Insulation and Ventilation
R-value, air, vapor
R-value measures resistance to heat flow; thicker and denser materials rate higher. But the practical hierarchy is air sealing first, then insulation, then vapor control. A vapor retarder belongs on the warm side of the assembly - in cold climates, the interior. Kraft facing should face the heated space. Faced insulation installed toward the cold side traps moisture and causes condensation, staining and eventually rot. Never stack two vapor retarders in one assembly.
Venting the attic and roof
Attics need intake and exhaust ventilation, balanced roughly 50/50. Common ratios are 1 square foot of free vent area per 150 square feet of attic floor when there is no vapor retarder, or 1 per 300 when one is present. Soffit baffles keep insulation from blocking intake air. Recessed lights must be insulation-contact rated, or insulation kept clear of them.
Moisture sources
Bath fans, range hoods and dryer vents must discharge outdoors - into an attic or crawlspace they dump moisture exactly where you do not want it. Vented crawlspaces use perimeter vents; encapsulated crawlspaces rely on a sealed ground cover and conditioning. Ground covers should overlap and be sealed to the stem wall.
Worked example
An attic of 900 square feet with no vapor retarder needs 900 ÷ 150 = 6 square feet of net free vent area - roughly 3 square feet of intake at the soffits and 3 square feet of exhaust at the ridge, after accounting for each vent's rated net free area.