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Exterior Wall Heat Loss

I'm concerned about heat loss through my exterior walls. My home shows frost patches on the exterior walls stretching from above the foundation to the attic. The areas vertically between the patches are bare of frost, looking as if that is where the framing runs. The patches are approximately 8" wide and the height of the siding. There is approximately 4" of bare space between each patch horizontally on the siding. This pattern manifests all the way around the house

The siding is concrete composite. Am I losing heat through my walls? They have R-21 insulation. What can I do about it?:(

Timothy Miller
Re: Exterior Wall Heat Loss

Hi thats allow of moisture . Have you had a broken pipe that flooded the home? If you have a humidifier on the furnace shut it off and have it checked to see if it is working properly. The baths have exhaust fans and are they used and left on 15 minutes after showers?

This is aggressive but you may want to have observation holes cut into the wall drywall to see if there is insulation in these areas but that would be the last not first thing i would do.

Re: Exterior Wall Heat Loss

Hi Tim, there are no water leaks, no humidifier and the bathroom fans are all on timers to run for twenty minutes after shower use.

The builder insists the walls all have R-21 bat insulation. It's hard to believe since it's such a dramatic pattern all around the exterior siding. Would a FLIR show if there is a problem?


Re: Exterior Wall Heat Loss

Does this happen on all sides of the house?

I'm certainly no expert but here is what I think. Frost is going to form on a cold surface, not a warm one. That means that the areas where the frost is occurring are well insulated. It appears that the warmest part of your wall is the framing. That is typical. You have insulated between the studs to R-13 (if it is a 2x4 wall), but the studs themselves are not R-13 and more heat is being transfered to the outside through them. This is called bridgeing. To reduce bridging you can lay foam insulation sheets over the framing. It can be done on the inside or the outside. This will certainly not be worth the expense unless it is done during construction or a remodel.

Re: Exterior Wall Heat Loss

Thanks Drooplug, I was afraid of that. The framing is 2x6 and it appears that what you described may be the case. It's amazing that it is so widespread around the exterior. There appears to be enough heat loss that it melts the frost.

Timothy Miller
Re: Exterior Wall Heat Loss

Hi consider applying a moisture retarding interior wall paint to reduce the moisture transfer into the wall cavity. Several paint companies sell it. Be sure and check the attic to be sure the bath exhaust fans are not vented into it but extend threw the attic and out of the roof. Be sure and check the attic for venting are there eve vents and how many roof vents near the top does the home have? Again you have way too much moisture in this wall and this is not normal but likely to create mold and rot issues and delamination of the siding.

Re: Exterior Wall Heat Loss

The frost is on the outside of the house. Interior moisture has nothing to do with it.

I think you shouldn't be too concerned with this heat loss. The fact that your walls are 2x6 with R-21 is really good. Better than the typical home. What you are witnessing is the improvement of having that R-21 over an R-13. You are retaining far more heat with your walls. It's retaining heat so well that you see frost on your siding and that is a good indication. The irony of this is that if you had R-13, you wouldn't see any frost and wouldn't have questioned anything about it.

Re: Exterior Wall Heat Loss

I did some more research after your intial reply and would have to agree with your diagnosis of the pictures. It's not a moisture problem at all. I just wish the builder had installed a thermal break with the exterior sheathing. That probably would have finished the job and not cost them much in the process. It's supposed to be an "Energy Star" home, so expectations were a little higher as to what we were purchasing.

I want to thank you for your help on this issue. You were right on the money.:)

Re: Exterior Wall Heat Loss

Typical exterior wall would have 2x4 studs with r-11 or r-13 insulation between them. The studs are pine which has an r-1.25/inch. A typical 3.6" stud has an r value of about 4.5. Since the studs make up about 10% of the wall, the average insulation is going to be somewhat less than the r-11 or r-13 that most people think they have.

There is also lateral heat transfer from the siding. That is that heat traveling thru the stud, then travels laterally because of the low r value of the siding. That is why the dry patches are about 4' wide instead of only 1.6" wide.

Now your walls are 2x6 studs with r-21. So you have studs with an r-7.5 making up about 10% of your wall. A lot of houses with 2x6 wall use a 24" center which reduces the % of wall that has the lower r value. The wall is just as strong as a 2x4 on 16" centers, but some local building codes do not allow the wider spacing. Apparently yours are that way or your builder wasn't as comfortable with the wider spacing, but it is a little more efficient.

My concern with the type of wall you have is that the insulation is packed in pretty tight and doesn't allow much ventilation of the fiberglass. A 2x8 wall would provide better ventilation of the fiberglass, which makes it more efficient. The insulation should have its air gap between the siding and the insulation instead of between the insulation and the sheetrock.

If this had been done, you would either have no frost on the outside or it would be a lot more uniform. A thermal brake between the studs and the siding would also have helped a lot.

At this point in time, there is no economic reason to make any changes. An added thermal break might save a few dollars in heating cost per year, you would never recover the cost of putting it in, not even close. If 50 years from now, the siding has to be replaced, then the extra cost of putting some furring strips in the outside of the studs for ventilation and then a layer of foam to make a thermal break might be recoverable.

Re: Exterior Wall Heat Loss

I hadn't considered that the insulation batts would have been packed in a bit too tight. That makes sense because an R-21 would likely be touching (or close to it) in a 2 x 6 cavity.

I also hadn't considered that the lateral heat transfer could be so great given the thickness of the studs and the amount of insulation.

You're right, there isn't a good economic reason to fix it. I guess at this point it's crying over spilled milk. I'll remember this lesson in the future; Energy Star can have limited flavors depending on the builder's commitment and integrity. Some do the minimum just for the marketing.

Thanks much for the knowledge. I really appreciate those more experienced taking the time to teach.

Re: Exterior Wall Heat Loss

I would agree with the information about this being common -- for one.
The exterior surfaces reaching dewpoint with night time cooling and thermal bridging --- second.
Interupted method of insulating between studs and rafters actually has a less R value averaged over the entire wall or roof assembly. --- third.

However, the comments about an air gap between the batts of insulation ( within the stud bay ) and the exterior wall sheathing is incorrect. There should never be a gap of any sort throughout the entire cavity. Fiber glass batts will have initmate contact with both the exterior and interior walls in order to completely fill the cavity. Any gaps will allow air movent ( stack effect ) within the space which has a negative impact on fiber glass insulation by reducing it's R value .

The main point of insulating a wall cavity --- in this case -- for preventing heat loss and condensation issues is to raise the temperature within the space. Having the insulation in direct ( intimate ) contact with the exterior wall provides the seperation barrier between the colder outside and the warmer inside. If you leave a gap from the exterior you allow the cold zone to move further into the wall cavity defeating the the intention of raising the temperature with the wall. Moving the cold zone in further means more heat loss and condensation potentials.

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