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Retrofit: Importance of considering moisture risk with internal wall insulation (IWI), on solid walls!

Trainer Insights - Retrofit Moisture Risk

<h4>Coaction trainer <a href="https://coaction.org.uk/people/eric_fewster">Eric Fewster</a> shares his personal experience on the challenges of considering moisture risk when retrofitting old buildings…</h4>
Internal wall insulation (IWI) is often required in retrofit, but it’s not just a case of considering u-values and heat
loss!
With old buildings, we also need to consider the moisture risk, as when you add more insulation on the inside, the original wall gets colder and the wall's moisture content tends to increase – adding more insulation, or insulation of the wrong type, means at some point there will be too much moisture and the wall can’t dry out as fast as it gets wet, giving rise to the risk of degrading the existing structure.
An example of this comes from a two-storey solid-walled sandstone house which had many different wall types, most of which were some form of solid sandstone . Some of the existing walls already had 50mm of either XPS or EPS insulation internally, sometimes together with an impermeable membrane – coring out some of the existing XPS insulation from a west-facing wall revealed humid conditions behind (Photo 2).
<img src="/uploads/3._ef_-_moisture_risk_blog_-_2025.jpg" alt="External photograph of a two-storey solid-walled sandstone house and core drilling" title="Core drilling solid wall @Eric Fewster">
Image: Eric Fewster - Core drilling solid wall
<img src="/uploads/1._ef_-_moistire_risk_blog_2025.jpg" alt="Core drilled hole to check moisture readings in external wall behind IWI and impermeable membrane" title="Moisture reading of wall behind IWI @Eric Fewster ">
Image: Eric Fewster - Moisture reading of wall behind IWI
While the project team were only considering the use of vapour-open materials in order to avoid or
exacerbate the moisture problem, it was not clear how much insulation could be applied, and what the airtightness strategy should be. As the house was in a fairly exposed location, it was felt that some WUFI hygrothermal modelling was needed. This involved getting stone samples cored out (Photo 1) from different walls and checking their absorptivity before running the analysis.
The WUFI model (Photo 3) helped to firm up the solution which was kept the same for all of the solid walls - a layer of lime plaster (to dub out the uneven walls), then 15mm of Diathonite, followed by 80mm wood fibre, with 10mm of lime plaster as the airtightness layer. This gave a U-value range of 0.31 – 0.34 W/m2K for most walls.
<img src="/uploads/wall_a1_-_wufi_graph.png" alt="WUFI Graph Relative humidity / Intervention - Time" title="WUFI Graph @Eric Fewster">
Image: Eric Fewster - WUFI Graph
You can read more about this phased retrofit project <a href="https://coldproof.co.uk/case-study/phased-retrofit-of-an-1800s-sandstone-detached-house-near-preston/">HERE.</a>
Coaction’s <a href="https://coaction.org.uk/courses/retrofit">RETROFIT DELIVERY</a> course covers the reality of tackling the practical challenges thrown up by retrofit
projects, approaches to minimise risk and reduce unintended consequences.

This is a comprehensive list of current funding opportunities for our courses.

Current Funding Opportunities

There are 2 funding opportunity currently available that can be applied to Coaction training courses.
We update this page when new grant schemes become available but if you know of any grant funding that we haven't listed, please send an email to support@coaction.org.uk telling us all about it!
<a href="https://coaction.org.uk/courses/contractorcornwall"></a><a href="https://coaction.org.uk/courses/contractorcornwall">__________________________________________________________________________________________________________________</a>
<h4>1. Wales Flexible Skills Programme Funding - Ends March 2026</h4>
50% grant for developing Net Zero skills in the construction and retrofitting of residential buildings.
To be eligible your business must be based in Wales, solvent and you must commit to release staff to undertake the requested training by the end of March 2025.
This funding can be applied to the following courses offered by Coaction Training CIC:
<ol>
<li>Introductory Level: <a href="https://coaction.org.uk/courses/bitesize">PASSIVHAUS LOW ENERGY DESIGN</a> for design teams.</li>
<li>Intermediate level:  <a href="https://coaction.org.uk/courses/tradesperson">CERTIFIED PASSIVHAUS TRADESPERSON</a> for contract managers</li>
<li>Intermediate level:  <a href="https://coaction.org.uk/courses/designer">CERTIFIED PASSIVHAUS DESIGNER </a>for professionals who will be designing, consulting or delivering buildings to the Passivhaus standard</li>
<li>Intermediate level:  <a href="https://coaction.org.uk/courses/practical">PASSIVHAUS IN PRACTICE </a>apply Passivhaus theory hands-on to real-world scenarios. Note this course is in-person at BE-ST, G72 0AH</li>
<li>Intermediate level: Introduction to <a href="https://coaction.org.uk/courses/therm">THERM</a> (16 PHI credit points)</li>
<li>Advanced level:  <a href="https://coaction.org.uk/courses/retrofit">RETROFIT DELIVERY </a>(16 PHI credit points) for designers and contractors working on deep retrofit projects.<a href="https://coaction.org.uk/courses/retrofit"></a></li>
</ol>
Funding is available until the end of March 2026.
More details can be found <a href="https://coaction.org.uk/blog/2024/Welshskillsfunding">HERE</a><a href=""></a><a href="https://coaction.org.uk/blog/2024/CITBfunding"></a><a href="https://coaction.org.uk/blog/2024/CITBfunding"></a>
<a href="">_____________________________________________________________________________________________________________________</a>
<a href=""></a><a href=""></a><a href=""></a>
<h4>2. CITB Local Employer Networks - 70% Funding - Currently no end date</h4>
70% grant for low energy and Passivhaus training courses.
To be eligible you must be a CITB registered business, contractor or subcontractor.
This funding can be applied to the following courses offered by Coaction Training CIC until April 1st 2026:
<ol>
<li>Introductory Level: <a href="https://coaction.org.uk/courses/bitesize">PASSIVHAUS LOW ENERGY DESIGN</a> for design teams.</li>
<li>Intermediate Level: <a href="https://coaction.org.uk/courses/tradesperson">CERTIFIED PASSIVHAUS TRADESPERSON</a> for contractor managers.</li>
<li>Intermediate Level: <a href="https://coaction.org.uk/courses/practical">PASSIVHAUS IN PRACTICE </a>apply Passivhaus theory hands-on to real-world scenarios. (in-person at BE-ST, Scotland G72 0AH)</li>
<li>Advanced Level: <a href="">RETROFIT DELIVERY </a>(16 PHI credit points) for designers and contractors working on deep retrofit projects.</li>
</ol>
CITB funding is applicable for GB regions only
More details can be found <a href="https://coaction.org.uk/blog/2024/CITBfunding">HERE</a>
<a href="https://constructionyork.co.uk/opportunities/"></a>

Simple Guide to Renewing Your Passivhaus Designer or Passivhaus Consultant Qualification

PHI Certification Renewal Guide

Passing the Certified Passivhaus Designer/Consultant exam is a significant achievement, but now that you are certified, it's essential to start planning straightaway how you will accumulate your professional development credits to retain your status, as the qualification is only valid for five years.
Follow our tips below to make the process as smooth as possible and avoid last-minute stress.
Quick Renewal Facts
<ul>
<li>The certificate expires five years from the date of the examination.</li>
<li>You can renew as many times as desired.</li>
<li>All renewal applications go through the <a href="https://cms.passivehouse.com/en/training/certificates/">PASSIVE HOUSE INSTITUTE.</a></li>
<li>Renewing your qualification by retaking the exam is not allowed (this encourages further self-development and learning).</li>
<li>The PHI renewal fee is 450 EUR. iPHA members currently receive a 20% discount</li>
</ul>
There are two routes to accumulate points:
<ul>
<li>Renewal Route 1 - Submit project documentation evidence where you are responsible for the building design and PHPP of a certified Passivhaus project.</li>
<li>Renewal Route 2 - Collect 100 PHI credit points (CP) through webinars, online courses, conferences, or teaching within five years immediately preceding your renewal. Each 45 minutes of learning time equals 1 PHI credit point (CP).</li>
</ul>
Top Tips
Here are a few top tips to quickly boost your points:
1. Record Everything!
You will need to submit the following details for your renewal application. We strongly suggest creating a simple spreadsheet and folder of evidence to keep track of them:
Event Details: Provider, Event name, Start date, Duration, Location (if applicable)
Documentation: Saved screenshot of the event website or certificate of attendance
Course Details: Start and end dates, Short explanation, Additional information on course content (e.g., PHI ID number)
2. Conferences = Credit Points!
You can accumulate up to 36 points in one go just by attending an approved conference. Not only is this big on credit points, but it's great for networking, accessing the latest research, and ensuring you are bang up to date with best practice. You are also guaranteed to bump into a familiar face or two from our cohort of trainers and course participants.
Notable European Passivhaus Events:
<ul>
<li><a href="https://passivehouseconference.org/en/">INTERNATIONAL PASSIVHAUS CONFERENCE</a>: Held most years, this event earns you 36 CP. It’s an excellent networking opportunity to meet the PHI team and other Passivhaus experts from around the world.</li>
<li><a href="https://ukphc.org.uk/">UK PASSIVHAUS ANNUAL CONFERENCE</a>: Typically awards 12-16 CP and is a hotspot for UK Passivhaus experts. Check the Passivhaus Trust website for upcoming details.</li>
<li><a href="https://aecb.net/events/">AECB CONFRERENCE</a>: Held every autumn, this event covers a wide range of sustainable building topics and features renowned experts from across the UK low-energy building sector, awarding 10 CP.</li>
</ul>
For a full list of accredited events, visit <a href="https://cms.passivehouse.com/en/training/directory/designers/accredited-trainings/?">THIS PAGE</a>.
3. Specialise!
Online courses are an accessible way to enhance your knowledge and earn credit points. We recommend the following courses as a natural progression for Designers.
Follow-On Courses for Certified Passivhaus Designers:
<ul>
<li><a href="https://coaction.org.uk/courses/retrofit">RETROFIT DELIVERY COURSE (16 CP)</a></li>
<li><a href="https://coaction.org.uk/courses/therm">THERM SELF STUDY COURSE (16 CP)</a></li>
<li><a href="https://aecb.net/the-aecb-carbonlite-retrofit-course/">RETROFIT FOUNDATION COURSE (35 CP)</a></li>
<li><a href="https://aecb.net/aecb-training-building-energy-modelling-using-phpp/">PHPP COURSE (50 CP)</a></li>
<li><a href="https://passivehouseschool.com/resources/software/designph/">DESIGN PH (12 CP)</a></li>
<li><a href="https://news.passiv.org/phppexperten">PHPP EXPERT (24 CP)</a></li>
<li><a href="https://cms.passivehouse.com/en/training/certificates/site-supervisor/">SITE SUPERVISOR &amp; CONSTRUCTION VERIFIER (40 CP)</a></li>
</ul>
4. Webinars
Webinars are often budget-friendly and come with easy-to-access recordings, allowing you to view the content at your convenience.
Many leading Passivhaus organisations offer free events:
<ul>
<li>International Passive House Association (iPHA) Webinars - 2 CP: If you are a PHT member, your iPHA membership is included, granting you free access to these webinars. You can also earn credit points from recordings if you can’t attend live, so it's worth registering your email anyway. (Note: All iPHA webinars are available on Passipedia, but you can't claim any points from there.)</li>
<li>PHT Masterclasses</li>
<li>Check our website for Coaction webinars.</li>
</ul>
5. Found a Relevant Event but It Doesn’t Earn Points?
Event organisers can fill out <a href="https://docs.google.com/forms/d/e/1FAIpQLScE74_FJ2OIV4xMiW-1uq5dPhzKqp42lFaVhiQg9ScEBVebbA/viewform">THIS FORM</a> to request event accreditation.
Feel free to contact our training team at (<a href="mailto:hello@coaction.org.uk">hello@coaction.org.uk</a>) if you have any further questions.
6. Further Resources:
<ul>
<li><a href="https://cms.passivehouse.com/media/filer_public/f1/4e/f14e2177-314d-4ea3-97e8-940c3e4d7674/appendix_iib_application_designer-renewal_cp_en_201908.pdf">RENEWAL SUBMISSION FORM</a></li>
<li><a href="https://cms.passivehouse.com/media/filer_public/f1/4e/f14e2177-314d-4ea3-97e8-940c3e4d7674/appendix_iib_application_designer-renewal_cp_en_201908.pdf"></a><a href="https://cms.passivehouse.com/en/training/certificates/designer-consultant/">PHI FULL RENEWAL INFO</a></li>
<li><a href="https://cms.passivehouse.com/media/filer_public/68/33/68338111-547a-4978-a7a4-9e481ea50223/exam_regulation_designer_en_de.pdf">PHI EXAMINATION REGULATIONS (SECTION 5: RENEWAL DETAILS, PAGES 18-23)</a></li>
<li><a href="https://cms.passivehouse.com/media/filer_public/79/71/797136b0-bfc8-4fd7-8ac0-918115e2d3c0/appendix_v_fees_designer-consultants_2016.pdf">PHI FEES DOCUMENT</a></li>
<li><a href="https://docs.google.com/forms/d/e/1FAIpQLScE74_FJ2OIV4xMiW-1uq5dPhzKqp42lFaVhiQg9ScEBVebbA/viewform">APPLICATION FOR EVENT CREDIT POINTS (EVENT ORGANISERS)</a><a href="https://cms.passivehouse.com/media/filer_public/f1/4e/f14e2177-314d-4ea3-97e8-940c3e4d7674/appendix_iib_application_designer-renewal_cp_en_201908.pdf"></a></li>
</ul>

Bath University

Cundal

East Ayreshire Council

ECD Architects

Edinburgh Council

Kier

Kingerlee

Royal Borough of Kesington and Chelsea

Southampton City Council

Stride Treglown

Willmott Dixon

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I now feel confident in my ability to assess individual thermal bridges if this is ever needed on a future project.

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The course also deepened my understanding of the impact of thermal bridging more generally.

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It was very inspiring. Great experienced tutors with a deep knowledge to share. The course gives you the tools to take real action on the design and construction stages to achieve energy performing buildings.

Learn from the UK's Passivhaus experts

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Proud to be official partners of the Passivhaus Trust.

Training construction professionals to meet the climate emergency.

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Trainer Insights - Retrofit Moisture Risk

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