Roof Insulation vs. Condensation

[fruitcakesa]

My shop has an occasionally heated, fully insulated first floor, ie: the walls floors and ceiling and an uninsulated attic.
The two spaces are isolated from each other by an insulated trap door.
The shop roof is made with plywood sheathing, with 1x strapping on 2 foot centers and grand rib steel roofing over all.
The 1x create an air space between sheathing and roofing and vents through a continuous ridge vent.
There has never been any condensation or moisture issues in 18 years of extremely variable weather and temp conditions.
The venting is the key.

 

[bugstruck]

Thank you for taking the time for that reply (twice, apparently ) Keep in mind that my question was specifically aimed at your "sun on the roof on a cold day" scenario, and how that could cause condensation on the underside of the roof deck.

My questions on this project relate specifically to our application - a building that is only heated for about 5 hours a week in the winter.

Also, some have suggested furring out the purlins to create a full air space above the foam board, but this wouldn't eliminate any condensation unless I can create air flow by means of some venting, right?

My question still gets back to whether or not condensation will be an issue in an unheated building.

Thanks.

You are certainly welcome. I may have over-emphasized the "sun on the roof on a cold day" somewhat. Was just advising that with trapped air it could add to the problem. So that condition is not usually a contributor of condensation in volumes that matter on it's own, as the temperature differential inside to outside is limited when unheated to begin with. What I was pointing out is that the reverse can occur to some extent with a hot roof and cold attic for example. I've seen truss fabrication plates sweat under those conditions in a conventional attic but it doesn't last that long. One reason that shingle heat doesn't contribute to the same degree conditioning a space does, is the heat transfer from the shingles through the substrate to any close proximity objects below the roof (that may be cold and condensation capable) occurs fairly quickly. Also those items below sheathing are not a membrane like the plywood which is a large collection surface. Compression and tension members and some truss plates (for example) don't offer much surface area and heats propensity to rise slows it too. No insulation is in the way in that hot roof condition direction so those objects below warm fairly quickly. Durations for a condensation building event are relatively brief with heat on the roof. The real nemesis is heat from within and no air gap or decent ventilation below the substrate. And this is obviously more apparent with a metal roof than with plywood. The process of condensation build on cold metal is certainly quicker and it can't absorb any water deposited to it's surface. Plywood is definitely less problematic in marginal situations but back to the ventilation. If you let that condensation keep accumulating just below the substrate and don't get enough air there, that is when problems occur.

I saw a lot more of this in the 1970's and early 1980's when the 1940's, 1950's and 1960's attics with poor to no ventilation had issues. By the mid 1960's they were figuring it out rather bettter. The common usage of plywood, tar paper, and asphalt shingles didn't help with trapping moisture. Prior to that roofs were battened or boarded (no glue to fail and often enough hardwood) with metal, slate, cedar or some such roof material. Attics were warmer, less insulation and so was any metal roofing.

Good roofs lasted longer a century ago. I thought this impossible until I saw it. We tore an 83 year old cedar shake roof off a farmhouse in about 1976. Guy who owned was 83 and that roof when on just before he was born. When you walked into the dark attic you could see many hundreds to maybe a couple thousand of points of light through the shingles. Batten construction on conventional rafters (of course) with 2-3" spacing between. They called us in as it had developed two small leaks, staining the ceiling was the extent. I could not imagine it wasn't leaking all over based on what my young eyes saw, but the light was coming from down shingle up into the attic. Granted wood was better then, more old growth. Mahogany was still used as pallet material at that time to give an idea. The reason those cedar shakes made that longevity was air could circulate all around them. They swelled when it rained and dried out after. If you've ever seen how 100 year old high altitude pine and other woods age, fully exposed and nobody maintaining, where it gets soaked but the air is arid, saw that 20 years later, you start to piece it together. We apply cedar over what now? Takes a long time to dry if it ever does. I've seen cedar blue label roofs 20 years old about shot and they had repairs in the interim.

I'll also remind that the ventilation at the ridge and soffits does not need to be continuous. Just somewhat judiciously placed. If you ran 8' of ridge vent and skipped 8' you'd probably have enough. Sure, more is better insurance but it's not always needed. The top venting is the most important. It will find ways to draw air from below easier than it will exit the top, unless well sealed up.

Good luck. Based on your question I'd say you've got this now.

 

[bugstruck]

One thing I failed to address is the duration of your particular heating. That is a short condensation building period each week. You may get away with far less than I'd recommend to most per above. I'd still batten below the purlins at minimum so you have the option to vent well. You can always vent as and if needed later. Just allow a space to inspect it after an event. That air that is trapped has some volume and reserve to suspend added moisture for short durations. And it also changes out over time, as in a week probably. Something to consider in your particular instance.

 

[Welshman]

Putting condensation aside for a bit, is there an potential for excessive heat in the cavities, in the summer?

 

[bugstruck]

Wouldn't worry that. It will trap heat in the summer but nothing the materials aren't able to handle. And that heat trapped heat won't transfer to the inside space to the extent it does now. If you vent it that cools it down obviously but it will still be hot enough.

Not to your question and a bit of subject, but I know there has been discussion on lower underside roof temps extending shingle life somewhat. Not sure I buy into that but it may have some merit? I can't imagine it changes the actual shingle temps by much at all. Lighter color roofs with lower shingle temps might be of some help in reducing the thermal cycling range but I don't think venting gains that much in life expectancy of a roof. I've replaced 30 year 280 or 300 # asphalts at 10 years (mine and I don't recall the exact weight but one of those) and 20 year 225# asphalts at 30 years (others). Both in similar condition. Backwards of what you would expect. 20 year 225# shingles currently on my rental replaced those 300# early failure shingles I paid a premium for. Those current 225#'s went on in 1988. I'm on that roof every year, well for as long as I'm able anyhow as you can't see it from the ground. It has 2-3 years left in it. Nothing changed between those two sets of shingles, just a different run and manufacturer. I take roof warranties as a grain of salt now obviously.

 

[Cord]

This isn't a tractor project, but since there are a number of other construction threads here, I'll ask my questions.

My sportsmen's clubhouse is 30'x48' and of pole barn construction. Currently there is no insulation and our plan is to finish the inside as funds permit. We shoot year round and need to keep the place warmer in the winter.

We have an opportunity to buy some 4'x8' sheets of 2 1/2" thick polyiso foam board that is faced with a fiberglass layer on both sides, at a very good price. Our plan is to install this on the roof purlins, which are 2x4's on end, that run parallel to the ridge.

One of our members, a former roofer, has concerns that condensation may form in the cavities (24" x 3 1/2") formed by the covering of the purlins. The building is heated as best we can, about 5 hours a week, in the winter.

Should we be concerned about this, and if so, is there a cheap way to mitigate it?

Thanks in advance.

No need to b concerned because the dewpoint will cross over inside of the foam. The foam being of closed cell construction will not absorb the condensation so there is no resulting damage. If this was an open cell product or batt insulation, then the roofer would be correct. For your installation, do not install a vapor barrier on the inside face and don't try to seal up the metal panels. Do seal the joints between the insulation panels and adjoining surfaces. Although messy, cans of spray foam work great for this.

The secondary benefit of the iso foam is that it's flame resistant and won't generate much smoke when burned. Perfect for an interior overhead application like yours.

You'll want to have two layers of foam with the joints staggered. You'll be a little short of modern energy codes, but close enough to suffice.

 

[Welshman]

No need to b concerned because the dewpoint will cross over inside of the foam. The foam being of closed cell construction will not absorb the condensation so there is no resulting damage. If this was an open cell product or batt insulation, then the roofer would be correct. For your installation, do not install a vapor barrier on the inside face and don't try to seal up the metal panels. Do seal the joints between the insulation panels and adjoining surfaces. Although messy, cans of spray foam work great for this.

The secondary benefit of the iso foam is that it's flame resistant and won't generate much smoke when burned. Perfect for an interior overhead application like yours.

You'll want to have two layers of foam with the joints staggered. You'll be a little short of modern energy codes, but close enough to suffice.

Thanks for this, but I'm unclear on a couple of things. What do you mean by "the dewpoint will cross over inside the foam"? Also, "don't try to seal up the METAL panels"?

What do you mean by using spray foam to seal between the panels and the "adjoining surfaces"?

Are you suggesting that we use two layers of foam board strictly because of the R factor?

Thanks.

 

[Cord]

Thanks for this, but I'm unclear on a couple of things. What do you mean by "the dewpoint will cross over inside the foam"? Also, "don't try to seal up the METAL panels"?

What do you mean by using spray foam to seal between the panels and the "adjoining surfaces"?

Are you suggesting that we use two layers of foam board strictly because of the R factor?

Thanks.

The dewpoint is the point where the air is no longer able to support the moisture so the moisture will settle out. Generally, the dew point will cross this point inside of the insulation. When you have an absorbant insulation like batt or blown you will need a vapor barrier to prevent the moisture from entering the insulation and then condensing. With a closed cell foam product this is no longer an issue because the foam will not absorb the moisture.

You won't want to seal up the metal panels because this will cause the panels to start acting like a vapor barrier and could potentially trap moisture. Because you are using a closed cell product, just let the panels breathe.

This is related to my second paragraph above. Fitting foam panels tight to surfaces is pretty much impossible. Get the panels reasonably close and then use spray foam to fill the gaps in and create an air tight seal. This will help prevent air bleeds that will result in moisture getting between the insulation and the metal panels. If some should get in there, then the gaps in the panels will allow the moisture to escape.

No, you want to layers so the joints are staggered. This helps to improve thermal efficiency.

 

[Welshman]

The dewpoint is the point where the air is no longer able to support the moisture so the moisture will settle out. Generally, the dew point will cross this point inside of the insulation. When you have an absorbant insulation like batt or blown you will need a vapor barrier to prevent the moisture from entering the insulation and then condensing. With a closed cell foam product this is no longer an issue because the foam will not absorb the moisture.

You won't want to seal up the metal panels because this will cause the panels to start acting like a vapor barrier and could potentially trap moisture. Because you are using a closed cell product, just let the panels breathe.

This is related to my second paragraph above. Fitting foam panels tight to surfaces is pretty much impossible. Get the panels reasonably close and then use spray foam to fill the gaps in and create an air tight seal. This will help prevent air bleeds that will result in moisture getting between the insulation and the metal panels.

Thanks for your reply. I understand what the dew point is, what I don't get is if the "foam will not absorb the moisture", how does the dew point "cross this point inside the insulation"?

Are you saying I should use the spray foam on the edges of the foam board?

We don't have any "metal panels".

Thanks.

 

[Cord]

The inside face of the insulation will be 70 degrees and in winter the outside face will be 30 degrees. There is a 40 degree temperature drop through the insulation. If the dew point is at 50 degrees, then the dew point will occur in the center of the insulation. This is acceptable because the closed cell foam will not absorb any moisture.

Yes, spray foam the gaps after installation.

So you have a pole building with wood sheathing for the roof and walls? If so, that's ok because won't change the assembly.