Typical drylined hollow block construction
Hollow block buildup:
The typical buildup of a typical drylined hollow block wall is shown above, as can be seen the two faces of a hollow block each about 40mm wide are linked by three cross members of 35mm thick. When built the resulting large hollows of each block become vertically linked. Given that moisture can penetrate through the outer 40mm of hollow block in driving rain conditions, it is a saving grace of their design that the air movement that occurs encourages evaporation of this moisture. However this air movement also reduces the thermal performance in that any warmed air will rise naturally to be replaced by cooler below.
Typical Drylined block walls:
In a typical buildup of a drylined hollow block wall treated timber studs are mechanically fixed to the hollow blocks wall. Mineral wool insulation is then friction-fitted between the timbers and a sheet of polythene is stapled to the studs to act as a vapour barrier. Where required a "hole in the wall vent" or a back box for a socket etc. breaks through the vapour barrier all contributing to a very leaky membrane. Water vapour can move with relative ease through the plasterboard into the insulation zone and condense where the temperature drops sufficiently.
Overcoming the problem:
The typical approach could be improved if the following steps where followed, firstly a fibrous insulant more resistant to sag and better able to absorb moisture without collapsing such as hemp wool as used, next an intelligent vapour control layer (VCL) such as "intello" could be installed. The membrane could then be separated by a services zones from the plasterboard, a limited amount of insulation can be installed there and services be run through.
Hygrothermic study of drylined walls:
The image below was generated in a specialist simulation software called "WUFI" which looks at temperature and moisture fluctuations (In this case in a drylined concrete block wall) this new kind of simulation can go beyond considering a wall buildup of dry materials, it can investigate the performance of that wall under different design scenarios, if the simulation shows that the resulting moisture content reaches a critical level or simply grows year on year we can be sure that we will have some level of building failure on our hands, it can test for reduced thermal performance, structural damage or even mould growth.
Dynamic display of moisture movement in the wall
Conclusion:
The external envelope of any building is a place of seasonal and at times weekly change in temperature, moisture content and vapour movement. In certain cases of inappropriate knowledge, installation or materials the thermal performance of the insulation can change and so to can the likelihood of that building element hosting mould growth. Designers and builders need to build with far more awareness of moisture movement to ensure the structure and the occupants health are safeguarded. In some cases it may mean clients need to pay more, in others there would be no cost difference gut the pay off for their health should be clear.
Excellent article Alan. I would guess that few practicioners realise how incorrectly detailed drylining can impact the performance of hollowblock walls and quickly negate any insulation benefits. Probably a very common issue in Ireland due to the prevalence of this type of construction and the also the climate. Look forward to reading future articles!
ReplyDeleteAlan this is a very interesting blog while I would say that External wall insulation is much better for hollowblock walls what would you recommed if you had to insulate a solid wall internally?
ReplyDeleteHi Tommy,
DeleteI agree 100% with you I would prefer External Insulation, the blog post points out the problems of using Internal Insulation for retro fitting a single leaf wall.
Alan
Hey Alan,
ReplyDeleteLoved the blog. It made for a very interesting and informative read.
Drawing was excellent and really help to understand exactly what you were taking. I'd copy write it quickly or Owen will be using next year in the Building Surveying module. :-)
Alan,
ReplyDeleteAn excellent article, with good detail and content, on something that's constantly evolving.
How long will it be before we see articles like this on external insulation and the solutions used in today's building environment?
Very good Alan. Very informative. Well done.
ReplyDeleteAlan,
ReplyDeleteGreat blog. Might use it next year in Building Surveying!
Hello,
ReplyDeleteThank you for the interesting and well presented article.
I would like to know, what do you think is the deal with solid concrete walls?
Regards
External-Wall-Insulation is one of the largest insulation companies in Ireland, established for over 30 years we are experts in the industry. We pride ourselves in providing high quality of service and workmanship
ReplyDeleteGreat knowledge, do anyone mind merely reference back to it attic insulation removal and replacement
ReplyDelete