the chimney lead-through
• Reduces heat loss and improves energy efficiency
• 19.7″ × 19.7″ (50 × 19.7 in) plate with 8.1″ (8.3 in) inner opening
• Sized for the 8.7 in Harvia WHP insulated chimney casing
• Limited 2-year warranty (consumer use); limited 1-year warranty (commercial use)

What’s Included

• Vapor barrier plate
• Installation instructions

Specifications

Compatibility

Designed for Harvia WHP steel chimneys such as the WHP1500 / WHP1500M kit. Install at the sauna ceiling lead-through together with the lead-through flange, insulation, and protector supplied with your chimney kit.

Planning note: Harvia’s WHP steel chimney system installs vertically only and is designed for wood-burning sauna stoves in standalone outdoor sauna buildings. Keep at least 4″ (4 in) of clearance between the insulated outer casing and any combustible material, and confirm your installation with your local building authority before you build.

Good to Know

Why does a sauna chimney need a vapor barrier?

Sauna air is hot and humid. Without a sealed vapor barrier around the chimney lead-through, that moisture condenses inside the ceiling insulation and roof cavity, which can cause mold and rot over time. The WHM25 restores the vapor-tight layer around the penetration.

Does it touch the hot chimney?

The plate’s 8.1″ opening fits around the insulated 8.7 in outer casing, the cool-touch layer of the chimney, not the hot inner smoke pipe. Always maintain Harvia’s required clearances and use the lead-through insulation and protector.

Is it included with the WHP1500 chimney kit?

No. Harvia sells the WHM25 separately, the chimney kit includes the lead-through flange and insulation, but the vapor-barrier plate is an add-on.

Designing a Harvia Chimney System

Every part on this page belongs to Harvia’s modular WHP steel chimney system: a 4.6 in (4.5″) smoke path inside a 8.7 in (8.6″) insulated casing, assembled with slot-and-lock joints. Here is how a complete system comes together:

1. Start with the chimney. For pitched roofs, the WHP1500 kit (59.1 in chimney plus rain cap, tilted rain flange, smoke pipe, and ceiling lead-through set) is the core of the system. For flat roofs and barrel saunas, the WHP1000500 package bundles the chimney with the symmetric roof kit.
2. Connect the stove. Bridge the stove collar to the insulated chimney with 4.6 in WZ smoke pipes (7.9 in, 19.7 in, or 3.3 ft). Add the WZ115030SP damper plate at the stove to control draft while firing, a WZ115120 reducer where pipe sections step down, or the WZ115155 adapter if you are transitioning to 6″ US Class A pipe.
3. Reach your height. Add WHP500 (19.7 in) or WHP1000 (39.4 in) extensions. Stay within Harvia’s limits: 16.4 ft total chimney height, no more than 6.5 ft above the last support, lateral supports every 9.8 ft.
4. Go through the roof. Choose the roof kit that matches your roofline: WHP-AS for tilted/pitched roofs, WHP-ASSYM for flat roofs and barrels. Replacement rain collars: WHM22 (tilted) and WHM24 (symmetric). Add a snow barrier above the chimney where sliding snow can load the flashing.
5. Seal and control. The WHM25 vapor barrier restores the vapor-tight layer where the chimney passes the sauna ceiling; the WHP270 insulated damper closes the flue after the fire is fully out so the sauna holds its heat.
6. Finish the stove side. Stone-filled covers (WL300 for Legend stoves, WL300PC for Cilindro) absorb radiant heat from the smoke pipe and significantly reduce its required safety distances.

Key clearances and rules: vertical installation only; 4 in (4″) from the insulated casing to combustibles; 19.7 in (stainless) or 14.8 in (black) from non-insulated smoke pipe unless shielded; designed for wood-burning stoves in standalone outdoor sauna buildings. Confirm your plan with your local building authority, and if you would like help speccing the system, talk to us; we design these daily.

Shop the Complete Harvia Chimney System

1. Start with the chimney

• WHP1500 Chimney Kit, 59.1 in (stainless or black), the system core
• WHP1000500 Complete Chimney Package for flat roofs & barrel saunas

2. Connect the stove

• WZ Smoke Pipes, 4.6 in (7.9 in / 19.7 in / 3.3 ft, stainless or black)
• WZ115030SP Stove-Side Damper Plate (draft control while firing)
• WZ115120 Reducing Pipe (joins mismatched pipe sections)
• WZ115155 Adapter, 4.5″ Euro to 6″ USA (hybrid Class A transitions)

3. Reach your height

• WHP500 Chimney Extension, 19.7 in
• WHP1000 Chimney Extension, 39.4 in

4. Go through the roof

• WHP-AS Roof Accessory Kit for tilted / pitched roofs
• WHP-ASSYM Roof Accessory Kit for flat roofs & barrel saunas
• WHM22 Replacement Rain Collar, tilted roofs
• WHM24 Replacement Rain Collar, flat roofs & barrels

5. Seal and control

• WHM25 Chimney Vapor Barrier (seals the ceiling penetration) (this page)
• WHP270 Insulated Smoke Damper (retains heat after firing)

6. Finish the stove side

• WL300 Smoke Pipe Cover for Legend stoves (cuts clearances)
• WL300PC Chimney Cover for Cilindro stoves (cuts clearances)