Damp in an old solid-walled house

How old walls handle water

SPAB says buildings from before about 1919 tend to have solid walls, built from materials such as stone, soft brick, unfired earth, lime mortars and plasters, and limewash ¹. These materials take in more moisture than modern ones but let it evaporate again when conditions are drier ¹. SPAB compares the two kinds of building to an overcoat and a raincoat: old buildings usually become damp when barriers to moisture are added, while new ones become damp when their barriers fail ¹.

Old buildings are not inherently damp, SPAB says, but where damp exists it should not be ignored, and any decision to do nothing should be made only after investigating it carefully ¹.

Find the cause first

SPAB says addressing the cause of dampness is always preferable, where possible, to dealing merely with the symptoms ¹. It groups the sources of damp into two kinds, and each needs a different fix ¹:

• Liquid water: rain getting in, water from the ground (including rising damp), and plumbing leaks.
• Water from the air: condensation, high humidity, and salts in the wall that draw moisture from the air.

Common causes SPAB describes are:

• Gutters, downpipes and drains. Blocked or cracked rainwater goods and gullies soak walls ¹. SPAB notes that problems put down to rising damp are often caused by defective drains or leaking water pipes near the base of a wall ¹.
• Hard surfaces against walls. Where patios, paths and other hard surfaces are laid up to walls, inadequate drainage or rainsplash commonly soaks them ¹.
• Cement, plastic paint and other impervious materials. Cement render and pointing, dense plasters and plastic-based paints stop a wall drying out ¹³.
• Less ventilation. SPAB says excessive draught-proofing and double glazing have contributed to condensation problems in old houses, and blocked air bricks and flues make it worse ¹².
• How we live. Cooking, washing, drying clothes indoors and flueless bottled gas heaters all add moisture ¹².

Condensation is often mistaken for rising damp

SPAB says condensation usually appears in winter, as droplets on hard surfaces or mould on absorbent finishes ². Moisture meters may show diffuse damp on plaster, especially on north-facing walls, and this is often wrongly blamed on rising damp ². SPAB warns that misdiagnosis can lead to unnecessary and expensive remedial work ¹.

SPAB’s suggestions cost little ¹²:

• Make less moisture: lids on pans, dry clothes outside, close kitchen and bathroom doors in use.
• Ventilate: open windows, and don’t draught-proof kitchen and bathroom windows. Keep lofts, floor voids and unused chimneys ventilated.
• Heat steadily: low, constant heating keeps surfaces warmer than short bursts.

Rising damp, and why moisture meters mislead

SPAB says rising damp exists but is much less of a problem than commonly supposed ¹. Rising damp is widely misdiagnosed on the basis of high electrical moisture meter readings alone. Salt left by a previous damp problem, certain timber preservatives, foil-backed wallpaper and carbon-containing materials can all give high readings in a wall that is not unduly damp ¹. If rising damp is present, there will be visible signs too, such as a tidemark ¹.

Get an independent diagnosis

SPAB strongly advises getting a diagnosis separately from quotes for the work, for example from a chartered building surveyor, so that no one with a commercial interest decides how much work is needed ¹. It says free surveys are best avoided ¹. In its experience, mortgage lenders sometimes demand damp-proofing that isn’t needed. It suggests challenging a questionable diagnosis and getting a second opinion in writing from an independent chartered building surveyor or consultant, not a contractor ¹.

Damp-proof courses, membranes and tanking

SPAB says there should be a strong presumption against inserting a retrofit damp-proof course into a pre-1919 building. Put in inappropriately, it can be damaging, ineffective and an unnecessary expense ¹. Be wary of anyone who proposes one on the strength of moisture meter readings alone ¹. Guarantees offered with such work are often loaded with get-out clauses and may have no insurance backing, SPAB warns ¹.

Other points from SPAB’s guidance ¹:

• A damp-proof membrane installed in the floor of a cob or other earth-walled building can increase dampness in the walls to the point that they collapse.
• Physical and chemical damp-proof courses should be avoided in earth buildings.
• Attempts to seal walls, for example with dense plasters, PVA or impervious paint, impede evaporation and trap moisture or move it elsewhere.
• In cellars, a sump kept dry by a float-operated pump is far more reliable than traditional tanking, which tends to be expensive and depends on faultless workmanship.

What SPAB suggests instead is helping the wall dry: lowering the ground immediately around the building to at least 150 mm below the inside ground floor where that is safe to do, improving drainage, and replacing hard cement render or pointing with a more suitable lime-based mortar ¹. SPAB cautions that groundworks can cause foundation problems and have archaeological implications, so get advice first ¹.

Listed buildings

SPAB says listed building consent may be required for work that exceeds like-for-like repair, and that carrying out work that needs consent without getting it beforehand is a criminal offence. If in doubt, consult the conservation officer ¹.