Exterior foam creates a thermal barrier between the studs and the external environment.
Not only does that create a warmer house than using batt insulation alone, it greatly lessens condensation problems is walls.
The reason is that by insulating the sheathing over studs, you can usually keep the stud spaces above the dew point so that condensation will not occur.
Foam exterior sheathing also eliminates convection currents within stud spaces for more uniform insulating tempertaures. This also reduces the potential for condensation.
Condensation in fiberglass reduces the ability of fiberglass to insulate.
Therefore foam over studs is much more effective than just fiberglass between studs alone.
But in all fairness, you could get a greater insulating factor using 2x6s with exterior foam compared to 2x4s with exterior foam, but the cost to benefit ratio may still not be there. |
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