A lot of people are still using aerated concrete blocks to build with. In my opinion these are naff! They crack and move arround too easily, are brittle, insubstantial and of too low thermal mass. I wont use them, instead I use medium or heavy weight blocks. But when I use mine I use mortar of the same thermal properties as my blocks but them that use aerated blocks use thermally bridging mortar-- crazy or what?

Is there such a thing as insulating mortar? Surely if the blocks can be insulating then shouldn't the mortar be as insulating? (even though I hate aero blocks it only seems common sense to me to have matching mortar) then again there it a sad lac of common sense in the way we build so it is not a surprise.

Tony, I agree with comments about the blocks. I have them in my extension, erected by a builder to plans passed by council. Never again.

As the bricklayers like to build the inner and outer leaves together, they aren't going to use two different mortars. Weak mortar just makes repairs easier to do. There are several cracks here, time-consuming to rake out and repoint, but no other option.

Chris ref. thin joint mortar, 1) it is a very minority sport, 2) the ratio of joints to block make it far less important, 3) still get cracks and no thermal mass, 4) the mortar is generally too strong and is not insulating.

Tony, I am interested to hear more about the cracking. Do you mean the blocks crack after the walls have been built? I have not experienced or seen this.

Mike, have a look at any house or extension built with lightweight blocks there are always cracks above the windows often both sides, below the windows often in the middle, above internal doors, in corners, on flank walls, half way up the stairs wall horizontally. Of late these are commonly hidden behind the dreaded blob and dob but easily visible if wet plastering is used.

Biff. I too think architects are a problem but it is neither group who should decide what goes on in this case.

Lime is added to most mortars to make it more workable. 1:1:6 is also known as Type N. From:
http://www.free-ed.net/sweethaven/BldgConst/Masonry/lessonmain.asp?iNum=fra0104

1-11. Type M. Type M mortar is composed of 1 part cement, ¼ part lime, and 3¾ parts sand. This mortar has a high cement-to-lime proportion that makes for a very strong mortar. This type of mortar is suitable for general use. It is recommended specifically for structures that are below grade and in contact with the earth, such as foundation, retaining walls, and walkways.

1-12. Type N. Type N mortar is composed of 1 part cement, 1¼ parts lime, and 6¾ parts sand. Type N has excellent workability because of its high lime content. It does not have the strength of Type M or Type S mortar. Type N-

* May be used in bearing walls that are above grade if stress is not too great.
* Is widely used in veneers.
* Is used in partitions and some exterior walls where climatic conditions are negligible.

1-13. Type O. Type O mortar is composed of 1 part cement, 2 parts lime, and 9 parts sand. It is extremely plastic and workable (notice the lime content) but has relatively low strength.

Type O should not be used if it will be subjected to freezing and thawing in the presence of excessive moisture. Many building codes do not allow Type O mortar for construction. However, some local building codes allow Type O to be used for load-bearing walls of solid blocks when the compressive stresses do not exceed 100 pounds per square inch. Before using this type mortar, check local building codes.

1-14. Type S. Type S mortar is composed of 1 part cement, ½ part lime, and 4½ parts sand. It is a very good general-purpose mortar used in above-grade exteriors that are exposed to severe weathering. Type S is also used in interiors and all load-bearing structures unless only Type M is specified. Type M and S are usually interchangeable.

Paul in Montreal

Posted By: tonyMike, have a look at any house or extension built with lightweight blocks there are always cracks above the windows often both sides, below the windows often in the middle, above internal doors, in corners, on flank walls, half way up the stairs wall horizontally. Of late these are commonly hidden behind the dreaded blob and dob but easily visible if wet plastering is used.

tony, I don't think the cracking you describe is limited to lwt blocks. It can be seen in many buildings, especially in the areas around windows and doors. It is often a result of the expansion and contraction of disimilar materials and can be minimised by the use of appropriarte materials [ie soft blocks complimented with soft mortars] and expansion joints.

Agree about the blob and dob though

okay, what mix proportions do you use?

Substituting sand and/or cement for anything with a lower density would no doubt be more insulating , but tends to compromise the effectiveness and/or workability of the mortar itself.

I don't see the point of doing this if you want thermal mass anyway. If you have a dense concrete wall then why bother?

1:8 must be bloody awful to work with. If the sand you use the quarried loamy stuff?

Yes, perhaps the thin joint adhesive produced by the manufacturers is of comparable properties to the blocks - anyone know? I looked at this as an option once- the bricklayer I asked refused to even consider it so that was the end of that.

So what we need is someone who has used the thin joint system to tell us whether the blocks crack or not.

Not all, there are still some of us who render over blockwork, regardless of what type they are.

Here in France the think joint system is used quite a lot.

the size of the joint is 1mm! so insulating isnt a factor so much

if it isnt a thin joint system they sometimes use 2 seperate tracks of mortar to bed the blocks to break the thermal conductance, think of cord wood constructon, there is a cavity inside the mortar bed.

there doesnt seem to be a problem with these blocks cracking.

regarding thermal mass, all the recent studies I have seen are saying that it is the density x total surface area of the thermal mass material that is most important, not the thickness. I think using block work for thermal mass, especially in northern latitudes might be over kill.

Strawbales provide the best model for the insulation/thermalmass balance. Large amounts of insulation on the outside making up the majority of the structure of the wall, and a thick coat of wet plaster on the inside to give the required amount of thermal mass over a large square meterage.

Hi bot de paille. Regarding the twin mortar tracks, do they use this in a wide[225mm trench block] type wall, or are we talking about conventional [UK] cavity type walls

Also, do you have a link to the studies you referred to, thanks,

Mike