Non-vented Conditioned Third Floor and Metal Roof

Natalya Pinchuk
3/2/2018
  1. When my husband and I gutted a 110 year old house in Pittsburgh, we were advised to spray the roof deck with 4 inches of closed cell insulation foam. We were told that venting was not necessary since we were going to use the third floor as a conditioned space. The foam was sprayed on the underside of the roof deck and it went over the rafters. Then we installed battens to level the ceiling planes all along the third floor for drywall. In relationship to this being a non-vented roof, does it matter if we go with metal shingle or standing seam? Adding more insulation or battens to the top of the roof deck is not ideal in our situation.

From everything I read the 4 inches of closed cell foam is enough to prevent moisture reaching the roof deck boards from the inside of the house. I know that the low pitch roof in the back will need to be standing seam. Would it be a problem if we went with standing seam all around? Or could this create condensation problems due to low R values in our cathedral ceiling? We could potentially add more insulation from the inside by blowing it in between the drywall and the closed cell foam, thus getting pretty close to the recommended R values. Or would metal shingles on the main roof be a better solution because there will be a built in thermal break between the roof deck and the metal? My main concern with metal roof in our situation is condensation.

  1. The house settled in the center over the years. As you can see from images below, the back roof has a dip in it. From looking at pictures I took on the inside when we were leveling the ceiling for drywall, I do not anticipate that the dip is more than 4-5" but I guess that is a question mark until someone goes up there. We know that the back of the house will need to have standing seam roof due to low pitch. What would be the best metal roofing product for the aesthetics with this dip, standing seam or metal shingle?

  2. I have spoken with contractors who install aluminum and steel. Each says that their product is better. Would aluminum be better in my situation because of our unvented roof? Or would steel be just as good? Being able to walk on the roof in the future for a knowledgeable contractor is important. Is that really an issue with aluminum, especially if we need to go with shingles on the main roof and standing seam on both porch roof and the back addition roof?

Any help in answering these questions would be greatly appreciated!

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Todd Miller
Isaiah Industries, Inc.
3/2/2018

What a great looking older home. We never know in the case of condensation what will be the "straw that breaks the camel's back." My hunch tells me that getting an airspace between the metal and the roof deck as a thermal break would be wise. This could be done with metal shingles or it could be done with a vertical seam metal roof installed on battens.

I really do not think that, from a thermal conductivity standpoint, there is much difference between aluminum and steel for your situation.

Anytime you have a metal roof installed with an integral or added airspace (through battens), extra care must be taken for walkability. Many metal shingles have optional foam inserts which fill up some of the airspace but most of them still leave some small thermal break. Regardless, though, the insulation value is also good at creating a thermal break.

As far as the aesthetics of the dip in the roof, battens for a vertical seam roof may do the best job of removing it. Generally, dips are more problematic for vertical seam roofs (both in terms of aesthetics and watertightness) than they are for metal shingles. The interlocks of the metal shingles help a great deal in adjusting the new roof to go over the dip. Regardless, if there is a dip before the new roof is installed, it will likely still be apparent afterward as well.

I hope this helps. Thanks for considering metal.

Natalya Pinchuk
3/2/2018

Todd, thank you for your answer!

Which standing seam manufacturers recommend installation over battens?

If I understood you correctly, we should only install standing seam if it goes over battens in order to create thermal bridge between the roof deck and metal. What kind of underlayment should be used in this case? And should the whole roof be covered with Ice Guard for extra protection or is that silly? Also, if there is condensation, should the battens be protected in any way?

If we decide to go with metal shingles and there is condensation, would it be better to have the asphalt shingles that are already on the roof? Same questions with underlayment and Ice Guard as with standing seam.

Thank you so much.

Todd Miller
Isaiah Industries, Inc.
3/2/2018

Thanks. You have a lot of questions there. I will try to help.

First of all, the condensation concern is not about condensation on the back of the metal panels. The concern is that the metal roof, if in direct contact with the structure, will conduct cold and cause condensation in the attic. Again, this is a situation where you don't know what might be the straw that breaks the camels back. You might be fine without any of this.

Most standing seam manufacturers make products that CAN be installed over battens. That question comes down to the particular panel and how it was designed, not what the manufacturer recommends.

If battens are installed, my recommendation is underlayment before the battens. Some folks prefer it on top of the battens.

One option is actually vertical battens and then horizontal battens, leaving vertically oriented chambers that can be vented. This is good in hot climates and also can be good if you have had issues with ice dams in northern climates like yours.

I generally am a fan of synthetic underlayments. If you remove the old shingles, then ice and watershield is required by code on the roof perimeter and in the valleys.

If you have one layer of shingles, they could probably be left, with underlayment installed on top of them. I generally do not advise going over multiple layers of shingles. Leaving the old shingles does help lessen the chance of the condensation in the attic I was warning about earlier.

Natalya Pinchuk
3/3/2018

Todd,

To make sure I understood you correctly, the fear with condensation is that A. condensation could form on the inside of the third floor where the room is conditioned (in my case it is a finished room with HVAC intake and registers) OR B. that the wood roof deck could gain moisture as a result of being in direct contact with cold metal standing seam (assuming 4" of closed spray foam against roof deck on the inside is not enough), and, therefore, possible rot of the roof deck could be expected at future time? I really appreciate all your answers! If it is best for us to have an air gap to prevent roof deck absorbing moisture, it sounds like metal shingles are winning over standing seam.

Natalya Pinchuk
3/7/2018

Todd,

To make sure I understood you correctly, the fear with condensation is that A. condensation could form on the inside of the third floor where the room is conditioned (in my case it is a finished room with HVAC intake and registers) OR B. that the wood roof deck could gain moisture as a result of being in direct contact with cold metal standing seam (assuming 4" of closed spray foam against roof deck on the inside is not enough), and, therefore, possible rot of the roof deck could be expected at future time? I really appreciate all your answers! If it is best for us to have an air gap to prevent roof deck absorbing moisture, it sounds like metal shingles are winning over standing seam.

Todd Miller
Isaiah Industries, Inc.
3/7/2018

The condensation that we are talking about is from moisture inside the structure trying to drive its way to the outside. So, warm moist air is inside a building trying to get out. If it hits a surface cold enough for dew point to occur, then condensation occurs. There are tables on the web showing dew point and the combination of temperature differential and moisture level that must be reached in order for dew (condensation) to occur.

Many people when they think about condensation and metal roofing assume it's from outside moisture condensing on the back of the metal roofing. Rarely does that ever happen because you don't have the temperature differential to cause it. That said, you will have dew form on the outside of the metal roof.

Anything that can be done to seal the warm moist air inside the living space is helpful. Likewise, anything that can be done, such as a thermal break, to prevent the outside cold from reaching the interior humidity level will be helpful as well.

R.A. Knowlton
Certified Installation Trainer and Author
1
3/8/2018

As a contractor for over 30 years, I can say Todd is correct. Condensation is a concern. I have installed at least one metal roof over a closed foam roof deck insulation and it did not go well. The condensation became tapped in the roof decking and rotted it out. It really does not matter what kind of roofing you install, getting the moisture out or eliminating it inside the home is key. As for the sway in the roof. This is an easy fix. My owe home in Kansas City had the same problem. I pulled a sting line across the roof and determined the amount of say. In my case it was 4". I then went to the lowest point and custom cut lumber to pad it out moving from the center out. Then I put 7/16 OSB over the lightweight framing and then installed Rustic Shingle, metal shingles over that. The air space that the new framing created made for a great ventilation system. In the end we cut our utility bills buy
64% and no condensation.

Eric Novotny
An informed customer is the Best Customer!
3/9/2018

Shingles vs. standing seam doesn't really make a difference here. Once the roof plain is treated as conditioned, it needs to be insulated and airtight (which it should be with the CC SPF).

Make sure the sheetrock is airtight, and you are good.

https://buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach

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