frost on exterior walls

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  #1  
Old 12-20-06, 08:16 AM
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frost on exterior walls

I painted my house this summer -- deep red -- and now that it is getting colder, I am noticing frost on the exterior walls. The frost seems to be heaviest between the wall joists, which makes me think that the insulation is the culprit. But the previous year, I did some renovation on one side of my house, and the insulation on that side (south) is brand new. The insulation on the other side (north) is probably original to the house c.1975. There is no difference between the north and south side of the house. (the east and west sides are shaded by tress and I don't see frost there). The house is 2x4 framing, no exterior wrap, T1-11 siding.

This could be normal, and I just never noticed it when the house was painted a lighter color. But any perspective you have to offer would be welcomed.

Thanks.
 
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  #2  
Old 12-20-06, 09:20 AM
Join Date: Mar 2005
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frost on exterior walls

What you are seeing is the pattern of the heat loss. It happens every day but you are seeing in now probably because the color of the house. The dark color allows the lighter frost to show.

You have frost in the area between the joists because there is less heat loss, so it is cold enough for water to condense or frost to form. If you had less insulation, you would not have frost on the exterior.

In the areas of the studs, you do have heat lose directly through the studs. It is a graphic example of the problems of using insulation batts between the studs. It is, in effect, a "thermal short circuit". If you had rigid polystyrene, this effect would not occur.

You have R13 or R15 or so where you have insulation and only R2 or R3 where you have studs. That is why you do not get an R13 wall (probably R10 or R11) when you use R13 insulation. - The pink panther advertising strikes again!!

It could be worse - you could have steel studs, which really have "thermal short circuit" and you can lose up to 40% of the insulation depending on the wall thickness and stud spacing.

Dick
 
  #3  
Old 12-20-06, 09:29 AM
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Also, is there a vapor retarder and is it placed over insulation on heated side of wall? When warm, moist air hits cold surface, it condenses and can freeze.
 
  #4  
Old 12-20-06, 10:10 AM
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Thanks for the information about heat loss through the studs. Makes sense.

Just to be clear then, is this frost "normal" in a typically insulated house? Or is there something additional I could/should do to improve the insulation value of the walls? I am (very) slowly renovating the house, so a change could easily be added to my list of things to do.

As for the vapor barrier, the insulation I put in was the typical kind with paper backing, installed on the heat side. Based on what I have seen in my demolitions, the rest of the house is similarly suited. There is no tyvek or other wrap on the house. It is just T1-11 nailed to the studs.
 
  #5  
Old 12-20-06, 10:51 AM
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frost on exterior walls

This is still early in the season, but the fact you have frost on the walls means there was not enough heat loss to prevent frost or melt the frost.

Tyvek is not a vapot barrier, so it will no decrease the amount of moisture coming out of your house. Your vapor barrier should be on the warm side of the insulataion.

Dick
 
  #6  
Old 12-20-06, 11:17 AM
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Is the moisture coming out of my house, or condensing from the outside air? The vapor barrier would have effect on the former, and not the latter. Since I have the insulation installed correctly with the paper vapor barrier on the warm side, I think I should be OK with moisture coming from inside. Unless you are suggesting that I install a separate vapor barrier on top of the insulation, warm side.

The tyvek, I guess, is an air barrier. So it doesn't come into play here at all.
 
  #7  
Old 12-20-06, 11:51 AM
Join Date: Mar 2005
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frost on exterior walls

I gave you the scenerio based on the materials working as they should. I cannot imagine how you would get enough moisture coming through the 1-11 under the mild weather conditions so far this winter.

Fiberglass is a horribly over-rated insulator in comparison the the claims based on actual real-life situations. After seeing Katrina problems, I have seen how much moisture it can hold (not absorb), which dramaticially reduces insulation values, but I doubt you are in that realm of a condition.

I think you are just seeing the frost because of the color change and the heat loss pattern may be scary. If you do not have enough mass in the wall to store heat, it will escape.

One check on you home is to check your heating bills against others in the same area to see if you insulation is effective.
 
  #8  
Old 12-20-06, 12:03 PM
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Wink

Since I have the insulation installed correctly with the paper vapor barrier on the warm side, I think I should be OK with moisture coming from inside. Unless you are suggesting that I install a separate vapor barrier on top of the insulation, warm side.


This is why we put a 4 mil poly over the paper back R13 As a back up with no leaks in it .
 
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