Does My Old Cot­tage Need Damp Treat­ment?

Ex­pert Dou­glas Kent an­swers one reader’s ques­tion

Homebuilding & Renovating - - Contents -

QWe are hop­ing to buy an old stone cot­tage in York­shire. The damp re­port we have had done sug­gests the house is damp and rec­om­mends a damp treat­ment. How­ever, from read­ing around the sub­ject of damp in old build­ings I’m hes­i­tant about whether this is the right thing. A lot of what I’ve read is that this is the worst thing to do, and that the house should be re­stored to tra­di­tional fin­ishes. I’m not sure if we should just avoid it all to­gether or if we should get a lo­cal her­itage builder to go over and take a look in­stead. Can you help to ad­vise on what’s best?

An­gela Brook, York­shire

AYou are right to be wary about the ad­vice you have been given, but I would not let this de­ter you from con­sid­er­ing pur­chas­ing the cot­tage if it gen­er­ally matches your re­quire­ments.

A com­mon prob­lem is that ad­vice on damp is of­ten given by re­me­dial treat­ment con­trac­tors. They have vested com­mer­cial in­ter­ests in their rec­om­men­da­tions, which leads to self-serv­ing re­ports and the over-spec­i­fi­ca­tion of work. Ad­di­tion­ally, many in­di­vid­u­als ad­vis­ing on damp do not ap­pre­ci­ate how the con­struc­tion of older build­ings pre-dat­ing about 1919 dif­fers fun­da­men­tally to that of most mod­ern build­ings.

As a re­sult, un­for­tu­nately, rec­om­men­da­tions are fre­quently made for work that is un­nec­es­sary, dam­ag­ing and ex­pen­sive — es­pe­cially the retrofitting of damp-proof courses (DPCS) in­tro­duced to act as a bar­rier to ris­ing damp in walls.

Most pre-1919 build­ings are of ‘ tra­di­tional’ con­struc­tion with solid walls that need to ‘ breathe’. Con­trast that with post-1919 or ‘mod­ern’ build­ings that de­pend on bar­ri­ers in or­der to stay dry. The for­mer are anal­o­gous to an over­coat and the lat­ter a rain­coat. If bar­ri­ers are added to a tra­di­tion­ally con­structed build­ing it is likely to ex­ac­er­bate damp, whereas a mod­ern build­ing will be­come damp if bar­ri­ers are re­moved.

Retro­fit DPCS are of­ten rec­om­mended un­nec­es­sar­ily just be­cause they are ab­sent. Some­times ris­ing damp is mis­di­ag­nosed solely on the ba­sis of high elec­tri­cal mois­ture me­ter read­ings; ­this could be due to pen­e­tra­tion from rain­splash, for ex­am­ple. El­e­vated read­ings oc­cur not in­fre­quently in old build­ings that are vir­tu­ally dry, due to salt de­po­si­tion on breath­ing walls. Damp-proof mem­branes (DPMS) are sim­i­larly rec­om­mended with­out good rea­son as a mois­ture bar­rier be­low old stone floors. By re­strict­ing evap­o­ra­tion, DPMS dis­place mois­ture into the pre­vi­ously dry ad­ja­cent walls, caus­ing damp there.

If damp does ac­tu­ally ex­ist, aim to tackle the cause. Mea­sures that help the fab­ric ‘ breathe’, such as re­plac­ing a hard ce­ment ren­der or the point­ing in ma­sonry joints with a more suit­able lime-based mor­tar, may be the best so­lu­tion. If a floor has an in­ap­pro­pri­ate DPM, this might be sub­sti­tuted with a ‘ breath­able’ con­struc­tion or a strip for evap­o­ra­tion cut around the room perime­ter and in­filled with a ma­te­rial such as lime con­crete, or grated over.

Con­den­sa­tion is an in­creas­ing prob­lem due to laud­able but mis­guided at­tempts to im­prove en­ergy ef­fi­ciency in old build­ings. It can arise not only where in­com­pat­i­ble non­breath­able forms of in­su­la­tion are used (for ex­am­ple, PIR boards rather than vapour-open ma­te­ri­als such as hemp) but over-zeal­ous draught­proof­ing, in­clud­ing the in­stal­la­tion of re­place­ment win­dows. Con­den­sa­tion can pro­mote rot and ag­gra­vate hu­man health prob­lems, in­clud­ing asthma, but can be con­trolled by im­prov­ing ven­ti­la­tion, gen­er­at­ing less mois­ture and in­creas­ing heat­ing.

What’s more, the ne­glect of ba­sic main­te­nance can cause rain pen­e­tra­tion. Sim­ple tasks such as clear­ing out blocked gut­ters and down­pipes, and re­in­stat­ing slipped or miss­ing slates or tiles should be un­der­taken on a reg­u­lar ba­sis to pre­vent this. Rain pen­e­tra­tion may also oc­cur where hard sur­faces are laid ex­ter­nally im­me­di­ately along­side walls.

It is worth chal­leng­ing any rec­om­men­da­tion re­gard­ing damp that you be­lieve is ques­tion­able. If nec­es­sary, seek a sec­ond opin­ion in writ­ing from an in­de­pen­dent char­tered sur­veyor or con­sul­tant, not a builder. This will usu­ally sat­isfy mort­gage com­pa­nies. The So­ci­ety for the Pro­tec­tion of An­cient Build­ings (SPAB) may be able to ad­vise you on suit­able names.*

“If damp­ness does ac­tu­ally ex­ist, aim to tackle the cause”

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