WE have frost forming on the underside of the roof decking and on the nails there seems to have adequate insulation R-30 proper vents and ridge vent

  • 1
    Lack of air sealing of the floor below is the usual source of warm moisture. Gaps around pipes, can lighting, electrical cables. The worst offender is a bath fan not connected THRU to the outside.
    – HerrBag
    Jan 8, 2015 at 4:17
  • 3
    Some amount (light) of frost/condensation can occur normally during a rapid transition from warm (above freezing)to cold (much below freezing). This frost should be transitory and dissipate in a day or two. It should not reoccur during stable below freezing temps, unless there is moist air infiltrating (as described in my previous comment).
    – HerrBag
    Jan 8, 2015 at 4:28
  • I like HerrBag's answer. It is the most likely cause of the situation. Jan 8, 2015 at 5:03

2 Answers 2


Are you sure that the insulation is not blocking the soffit vents?

If the insulation is pushed too tightly into the soffit area, the vent won't work properly causing the moisture to build up, causing the frost on the underside of the roof.


Ridge vents are the best venting for a roof because they give continuous venting across the roof. Other vents, such as mushroom vents, can leave hot air trapped at the peak of the ridge. When working properly, airflow is drawn in at the soffits, up through the attic and exiting at the ridge vents. This keeps your roof cool (preserving the life of your shingles), and keeps your house cooler in the summer, because your attic isn't heating your house from the top.

Given that you have ridge vents, you should check that you have continuous venting in your soffits and that they are not blocked. If this is good, you should check that your attic insulation is not blocking the airflow from the soffits (as noted by Johnnyz).

The other thing to consider, if the ridge vents were retrofitted, you may have gable end vents that should be blocked up because they interfere with the airflow from the soffit->ridge.

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