Last Updated on June 11, 2025 by Ferg
From Donegal’s Atlantic spray to the damp east-coast drizzle that soaks Dublin’s red-brick terraces, Irish weather has a knack for sneaking indoors. Older block and solid-stone walls cool quickly under those grey skies, inviting two unwelcome guests: condensation and heat loss. Damp corners foster black mould, while draughty rooms drain the bank account every time the boiler fires up.
Homeowners often treat these issues separately—buy a dehumidifier for the wet patch, add another 50 mm of insulation board in the attic for the heat. Yet both problems share a single root: the temperature gradient running through your walls. Master that gradient and you knock out dampness and wasted kilowatt-hours together.
That’s where an idea borrowed from building physics—the isotherm—earns its keep. When you design internal wall insulation around key isotherms, you can:
- keep condensation off internal surfaces,
- lift average room temperature without cranking the thermostat, and
- cut the home’s BER rating by a band or two, unlocking larger SEAI grant payments.
Let’s unpack how it works in language every Irish householder—whether you live in a 1930s semi-d in Rathmines, a Victorian terrace in Limerick, or a granite cottage in Connemara—can follow.
How Dampness & Heat Loss Hit Irish Homes
Persistent maritime moisture
Ireland’s relative humidity rarely drops below 70 % for long. Even when it’s not lashing rain, the air carries enough vapour that interior walls sitting under 13 °C will gather moisture. Solid masonry (and many early cavity walls) land well below that mark on winter nights. Push a wardrobe tight to a gable wall and you’ve built the perfect micro-climate for mould.
The thermal-bridge curse
Concrete ring-beams, steel lintels, and party-wall junctions give heat a handy shortcut to the outside. These bridges pull the internal surface down a few extra degrees. Every Irish painter has seen the tell-tale “shadow line” that returns months after a fresh coat of Dulux Vinyl Matt.
Rising energy bills
SEAI data show that space heating swallows over 60 % of the typical Irish household’s energy spend. When walls leak warmth, occupants respond by nudging the boiler control from 18 °C to 21 °C—exactly where condensation risk peaks if surfaces can’t keep up.
Health knock-ons
Damp, chilly rooms aggravate asthma and arthritis. The HSE recommends 18–20 °C for healthy living spaces; many pre-2000 homes never reach that without double sweaters indoors. A smarter internal wall solution is cheaper, healthier, and greener than relying on electric fan heaters.
The Upside of Tackling Isotherms
Upgrading walls with isotherm insulation has delivered results across pilot projects funded under SEAI’s Deep Retrofit scheme and local authority retrofits:
- Condensation down—kitchen tiles, window reveals, and the cold back bedroom stay dry even on a sleety January morning.
- Comfort up—surface temperatures rise by 3–6 °C, removing that icy “cold-sink” feeling near external walls.
- Lower BER and bills—gas or oil demand drops 15–35 % depending on the original construction, frequently nudging the house from a D-rating to a solid B-rating. A higher BER brings better mortgage rates and resale value.
What Exactly Is an Isotherm?
Think of an isotherm as a contour line on a topographical map—but instead of marking equal height, it marks equal temperature. In a wall cross-section the 15 °C isotherm might hover just behind the plasterboard; the 5 °C isotherm lurks closer to the outer leaf.
Where things get interesting for Irish damp issues is the 9–12 °C band. That’s the typical dew-point range inside a lived-in house (RH ≈ 60 %). If that band touches interior plaster, you’ll see water beads and peeling paint. Shift it outward into the insulation layer and the problem disappears.
Building-science tools (for example, WUFI or the free Therm software used in Part L thermal-bridge calculations) let engineers draw those lines, showing exactly where condensation might prowl. The goal is simple: keep every indoor-facing material above the dew-point for 95 % of the year.
What Is Isotherm Insulation?
Conventional internal wall upgrades in Ireland typically mean 90–120 mm of foil-faced PIR on battens, plus vapour-control tape round every socket—a solid system, but one that robs you of floor area and can trap moisture if the VCL isn’t perfect.
Isotherm insulation (often marketed as thin internal wall insulation) flips the script. Rather than piling on thickness, it combines a high-performance core (phenolic foam or aerogel) with a capillary-active, breathable surface (calcium-silicate or lime-based board). Total build-up: as little as 25 mm.
Because the board itself buffers vapour and keeps the inner face warm, you:
- push the critical dew-point isotherm safely into the panel,
- leave masonry free to dry to the outside, and
- avoid the tight-tolerance VCL taping that leads many Irish DIYers astray.
Anatomy of a typical system
| Layer | Purpose | Typical thickness |
| Hygroscopic mineral topcoat (lime skim or silicate render) | Takes paint, evens moisture spikes | 3 mm |
| Capillary-active board (calcium silicate / aerogel mat) | Buffers humidity, insulates | 15–20 mm |
| High-λ-value foam core | Main thermal break | 5–10 mm |
| Breathable adhesive mortar | Full-surface bond, no air gaps | 3–5 mm |
All told, you reclaim nearly the full room size, dodge expensive alterations to window boards, and still meet the TGD Part L surface-temperature criteria (fRsi ≥ 0.75).
How Isotherm Insulation Prevents Damp
- Raises surface temperature
Field tests in Cork retrofits show internal plaster jumping from 12 °C to 17 °C on a 4 °C external night—enough to stay clear of dew-point even after a steamy shower. - Seals thermal bridges
Continuous adhesive covers masonry joints and concrete angles, flattening isotherms that once dipped around ring-beams and junctions. - Controls vapour without a rigid VCL
The board’s microporous structure soaks and re-evaporates moisture swings, reducing reliance on a polythene sheet that must be stapled perfectly—a known weak spot in many grant-backed dry-lining jobs. - Lets masonry breathe outward
Older rubble-stone or mass-concrete walls in Mayo cottages dry gradually through lime pointing. A vapour-open inner system respects that heritage principle while still meeting modern comfort expectations. - Minimal floor-space loss
A 25 mm layer on three walls of a 3 × 4 m bedroom steals under 0.1 m²—far less than the bulkhead needed for 100 mm PIR. - Faster, cleaner install
Two days to bond and skim a standard Irish semi-d sitting room; you’re painting by the weekend. No stud walls, no shifting sockets 100 mm forward, no cutting skirting boards to fit. - SEAI-grant ready
Because the system improves U-value and surface temperature, it qualifies for the SEAI Dry-Lining Grant (up to €4,500 for detached houses), provided you hit the required post-works BER and airtightness targets. - Backed by hygrothermal modelling
Manufacturers submit EN 15026 calculations demonstrating zero long-term moisture build-up—often a hurdle when bank valuers check paperwork for mortgage switchers.
