Tag Archives: condensation

What Causes Roof Condensation And How You Can Prevent It

What Causes Roof Condensation?

Moisture can be sneaky.  If you’ve got a roof that doesn’t leak, you may think you’re protected from moisture damage.  But the truth is that condensation is one of the chief forms of moisture damage in roofing systems.

The straightforward way of determining roof condensation causes is lack of ventilation.  But that answer raises many questions, like how to vent roofs well enough or what to do when roofs can’t be vented.

There are two different roofing paradigms when it comes to these issues: one for steep slope and one for low slope.  The physics are the same for each (obviously), but the concerns and practices for each differ.

In steep slope roofs with attic space, ventilation is a reasonable tool to counter the risk of condensation.  But in low slope roofs with no attic space, ventilation is typically inadequate.

The Physics of Condensation

The air around us contains water in its gas phase (water vapor).  Temperature determines the amount of water vapor air can hold.  When temperature decreases enough, the water vapor condenses into its liquid phase.

Remember how the mirror fogs up when you step out of the shower?  The mirror is a cold surface (condensation needs a surface), and the warm water vapor condenses as it lands on this cold surface.  The same thing frequently happens in roofing systems and attics, causing all sorts of damage.

The temperature at which vapor condenses is the “dew point.”

Imagine outside temperature is 45 degrees and inside your home is 70 degrees.  There will frequently be a dew point between those two temperatures, which means that at some point between the inside and outside of your home, water vapor condenses as hot travels to cold.

Roofs and attics are of particular significance since vapor can be carried by air, and air rises.  Without adequate air barriers enveloping your home, warm air with vapor will contact a cold surface and condense.

Vapor diffusion is the method by which vapor flows through solid material.  So, even if air travel is blocked, vapor can still travel through objects and condense on their cold surfaces.

For steep slope homes, the strategy for reducing the risk of condensation is by ventilating the attic.  We want the attic temperature to be as close to the temperature of the outside as possible.  If it is not, there is likely a dew point at some point in the attic (often noticed on underside of nails holding down shingles).  This can result in a roof going bad before the roofing shingles go bad, because the sheathing can rot.

With enough attic ventilation (both intake and exhaust), the risk of condensation on the roof underside can be significantly reduced.  But in that case there is still a dew point somewhere between the conditioned living space and the attic.

With a vapor barrier enveloping the living space and enough insulation in the attic for heat transfer resistance, the amount of vapor that gets into the attic can be minimal.  And the vapor that does can be swept away by the ventilated space.

Signs your roof may have condensation issues

  • Get into your attic.  Is it warm?  It should be really close to the same temperature as the outside
  • On a frosty morning, can you see the rafters outline from the outside?  Is the frost on one part of your roof melting that probably shouldn’t (not in direct sunlight)?  Are the overhangs frosty while the roof above living space is not?
  • Perform the smoke candle test.  Light a candle and put it up to your soffit intake ventilation (usually bird blocks beneath your gutters).  Find where that smoke goes.  If it exits your mushroom vents near the ridge of your roof or out of a ridge vent (depending on which you have), then ventilation may be good.  Otherwise it is probably not.
  • Is your roof overly mossed?  Sure some moss can be normal, but if you have a lot or if it seems to be in spots it maybe shouldn’t be, condensation can be feeding it from below the roofing shingles.
  • Do you have bird blocks?  Sometimes these are adequate intake ventilation, but often they’re not. Smart intake vents like Cobra IntakePro may be needed.
  • Is insulation blocking your soffit vents along the eave line in your attic?

Addressing flat roofs with no ventilation

Most commercial roofs (and some residential) are flat or low slope and don’t have much ventilation.  These roofs require a bit more sophistication to counter condensation.

As the roofing industry improves, understanding of condensation in roofing systems increases and our ability to solve it improves.

The strategy is to use a vapor retarder on the roofing substrate.  There is some difference of opinion on whether a retarder or barrier is best.  Because vapor does get through even the most well designed systems, it is believed a retarder is best because they have some level of breathability that can allow vapor to exit more easily.

Overlapping rigid insulation should go in the middle and the roofing membrane over the top.

This creates a roofing system with vapor retarder/barrier on each side and strong thermal resistance through the middle.  With the dew point in the insulation, the idea is for there to be little diffusion of vapor to a surface on which to condense.

And when that does happen (because no system is perfect), the roofing membrane itself plays a pivotal role in reducing damage by how it heats up.

It is well understood that some amount of water just finds its way through so many large, flat roofing systems.  But that water can dissipate seasonally (or daily) when the roofing membrane reheats by absorbing solar rays.

In recent years, white roofs for large, flat buildings have become wildly popular.  White is very reflective and much of the solar radiation that hits it isn’t absorbed.  The popularity of white roofs may be for environment-related concerns, but there’s an irony in that the reflective nature of white roofs may be causing unnecessary damage, which in turn costs energy to fix.

Gray is an option growing in popularity because of this.  It is less reflective than white and does have some ability to reheat.

But it may be the case that the only way to robustly protect against condensation in most flat roofs is by using a dark material, like modified bitumen.

Even then, there isn’t true certainty.  For homes and businesses alike, ventilating the living/working space may be required.  Even the smartest roofing system can’t avoid condensation if there is too much vapor in the conditioned space.

Extra special thanks to the Roofing Contractor’s Association of Washington (RCAW).  Their series of condensation and ventilation related classes at the annual trade show of 2019 was a brilliant move.  When we saw the schedule, we were expecting them to be great.  And the classes turned out better than that.  Kudos to GAF’s James Willits, Owens Corning’s Rollo Gallion, and Wetherholt and Associates’ Ray Wetherholt for their informative presentations!

Ready for a beautiful new roof?  Call Chase Construction North West, Inc. today (253-445-8950) to schedule a free estimate.  Or complete our quick estimate form.  Join thousands of happy Puget Sound customers with gorgeous roofs that protect their homes!

The Uncool Effects of Cool Roofing

Cool roofing may not be as “cool” as we think!



Beginning in the 1980’s research on the benefits of cool roofs was conducted by the Department of Energy in hopes to find a way to save on energy costs and reduce our carbon footprint on the environment. The widely accepted belief that has developed from this research is that white roofs are the most environmentally friendly and energy efficient choice when it comes to roofing. This “cool roof” solution has become a quick and low cost solution in hopes to reduce carbon emission which is known to effect climate change. However, it has been said that cool roofs are not a “quick fix” to save on energy costs and that in certain climates they may actually increase energy consumption.

Due to the belief that reflective roofing is the easiest solution to lowering energy costs and reversing the effects of climate change, there has not been much consideration of geographic location . Although roofing professionals know that depending on geographic location and weather conditions there is a roofing system required for each roof.

Reflective, or what is known as “cool roofs” have been proven to reduce air conditioning costs during hot summer months, which in return provides home and building owners a great deal on energy savings. However, take into consideration colder, northern climates where people must spend more money heating a building rather than cooling them, the cool roof solution may not be a smart choice for them. In return, a reflective roof may actually increase carbon emissions due to increased energy consumption.


Effects of Cool Roofing in Cool Climates

Aside from inefficiency, reflecting roofing systems can have negative consequences in cool climates. The roof cover only concept regarding white reflective roofing implements only changing the color of the roof top from black to white. However, this concept ignores the components that are underneath the surface and how they are effected by the transition from black to white. One issue that can arise from cool roofs being installed in cooler climates is condensation. During the summer surfaces stay cooler, however when temperatures are colder the surface remains cooler for a longer period of time. In return it will fall below the dew point and remain there longer compared to darker membranes, which leads to condensation formation.

Condensation is one of the most potentially hazardous consequences of cool roofing. How does condensation occur? Warm air begins to rise toward the roof when temperatures outside drop. With no air or vapor retarder the warm air that rises makes contact with a surface that has dropped below the dew point and forms condensation.










Warning Signs

There are several signs that can reveal the presence of condensation. One way to tell is when you walk on top of the roof, if there is a crackling noise below your feet it is a dead give away that there is frozen moisture in your roofing system. You can come across this situation typically when a roofing system is insulated with a single layer opposed to multiple layers. When multiple layers of insulation are implemented in a roofing system, the joints end up staggered which in turn makes warm, moist air to reach the underside of the roof membrane. Therefore, using multiple layers to insulate your roof, although not foolproof, can help to decrease the level of moisture collecting beneath a roof membrane. Another issue that can arise from the use of a single layer of insulation is that moisture can free between the insulation joints, which causes it to expand as it begins to freeze and pushes the edges of insulation boards apart. Moisture will also lead to the insulation warping. Therefore, when frozen inflation experiences warmer temperatures, the frozen moisture will begin to drip inside the building.  Another unfortunate result of condensation is adhesion loss. When moisture accumulates and does not have the opportunity to dry, insulation facers can weaken, and therefore the membrane can detach from the insulation which causes reduced wind uplift resistance.


How to avoid condensation

When choosing  a roofing membrane color make sure you take into consideration the climate in which the roof will be installed. Typically in northern and colder climates, a dark membrane is used, where in warmer southern climates, a white more reflective membrane is used. However, if you are to implement a reflective membrane in a cooler climate, one way to try and avoid condensation is to use a continuos air or vapor retarder, as well as using the multi layer insulation method.

Recycle Building

Although reflective roofs are installed with good intentions, in hopes to reduced energy costs as well as our carbon footprint. Do not forget that these benefits are typically seen in warmer, southern climates. Therefore, if you are going to install reflective roofs in cooler climates make sure you implement certain design modifications to help reduce the presence of condensation, because when installed cautiously and correctly, cool roofs can perform quite well in cooler climates!

Chase NW Roofing is factory trained and certified with GAF, IKO, Versico and Custom-Bilt Metals to install almost every roofing application. We specialize in composition shingles, torchdown, single ply, metal roofing, wood shakes, slate and concrete tile. We have been providing quality re-roofs in the North West for over a decade. If you are planning to re-roof your house, need an inspection give us a call at 253-445-8950, or fill out our free estimate form!