Insulation R-Value and Humidity
Some types of foam insulation are blown with heavy gases such as chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HFCs). However, over time these gases diffuse out of the foam and are replaced by air, thus reducing the effective R-value of the product. This has been shown to have even worse effect when the insulation becomes wet, sometimes lowering the R-value by up 25% or more.
Insulation manufactures definitely avoid emphasizing this type of thing in their literature – finding any data on this is difficult. Finding data on claimed R-values for new insulation installed under perfect conditions is trivial, in fact they beat you over the head with it. As bad as the lack of data is for foams – it’s infinitely worse for blow in insulation products.
I would really like to see a chart showing the R-value per inch for both blow in cellulose and fiberglass showing what happens as the moisture level increases and the temperature drops at various blow in densities. It’s well known that cellulose can adsorb up to 135% its own weight in water. Cellulose is commonly called a moisture buffer these days. What happens to the R-value in humid climates after this so-called buffering? What happens with fiberglass? Where’s the data?
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I am also curious to know this. What we call cellulose in reality contains also borates which are highly hygroscopic salts so the situation only get worse.
I would be interested in seeing a study like that as well but as you can imagine it would be a difficult one to design and implement. It's the air in between cellulose fibers that does the insulating, so if those spaces are filled with water--a very good conductor of heat--it would reduce the effective R-value. But cellulose is very light, so 135% of its weight isn't much, and adsorption means that the moisture sits on the surface of the fibers, not filling the air spaces, so I doubt it would change the R-value much unless it was so saturated that the air spaces were filled. Further, moisture changing from liquid to vapor absorbs a lot of heat energy, so there is a phase-change benefit to insulation that can take on some moisture, raising its effective R-value.
MICHAEL: I see the logic of your thinking, but my suspicions run the other way. Unfortunately, I can't see any way to prove this without actual measurements and there is no incentive (negative incentive actually) for the manufactures of these products to do any such study.
Imagine this scenario - you live in climate zone 6 in a very wet area and it's an unusually wet year - the humidity is above 90% for months, then you have a record cold winter - the cellulose you carefully blew in at 3 lbs per cubic foot now weighs 6 lbs per cubic foot - does the heat loss due to conduction become significant? Does the cellulose undergo significant settling leaving voids?
I don't know the answers to questions like this and I'm suspicious and would like to see some real data.
I would be more interested in why this (theoretical) wall would be taking on this much moisture. A properly designed wall should maintain within reason a constant level of humidity. I would think BSC has done some research on this.
Here is a picture that I took in my attic last summer. Im in midcoast Maine but not on the water. It is pretty clear that the water absorbed by the cellulose does cause some settling. I think in this case the accuvents were to blame. It looks like the moisture in the winter collects on the thin plastic and causes wetting of the cellulose. in the areas not around the plastic the settling was minimal. You can see how the moisture causes the insulation to pull away from the vent. The cellulose was hard and came out in on large piece.