Spray foam hybrid system/dead air R valu
I am considering using a polyurethane spray foam in my walls and roof. Costs of the product are forcing me to give consideration to a hybrid spray foam/batt combination in the roof. I would like to achieve a R-50 in the roof. Is there any condensation issues with using the foam on the first layer and then batt insulation under it sealed with a vapor barrier. If it is an expectable building practice any reference material would be useful to show the building inspector who is considering it at the moment.
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Does dead air space have a measured R- value? In my walls I plan to strap the 2×6 stud on the exterior with a 2×2 and spray 4′ of foam. This leaves me with 3′ of dead air space for electrical. If this air space is sealed, is there a measured R-vale for this space.<!—-> <!—->
Trying to get an unbiast opinion for the R-value of polyurethane spray foam. What value should I use?
Thanks for your insight
Replies
Amost any and all insulation is a way of creating dead air space.
The space you envision is not dead air.
Even in stud cavities filled with batts, a well known pphenomenon called a convection loop exists. It works like this - Th ecold surface near the exterior chills the air in contact with it. At the same time, the warm interior surface is warming the air in contact with that surface so it is rising, while the outer slice of air is falling because it is denser.
Sop the air in a stud cavity is in constant motion, transferring heat energy to the exterior surface.
This would be reduced somewhat since you plan 4" of foam. If this is urethene foam, you will have penty of insulation anyways.
same with roof. I have mixed a couple inches of foam with 9" of FG. I think th efoam did 90% of th einsulating.
Don't forget the sills at foundation. Infiltration control is a large part of insulating ahouse and that is one thing that spray foams excell at.
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Does dead air space have a measured R- value?
Yes, for all intents and purposes, it's R1 per inch.
Except around engineers, who might quibble that it's 0.987-something R per inch; and that it's defined by at least two boundary layers which are different R values, and that the convetion, conduction, & radiation mean that it's never really "dead," . . .
R values for truly dead gas spaces at +30 F:
Air: 6.0
CO2: 10.0
Argon: 8.8
Krypton: 16.4
As pointed out, getting truly "dead" air is elusive. For air, density difference between room temperature and 0 F is about 15%. Any density difference at all introduces convective currents in open gas space. Measurements done by Oak Ridge National Lab early 90s show that FG will start displaying convective air movement within the batt at around 25 degrees temperature difference. Temperature profile across a hybrid of two different kinds of insulation can be calculated. The drop across any layer of the assembly is proportional to its fraction of the total R value of the assembly. For a really cold climate, you probably don't want the FG batt to provide any more than about a third of the total R value, for the FG part to be most effective. Cellulose would be far better in the respect. Also, if you have an impermeable foam plus a vapor retarder on the warm side of the FG batt, then you have a double vapor barrier situation. Sealing will be important, as any moisture that gets past the inner VR must be able to diffuse readily past the outer VR (foam), or accumulation may occur. Since inside air at 70 F and 40% RH will have a dew point of 46-47 F, if you have any convective leakage of air past the inner VR then you have a potential problem. You could theoretically skip the inner VR as long as the foam keeps the temperature at the warm side of the foam above the dew point, meaning you wouldn't want a little foam and a lot of FG situation.
Our building inspector is saying that we need a vapor barrier on the warm side of the wall if we use foam, irrespective of using batt or any other kind of hybrid system. Is this a bad practice because of the potential of moister penetration into the wall cavity? The inspector comes from the SIP industry were he experienced moister penetrating the joints of the SIP when a vapor barrier was not used. Is there parallel logic to apply the same mitigating measures to a spray foamed house?<!----><!----><!---->
If you are using Corbond spray, they will help you with the inspectors and codes.
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Urethane foam has an R-value of 7 per inch. This is the long term R value, initially it is a bit higher.
We use a cimbination of the two often. In addition to the cost savings, it seals the building against air infiltration. With the foam on the exterior side, the benefit is that no moisture can condense in the wall assembly because it can't penetrate the closed cell urethane and reach the spot where the temperature is at the dewpoint. A 2" foam application is usually enough to achieve this goal.
Thanks for your insight. Do you use a second vapour-barrier on the inside in the hybrid batt/foam system? Does the due point fluctuate with temperature? I live in the Canadian North where -50, although extreme is a possibility. I will be making a case to the building inspector on this subject, are you aware of any studies on this?<!----><!----><!---->
Thanks again for your help.<!----><!---->
Mark
You want studies these folks have all kinds:http://www.buildingscience.com/buildingphysics/thermalcontrol
Yes, we do use a 4 mil vapor barrier. I'm not clear on your dew point question. The dew point does change with the temperature. I think you are asking whether the dew point location changes within the wall, moving further inward as the outside temperature drops. I have no doubt that it does. It would be best, of course, if the foam was thick enough so that the dewpoint always occurs within the foam. Perhaps the link you were given above will give you some insight on this. I'm sure you know that at low outside temps. you don't want too much humidity inside. I don't think in Centigrade, but at -20º Farenheit, you don't want much more than 15% relative humidity inside.