What is the Advantage and Disadvantage of Render Retarder

Vapor barriers in walls, why polyethylene can be problematic

It would probably surprise many home builders to hear what really causes moisture accumulation in walls, and what to do to prevent it. An understanding of how water vapor moves through walls is important, so a good place to start would be with our page explaining moisture movement in homes (see related articles below).

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The traditional approach to preventing water vapor from penetrating walls in homes is a 6 mil polyethylene vapour barrier, or 'vapor barrier' for our southern neighbours. This is an ideal building practice in the extreme northern communities of Canada, less so as you come further south. Despite it being used extensively in residential construction, it can be overkill in most Canadian homes, and can cause problems of its own.

"One of the problems in the building industry is that we have a spreading 'cult-like' mentality that worships at the 'church of polyethylene'. This cult views the answer to all moisture problems as the installation of a polyethylene vapor barrier on the inside of buildings. This cult is responsible for many more building failures than building successes. It's time that the cult deprogramming started."

- Joe Lstiburek, Principal of Building Science Corporation

The USA & Canada has many climatic zones, so there is not one building envelope that can possibly serve them all. The automatic installation of a polyethylene vapour barrier in every home from the Hudson Bay to the vineyards of Southern Ontario to the deserts of Arizona meets the state & provincial building codes, but completely ignores the reality of how different those climates are.

Many parts of the country can range from extreme cold to extreme heat and humidity, with temperatures that vary as much as 60 degrees Celsius or more. In areas like that, the vapour barrier that works great in February isn't doing you any favours in July. During those 30+° Celsius days with relative humidity levels upwards of 80% and an indoor air-conditioned environment some 10 degrees cooler, that vapour barrier is on the wrong side.

Is the solution then to not install a vapour barrier? No, but since there isn't a perfect solution that meets the needs of both climatic extremes, we should find a solution that at least takes them both into account.

The vast majority of Americans & Canadians live in a temperate climate, so for most of us a vapour barrier (or more accurately semi-permeable vapor retarder) that allows a certain amount of water vapour to pass through a wall could actually serve us better over the course of the year.

As warm, humid air cools, air molecules shrink and squeeze out the moisture. This can be a problem if it happens inside your walls, so vapour barriers are there to mitigate that.

In order to prevent condensation from forming, a vapour barrier should be placed on the warm side of your insulation to stop warm, moist air from condensing on a cold surface inside your wall.

In cold climates like Canada, for most of the year the vapour barrier should be on the inside of the insulation. In hot climates like the southern U.S. for example, it should be installed on the outside of the insulation. 

In both cases, the vapour barrier is tasked with preventing warm, humid air from shedding its moisture as it meets a cool surface, no matter which direction it is travelling.

The most important thing to realize is that there is no fixed rule regarding vapor barriers. Building practices should always be determined by the climate zone in which you are building.

Understanding vapor barriers:

The National Building Code of Canada stipulates that for residential buildings, a vapour barrier must have a water vapour permeance of less than 60 ng/Pa*s*m2 or 1.0 Perm. That means that no more than 60 nanograms of water vapour can pass through a square meter of the material in one second. Nanograms are pretty small by the way, that's one billionth of a gram.

Traditionally, a polyethylene vapour barrier (with a vapour permeance rating of 3.4 ng) is installed behind the drywall in new Canadian homes. In fact, you would be hard pressed to find a home being built in Canada right now that does not have it, or something equally impermeable to moisture. This doesn't mean that there aren't other options out there, they just aren't being applied.

In the US any material that has a perm rating of 1 or less is considered to be an adequate vapor retarder for residential construction. As requirements vary between states we suggest giving your local permit office a call to establish their recommendations. The perm rating is a measure of the diffusion of water vapor through a material & the table below shows the perm rating of some common building materials that are consistent with the ASHRAE Handbook of Fundamentals and other industry sources.

The problem is largely because the 6 mil poly that gets installed as a vapour barrier is mistaken for, and almost entirely relied upon to act as the air barrier. The purpose of the two barriers should not be confused - the job of the vapour barrier is to control vapour diffusion, the job of the air barrier is to control air leakage.

6 mil poly can work effectively as an air barrier if it is carefully sealed, but so can other materials. Well-sealed drywall in itself makes a great air barrier. But unless you install polyethylene with the express purpose of it being an air barrier, it likely isn't doing the job. And in fact, the term 'air barrier' is rarely if ever used in mainstream residential construction, and it really should be.

Vapour retarder latex primers:

Firstly, the classification of a material as either an impermeable ‘vapour barrier’ or a semi-permeable ‘vapour retarder’ is determined by how much water vapour passes through the material under specific conditions.

There are vapour retarder primers on the market that exceed the requirements of the National Building Code of Canada & local US building code regarding water vapour diffusion, with a vapour permeance in the area of 30 to 36 ng, which is about half of the 60 ng often allowed by code.

So concerns that primers are insufficient to control vapour diffusion are unfounded, they just aren't widely used. But keep in mind that the construction industry can be slow to adopt new practices, regardless of the merits. So don't be intimidated if you want to break the norm.

Air leakage:

Now that we have looked at some options regarding vapour barriers time to understand the difference to air barriers, and firstly it should be pointed out that the water vapour permeating through building materials - the reason for installing a vapour barrier - is not the monster it has been made out to be. 100 times more water vapour is carried though a wall assembly by air leakage, than is carried by vapour diffusion. So the air barrier is 100 times more important than the vapour barrier.

Therefore, we really don't need to go to the extremes that we do regarding vapour barriers, as it actually takes the focus away from what we should be thinking about, which is creating an effective air barrier.

So here is the summed-up case for the “poly-free” house, and a bit of perspective:

• Water vapour diffusion through building materials accounts for only about 2% of moisture penetration through walls, and a vapour retarder primer can be twice as effective as it needs to be.
• Polyethylene is some 15 times more resistant to water vapour diffusion than it needs to be; it's expensive to buy and install; is environmentally questionable; and it can actually cause problems in the summer months.

In much of the country, you could take the time and money you would have spent on installing polyethylene on the entire exterior wall of your home, and instead put those resources into a latex vapour retarder paint on primer and a properly sealed air barrier. There are hard cost savings to be had doing this, and an improvement in both performance and durability.

The one glitch in the system, is that building inspectors can also be subject to the same conditioning that a lot of builders are, and do not realize that in many cases there are better options available than polyethylene for controlling water vapor in homes. When you take your plans in to get a permit, make sure it is clear what material you plan to use for water vapor control, so that you can go into battle then, and not during a home inspection after construction is complete.

References:

Lstiburek ():

U.S. Building Code requirements for vapor retarders are proposed based on climate and properties of other materials in the wall assembly. Identified hygrothermal regions include those applicable to Canada. Most assemblies do not use polyethylene and incorporate latex paint or vapor semi-permeable interior finishes.

The following main principles are recommended:

• Avoid vapor barriers where vapor retarders will work, avoid vapor retarders where vapor permeable materials will work.
• Avoid the installation of a vapor barrier on both sides of the wall assembly.
• Avoid using poly, foil faced batts, reflective barrier foils, and vinyl wall coverings on the interior of air-conditioned assemblies.
• Ventilate enclosures

Use of Water Reducers, Retarders, and Superplasticizers.

Introduction

Many important characteristics of concrete are influenced by the ratio (by weight) of water to cementitious materials (w/cm) used in the mixture. By reducing the amount of water, the cement paste will have higher density, which results in higher paste quality. An increase in paste quality will yield higher compressive and flexural strength, lower permeability, increase resistance to weathering, improve the bond of concrete and reinforcement, reduce the volume change from drying and wetting, and reduce shrinkage cracking tendencies (PCA, ).

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Reducing the water content in a concrete mixture should be done in such a way so that complete cement hydration process may take place and sufficient workability of concrete is maintained for placement and consolidation during construction. The w/cm needed for cement to complete its hydration process ranges from 0.22 to 0.25. The existence of additional water in the mixture is needed for ease of concrete placing and finishing (workability of concrete). Reducing the water content in a mixture may result in a stiffer mixture, which reduces the workability and increases potential placement problems.

Water reducers, retarders, and superplasticizers are admixtures for concrete, which are added in order to reduce the water content in a mixture or to slow the setting rate of the concrete while retaining the flowing properties of a concrete mixture. Admixtures are used to modify the properties of concrete or mortar to make them more suitable to work by hand or for other purposes such as saving mechanical energy.
 
 

Water reducing admixtures (WRA)

The use of WRA is defined as Type A in ASTM C 494. WRA affects mainly the fresh properties of concrete by reducing the amount of water used by 5% to 12% while maintaining a certain level of consistency, measured by the slump as prescribed in ASTM C 143-90. The use of WRA may accelerate or retard the initial setting time of concrete. The WRA that retards the initial setting time more than three hours later is classified as WRA with retarding effect (Type D). Commonly used WRA is lignosulfonates and hydrocarboxylic (HC) acids. The use of HC acids as WRA requires higher water content compared to the lignosulfonates. Rapid bleeding is a problem for concrete treated with HC acids.

Increase of slump is different according to its type and dosage. Typical dosage rate is based upon the cementitious material content (milliliters per hundred of kilograms). The figure below illustrates the influence of dosage of Lignosulfonates and HC acid on slump. It is shown in the figure that HC acids give a higher slump compared to lignosulfonates with the same dosage.
 
 

Figure 1 Influence of Dosage of Retarders on Slump (Neville, ).  
 

WRA has been used primarily in hot weather concrete placing, pumping, and tremie. Careful concrete placement is required, as the initial setting time of concrete will take place an hour earlier. It is also shown that the use of WRA will give a higher initial concrete compressive strength (up to 28 days) by 10% compared to the control mixture. Other benefit of using WRA is that higher concrete density is achieved which makes the concrete less permeable and have a higher durability.
 
 

Retarding admixtures

The use of this admixture is defined in ASTM C494. There are two kinds of retarders, defined as Type B (Retarding Admixtures) and Type D (Water Reducing and Retarding Admixtures). The main difference between these two is the water-reducing characteristic in Type D that gives higher compressive strengths by lowering w/cm ratio.

Retarding admixtures are used to slow the rate of setting of concrete. By slowing the initial setting time, the concrete mixture can stay in its fresh mix state longer before it gets to its hardened form. Use of retarders is beneficial for:

• Complex concrete placement or grouting
• Special architectural surface finish
• Compensating the accelerating effect of high temperature towards the initial set
• Preventing cold joint formation in successive lifts.

Retarder can be formed by organic and inorganic material. The organic material consists of unrefined Ca, Na, NH4, salts of lignosulfonic acids, hydroxycarboxylic acids, and carbohydrates. The inorganic material consists of oxides of Pb and Zn, phosphates, magnesium salts, fluorates, and borates. Commonly used retarders are lignosulfonates acids and hydroxylated carboxylic (HC) acids, which act as Type D (Water Reducing and Retarding Admixtures). The use of lignosulfonates acids and hydroxylated carboxylic acids retard the initial setting time for at least an hour and no more than three hours when used at 65 to 100 oF.

A study performed on the influence of air temperature over the retardation of the initial set time (measured by penetration resistance as prescribed in ASTM C 403 – 92) shows that decreasing effect with higher air temperature (Neville). The table below describes the effect of air temperature on retardation of setting time:

Table 1 Air Temperature and Retardation of Initial Setting Time Admixture Type Description Retardation of initial setting time (h:min) at temperature of 30oC 40oC 50oC D Hydroxylic acid 4:57 1:15 1:10 D Lignin 2:20 0:42 0:53 D Lignosulfonates 3:37 1:07 1:25 B Phosphate-based --- 3:20 2:30

The use of retarding admixture has the main drawback of the possibility of rapid stiffening, where rapid slump loss will result in difficulty of concrete placement, consolidation, and finishing. An extended-set admixture has been developed as another retarding admixture. The advantages of this admixture compared to the conventional one is the capability to react with major cement constituents and to control hydration and setting characteristics of concrete while the conventional one will only react with C3A.

Careful usage of retarder is required to avoid excessive retardation, rapid slump loss and excessive plastic shrinkage. Plastic shrinkage is the change in fresh concrete volume as surface water evaporates. The amount of water evaporation is influenced by temperature, ambient relative humidity, and wind velocity. Proper concrete curing and adequate water supply for surface evaporation will prevent plastic shrinkage cracking. The amount of water needed to prevent plastic shrinkage cracking is given by the chart below:
 
 

Figure 2 Rate of Surface Moisture Evaporation  
 

The extended-set admixture is widely used as a stabilizing agent for wash water concrete and fresh concrete. Addition of extended-set admixture enables the reuse of wash water to the next batch without affecting concrete properties. This admixture can also be used for long haul concrete delivery and to maintain slump. Factors affecting the use of this admixture include the dosage rate and the ambient temperature of the concrete.
 
 

Superplasticizers (High Range Water reducer)

ASTM C494 Type F and Type G, High Range Water Reducer (HRWR) and retarding admixtures are used to reduce the amount of water by 12% to 30% while maintaining a certain level of consistency and workability (typically from 75 mm to 200 mm) and to increase workability for reduction in w/cm ratio. The use of superplasticizers may produce high strength concrete (compressive strength up to 22,000 psi). Superplasticizers can also be utilized in producing flowing concrete used in a heavy reinforced structure with inaccessible areas. Requirement for producing flowing concrete is defined in ASTM C . The effect of superplasticizers in concrete flow is illustrated in the chart below:

Figure 3 Relation between Flow Table and Water Content of Concrete with and without Plasticizers (Neville, ).

Another benefit of superplasticizers is concrete early strength enhancement (50 to 75%). The initial setting time may be accelerated up to an hour earlier or retarded to be an hour later according to its chemical reaction. Retardation is sometimes associated with range of cement particle between 4 – 30 m m. The use of superplasticizers does not significantly affect surface tension of water and does not entrain a significant amount of air. The main disadvantage of superplasticizer usage is loss of workability as a result of rapid slump loss and incompatibility of cement and superplasticizers.

Superplasticizers are soluble macromolecules, which are hundreds of times larger than water molecule (Gani, ). Mechanism of the superplasticizers is known as adsorption by C3A, which breaks the agglomeration by repulsion of same charges and releases entrapped water. The adsorption mechanism of superplasticizers is partially different from the WRA. The difference relates to compatibility between Portland Cement and superplasticizers. It is necessary to ensure that the superplasticizers do not become fixed with C3A in cement particle, which will cause reduction in concrete workability.

Typical dosage of superplasticizers used for increasing the workability of concrete ranges from 1 to 3 liters per cubic meter of concrete where liquid superplasticizers contained about 40 % of active material. In reducing the water cement ratio, higher dosage is used, that is from 5 to 20 liters per cubic meter of concrete. Dosage needed for a concrete mixture is unique and determined by the Marsh Cone Test.

There are four types of superplasticizers: sulfonated melamine, sulfonated naphthalene, modified lignosulfonates and a combination of high dosages of water reducing and accelerating admixtures. Commonly used are melamine based and naphthalene based superplasticizers. The use of naphthalene based has the advantage of retardation and affecst slump retention. This is due to the modified hydration process by the sulfonates

Admixtures Dispensers

The basic function of a dispenser as defined in ACI Bulletin E4-95 is:

• To transport the admixture from storage to batch
• To measure the quantity of the admixtures required
• To provide verification of the volume dispensed
• To inject the admixture into the batch.

Admixtures have been dispensed in liquid form to ensure proper dispersion in the concrete mixture. WRA should be dispensed with the last water batch. Proper timing is very important, as any delay ranges between one to five minutes after the water addition will result in excessive retardation of setting time. The Superplasticizers should be dispensed on to the batch immediately before discharge for placement (Type F) or with the last portion of the water (Type G).

References:

Chemical Admixtures for Concrete, ACI Committee 212.3R-91 Report.

Chemical and Air Entraining Admixtures for Concrete, ACI Education Bulletin No. E4-95.

Dodson, Vance, Concrete Admixtures, VNR, .

Gani, M.J., Cement and Concrete, Chapman & Hall, .

Komatska, S. H. and Panarese, W. C., Design and Control of Concrete Mixtures, PCA, .

Ramachandran, V. S., Concrete Admixtures Handbook, Properties, Sciences, and Technology, 2nd edition, .

Aitcin, P., Jolicoeur, C., and MacGregor, J., Superplasticizers: How They Work and Why They Occasionally Don’t, Concrete international, May .

Information compiled by Titin Handojo.

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