Residential Wall Air Gap Eliminating Moisture Walls

When insulation is placed inside the masonry wall, the air barrier has to be permeable to allow moisture vapor to pass through it, he says.

However, wall systems of brick masonry units and mortar joints can — and sometimes do — allow water penetration. A drainage-type wall requires an unobstructed air space, properly installed flashing, and adequately spaced and open weep holes to conduct to the outside any water which has penetrated the exterior of the masonry.

Section 1404.2, and a means for draining water that enters the assembly to the exterior. Because of the cavity, the inner wall resists rain penetration. The ventilation is provided by weep holes between the bricks in the bottom course. The insulation should be installed at a lower level, between the attic and floor joints, not the rafters, to allow for air circulation. Some part of the wall is structurally sound, holding up what is attached to the wall and carrying what is above the wall, including the roof with its changing weights of snow and wind. 

This often causes the outer layers of the wall to be both colder and wetter as the materials are no longer warmed and dried by the interior heating system.

If a wall has difficulties drying due to any of the above causes it is possible that over the course of several years the quantity of water within the wall will consistently increase.

Four (4) proposed wall constructions were analyzed to determine how different types, quantities, and configurations of insulation would impact the existing constructions.

CHAPTER 14 EXTERIOR WALLS – Residential Masonry Wall Air Gap


Thorough and puts things into proper perspective. Water repellents are not a permanent repair and will require re-application. Removal of brick for flashing replacement.
If tuck-pointing is required, it must be done before the water repellent is applied. Do not use surface-forming or acrylic-based sealers on exterior masonry walls.
The application of a water repellent may cause a slight change in appearance to the masonry wall. The application of any water repellent will not be effective if applied over poor masonry work. A water repellent will not bridge holes or fill gaps in mortar joints.
As can be seen in the chart, all of the simulations, including the base case showed some accumulation of water over the five year simulation.

The results indicate that even the base case is accumulating water over time. Solutions that used foam insulation performed better than those with batt insulation.
The analysis looked at the temperature within the middle of the masonry to determine how added insulation would impact the material. As can be seen the brick temperature remains consistent with the base case in all retrofit options. A constant and high relative humidity (above 80%) indicates the potential for mold growth. The addition of interior insulation caused the relative humidity within the layer to increase approximately 15%, from 65% (base case) to just over 80% (all options for added insulation).

The above analysis has indicated that there is a potential for mold growth, but has not confirmed its likelihood. Testing of the existing materials and specific data on proposed products should be used to refine this analysis and determine extent of mold growth risk.

The veneer ties shall have sufficient strength to support the full weight of the veneer in tension. Portland cement grout and pea gravel. Sections 2103 and 2512.2 applied to create a full setting bed for the back of the masonry veneer units. Such shelf angles shall be not less than 0.0478-inch (1.2 mm) thick and not less than 2 inches (51 mm) long and shall be spaced at approved intervals, with not less than two angles for each glass unit.

Other fasteners shall be installed in accordance with the approved construction documents and manufacturer’s instructions. Unless otherwise specified in the approved manufacturer’s instructions, nails used to fasten the siding to wood studs shall be corrosion-resistant round head smooth shank and shall be long enough to penetrate the studs at least 1 inch (25 mm).

Table 2304.10.1 or the approved manufacturer’s instructions.  This foam board should be attached tightly to the house sheathing.  In the same vein, if you put insulation panels over existing siding that was mounted on strapping, you are leaving a ventilated air space between your new insulation and the house — totally eleminating all the insulation value of your expensive work. 

It would work better if you sealed that air space but the best is to remove the old siding and strapping and then insulate tight to the wall.

Being on the outside of modern heavy insulation, it is too cold to help much with ventilation, and the convection currents in this air space can actually make condensation problems worse.

However, when insulating an open wall, don’t leave any air space.
If on these same walls you have an accidental space between the insulation and the vapour barrier, an air current can loop around the insulation taking heat directly from the warm drywall to the cold sheathing.

If the air space is between the insulation and the vapour barrier, the air will rise because of the warmth of the house.  This movement of air will find its way through or around the fiberglass insulation to the cold side, where it will fall because of the cold surface of the sheathing. 

When the insulation is installed less than perfectly, the looping force will accelerate.  When there is an air space between the vapour barrier and the drywall, nothing happens.  The temperature goes from cool on the bottom to warm on the on the top but the air in this space has no access to the cold side of the wall. 

It may circulate but it has no more effect than room air circulation.  Is there any plus value to having an air space under the drywall?  With modern construction and heavier insulation, there is no longer a problem of condensation on drywall caused by the studs being cold.   In fact they strapped walls a long time before we started using poly vapour retarders and their habit today is to install the poly first, then strapping, then the drywall. 

Placing their vapour barrier on the insulation side of the air space means that their strapping has no real effect on the performance of the wall. 

Those contractors forgot to read the electrical code that does not allow unprotected wires directly behind drywall!  This inversion causes warm possibly moist air to rise and deposit that moisture on the cold wooden floor structure, with the cold air falling to the bottom. 

What causes condensation on the wall when there is no air space? Of course water outside and a leak in the wall will cause moisture problems in any basement wall.  In new construction, heavy condensation is often seen on the inside of the vapour barrier in a basement.  This is simply the 600 or so gallons of water, used to make the wall, trying to escape.  Unfortunately in modern construction we are always trying to finish a house too quickly – in the old days we waited a full season before finishing a basement after construction, allowing it to dry out. 

Surprising as it may seem, even in 2016 basement moisture control is an evolving science.  Here is a link to some of the history and some of the progress.  The concrete of a basement wall insulated on the inside will have a very large temperature difference between the top of the wall and the bottom of the wall.

It is almost impossible to ensure that there is absolutely no space between the front of the insulation and the drywall.

Space Within Masonry Wall Needs Ventilation

Hence we find a very strong convection current that loops around the insulation.
This same mechanism does not happen in such a serious way with an above grade wall totally exposed on the outside because you have an evenly cold exterior, not the large temperature differences that exist from the top to the bottom of an internally insulated basement wall.

The convection loop will draw moisture both out of leaks into the wall from the house and out of the lower portions of the concrete itself.

Repeated freeze/thaw cycles can cause spalling or flaking of the outer surface of the basement wall. Convection loops around your insulation will essentially eliminate their insulating effect, carrying the heat around the insulation to the cold wall behind. With no air currents, the only moisture that can get through the wall is what can diffuse slowly up to the top of the wall and out through the wall without causing saturation conditions.

About 4 feet up the wall you should be close to the soil grade on the outside.  If you carefully slice open the wall paneling (don’t cut through the studs) at about 4 feet from the floor exposing about two inches, you can then run a knife through the insulation (foam or batts) right down to the air space. 

Pull out that plug of insulation carefully and set it aside.  When you can see the wall, or the moisture barrier on the wall, use the spray foam in a can to fill up the two inch strip behind the insulation. 

Be sure to spray behind the studs if they do not touch the wall.  Put back in your insulation, even put on the slice of paneling you removed.  You can either use drywall tape to smooth out the wall, or a piece of trim to look a bit like an old chair rail. 

You might even want to do this whole operation a bit lower than 4 feet so that it really is at chair rail height.  That practice disappeared with the advent of more durable “drywall”.
On the room side, this air space is generally fairly cold at ground level and much warmer at the top of the wall – just like in the house above. 

Because soil insulates, the very bottom of the wall is close to the same temperature as the bottom of the wall inside the basement.  The top of the wall behind the insulation is freezing cold.  So generally the air just doesn’t move and the air space between the insulation and the vapour barrier doesn’t cause any problems, as it can do in above grade walls.

Air will tend to rise and if it can get out the top of the wall it will simply circulate with the air in the room or in the heated joist space. 

If you want to add exterior insulation it is an unsettled debate as to where to install the air-gap membrane.  Generally it is installed directly against the concrete and the insulation installed over the air gap membrane.  Critics say that this reduces the effectiveness of the insulation — some say perhaps as much as 10%.  If the insulation is installed first and the air-gap membrane placed over the foam panels, critics say that the pressure of the soil will cause the dimples to sink partially into the foam, reducing the drainage effectiveness of the membrane. 

This is very effective, but of course two extra layers of material make this much more expensive.
Creating an air barrier is usually part of soundproofing, such as caulking all holes between the two living spaces, because much of the sound we want to block actually travels through the air. 

A sealed plastic sheet could be used as an air barrier but its vapour blocking properties has no positive or negative effect because there is no temperature difference between the two areas and the whole assembly is too warm to cause condensation.

When you put insulating or specific sound proofing batts into a ceiling/floor space, leave about 1/3rd of the space empty.  That air space actually helps to break up the reverberations and frequencies of the sound that is passing through.  Having an air space in a sound proofed division between two rooms usually preforms better than filling the whole space with insulation.

Historic Brick Insulation Retrofit