Free software · thermal bridges and moisture

BPA Junction

Thermal bridges for Passivhaus, from drawing to report, in a weekend in an afternoon. Free, for Windows and Mac.

Why it exists

Every Passivhaus project has its thermal bridges: model each junction, take off the flanking walls, convert to external dimensions, check the surface temperature, write it up, type it into the PHPP. Then do it again when the detail changes.

BPA Junction does the repetitive part, shows its working, and leaves the judgement where it belongs: with a competent professional who checks every result.

Built by a Certified Passivhaus Designer who has written one too many a great many thermal bridge reports.

A thermal bridge report rebuilt in Excel for every jobψ, fRsi and installation ψ straight from your THERM files and PHPP
External dimensions converted with a calculatorThe PHPP conversion done for you, and ψ sent back into the PHPP
Redrawing a CAD detail line by lineImport a DXF, a CAD PDF or a THERM file; trace a screenshot
“Is that leg long enough?”Model checks that say what is off and how to fix it
Mould risk as a hunchHour-by-hour heat and moisture, with the mould index on a four-colour scale
“Would a ventilated cavity help?”Suggest a cavity: each façade, 20–45 mm, from the site’s own wind

What it looks like

Slab edge with the ground: heat finding its way round the footing

Barge: the roof meeting a 170 mm timber-frame wall

Eave on a 170 mm timber-frame wall: the whole junction, cut to cut

Temperatures through each junction, cold (violet) to warm (red), as solved by the Junction solver. The badge shows which way is down.

What it does

Reports from THERM

Reads your PHPP and THERM 7.8 files and writes the thermal bridge report: ψ, fRsi against the PHPP limit, installation ψ for windows.

The Junction solver

Draw the section or bring it in (DXF, PDF, picture, THERM), paint materials, set boundaries and solve. Every step of the working is shown.

Moisture check

Transient heat and moisture (not Glaser): a surface check in seconds, each build-up in 1D, the whole junction in 2D, all in the background.

Projects without a PHPP

Start from a weather file and a few questions; link the PHPP when it arrives.

THERM packages

Send a drawing through THERM 7.8 and see THERM’s results beside BPA Junction’s, detail by detail.

Back into the PHPP

ψ values and installation ψ written into the PHPP (after a backup), or copied ready to paste.

Where it’s up to

BPA Junction is in use on real projects. The Junction solver has been checked against hand calculations and against THERM 7.8: on 69 sample details solved by both, the heat flow agrees within a median 0.42 %, and the coldest interior surface within a median 0.02 K. The moisture model passes the EN 15026:2023 Annex B benchmark.

Validation against EN ISO 10211 and EN ISO 10077-2, which Passivhaus certification needs, is in progress. Until it is done, reports from the Junction solver say so: for certification, confirm the result in a validated tool such as THERM.

A calculation aid, not a substitute for judgement: a competent professional must check every result.

Measuring airtightness...