Thursday, June 20, 2019
Condensation in Dense-Packed Cavities

Condensation in Dense-Packed Cavities
by Charlie Holly

Last week I got a call from one of my favorite Maine contractors, Bo Jesperson of The Breathable Home, located in Augusta, Maine. As part of a renovation of his own walk-in attic, he was tearing into some roof slopes that he had dense-packed with cellulose several years earlier. The interior of the slopes were plastered with no cracks and painted. The roof was nearly new; no leaks. The cellulose looked great with no voids or degradation of any kind. All looked perfect except that there was condensation on the underside of the roof sheathing. How was moisture getting into this assembly?

In this article we will use three RED Calc tools to argue that the moisture was already in the cavity, stored mostly in the cellulose, and that the condensation is to be expected. We will then show that, at least in this case, the moisture is probably not causing any harm.

Estimating the dew point inside the cavity

As we know, water vapor condenses on a surface because the surface's temperature is colder than the dew point temperature. The outdoor temperature was around 15°F that day. What should we expect the dew point in such a cavity to be?

Weather Station Data tool

The cavity being enclosed by vapor retarders (painted plaster and roofing materials) indicates that moisture will pass in and out rather slowly. Since most of the moisture that does get in will be adsorbed by the cellulose (more than 99 percent) we should expect that the dew point of the air in the cavity will change very slowly and will be close to the long-term average dew point of the air adjacent to the cavity.

Using our Weather Station Data tool, we found that the annual average outdoor dew point for Augusta is just under 36°F. Because this house does not use air conditioning or other dehumidification during the summer, or humidification during the winter, we expect that the average interior dew point will be a little higher than outdoors due to typical sources of moisture inside the home. So the dew point in the cavity should be close to 36°F, or a little higher, reaching its highest value at the end of summer and its lowest value at the end of winter.

If the dew point temperature inside the cavity is a bit higher than 36°F, it is likely we will find condensation on the underside of the roof sheathing whenever the outside temperature drops below 36°F.

But will the moisture cause harm?

The next obvious question is whether this condensation, or more generally, the moisture in the cavity, is likely to cause problems. One obvious problem would be the rotting of the roof sheathing. For wood to rot, it must be at a temperature between 40°F and 105°F and have at least 20 percent moisture content.

Wood Moisture Content tool

Using the RED Wood Moisture Content tool, we find that 20 percent equilibrium moisture content in the roof sheathing corresponds with 88 percent relative humidity in the air near the sheathing.

Moisture Metrics tool

Setting the dew point temperature in the Moisture Metrics tool to our 36°F and adjusting the (dry bulb) temperature until the RH is 88 percent, yields a temperature of 39°F at the roof sheathing. If the roof sheathing is above 39°F, it will be too dry to rot. So, given all our assumptions for this example, the roof sheathing never sees conditions favorable for rotting; it is always either too cold or too dry.

In other climates, or under other assumptions about the indoor conditions, the calculated temperature at which the roof sheathing has 20% moisture content can be somewhat higher than 40°F. In that case, the roof sheathing will have an increased potential for rot for outdoor temperatures in a narrow range above 40°F. The question then becomes, how much time is spent in that narrow range, and is it enough to actually cause problems.


  • Using selected RED Calc tools, we can explain finding condensation in this dense-packed roof by assuming that the moisture stored in the cellulose is the source of the condensation.
  • We are likely to see condensation (or icing) in cavities with dense-packed cellulose when it is sufficiently cold outdoors.
  • In many situations, it can be shown that the moisture in cavities with dense-pack cellulose will not cause problems.

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