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I just had a new roof installed and it looks decent enough. They replaced my metal ventilation with some plastic vents, which is fine, but I'm not sure if it was enough. I had a two layers of asphalt shingles before. It was a pretty thick roof.

Sometimes when I wake up in the morning, there is a very clear line of moisture that exists over the house, but not the garage. (it could be vice-versa, but I'm pretty sure it's over the house) This was not the way I remember it before.

Now he's mumbling something about insulation, but none of that has changed. What should I do to verify that the proper amount of ventilation or types of vents were used? Is this even my problem at all?

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  • maybe you just now notice it because it is new? The dew should burn off when the sun comes up.
    – Engineer2021
    Aug 17, 2010 at 12:26
  • Possible, but the stark nature of the line is a bit worrysome. How will this wear over the years if more moisture lies on one side versus the other. I'm in Michigan, so I'm also waiting to see how this plays out with snow. It will be obvious with snow if I see a lot more of it. Is it possible that the very thick nature of the double roof was keeping the roof temperature more consistent across the length of the roof?
    – cgp
    Aug 17, 2010 at 13:09

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You mentioned snow, and that's the only metric i know about. You want there to be more snow.

When there is frost, or snow, on your roof - it should not be melting from the heat of your house. The attic is supposed to be insulated, but the roof is supposed to be not insulated.

In my (southern Ontario) winters there will be frost on everyone's roof. If frost is melted where the roof joists are - i know their attics are not insulated enough (or the roof is over-insulated). In other words, this is bad:

alt text

The roof is supposed to be freezing, just like the outside. Melting snow means that heat is leaking.

Snow is how i know i can see when an attic is not sufficiently insulated, or the attic is not letting out enough heat, or the spaces between the rafters do not have those styrofoam channel things which let cold air reach the underside of the roof sheathing.

But all you have to go on is moisture, and dew. i don't know how dew reacts with an under-insulated attic/over-insulated roof.

i would assume everything's fine. i know that dark materials, while the best at absorbing heat, are also the best at releasing heat. i would think that the dark shingles have cooled off during the night, and water is condensing out on them.

But if it's the middle of the (hot August) day, and the shingles are still so cold that water is condensing on them - then something has to be freezing your shingles. That would have to be your air-conditioning; leaking to your attic and roof.

But insulation is not the job of the guy who replaced your shingles. And the fact that the roof can cool off enough during the night to allow condensation - makes me think that your attic is vented fine.

i'd say not worry about it, and wait for fall and check for melting frost, or snow, come winter.

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  • attics are pretty cold in the winter to wait and see if he needs to add / remove insulation :)
    – Engineer2021
    Aug 17, 2010 at 19:02
  • ok, these are both good points, they were asking me to wait, I was getting nervous because hey, I just spent a decent chunk of cash on a new roof, and naturally, I'm going to be edgy about anything that seems out of sorts. I feel a bit more reassured that waiting until winter might be ok.
    – cgp
    Aug 18, 2010 at 11:17

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