Help me understand attic insulation and moisture

[iamsoners]

Slowly and steadily we're air sealing and insulation our 1920's bungalow. We want to get started on the attic insulation but also plan to eventually finish off the attic and need the insulation to be compatible.  We have a steeply pitched roof and we'll put cathedral ceilings in when we finish off the room.  But everyone who looks at the room and everything I read has differing opinions on how to insulate the roof/ceiling/attic.  And I feel like my handle on the moisture issue is tenuous at best

Our roof is ventilated with ridge vents. The soffits are closed. Currently there's about 8 inches of cellulose blown on the floor/joists of the attic. 

The main suggestions I've seen are:

-Create vent channels on the underside of the roof, then put in bat insulation, then the ceiling drywall? (Do the vent channels make sense if my soffits are closed?)

-Use foam insulation to insulate the underside of the roof sheathing then drywall (how is moisture controlled in this situation?)

-Spring for spray foam and spray foam the underside of the roof (but will this hold moisture in?)

-Or something else?

We're experiencing our first cold days here and getting significant condensation on the inside of the windows so there definitely moisture in the house.  Anyhow, any ideas, explanations or links to people who explain this well would be greatly appreciated!!

Thanks!

[OptimusFrugal]

I found an interesting article about this here

http://www.applegateinsulation.com/Product-Info/Technical-Pages/249234.aspx

[iamsoners]

Interesting read Optimus and I've seen others to the same effect.  Thanks

[Greg]

You need to carefully evaluate your roofing to see if it's 100% moisture proof and how non-venting construction may affect the roofing and any warranty.  With shingles, you have thousand of potential moisture intrusion points and some under-sheathing ventilation as is traditional helps with that.  If you have metal or some other less-permeable roofing you could probably just abandon the roof ventilation altogether, but If it were me I'd check with my local building department to see what they recommend or require. 

Assuming you have asphalt composition or wood shingles, I'd opt for 1" airspace and then either rigid foam insulation or high density batts, depending on your rafter depth.  With high density batts you can get R-30 in a 10" rafter.  In my area of the PNW R-30 is the minimum for vaulted areas.

[DoingMe]

Be sure to check your local building codes.  The depth of your roof rafters may limit the choices you have that result in code-compliant R-value.

[Rural]

If you insulate the roof deck with spray foam, you should close off the ventilation; the space under the ventilation becomes climate-controlled space.

Yes, you want to avoid moisture intrusion in that case.

[paddedhat]

You need to carefully evaluate your roofing to see if it's 100% moisture proof and how non-venting construction may affect the roofing and any warranty.  With shingles, you have thousand of potential moisture intrusion points and some under-sheathing ventilation as is traditional helps with that.  If you have metal or some other less-permeable roofing you could probably just abandon the roof ventilation altogether,

Not quite sure where you developed this concept, but it's completely wrong. Shingled roofs, are watertight when properly installed and maintained. This holds true for any modern roofing material. Ventilation is typically required to prevent the accumulation of moisture on the underside of the roof structure. This is typically the result of air introduced by leakage in the thermal envelope of the home, and air intentionally or incidentally introduced from the outside of the home via vents, attic windows and construction gaps. This air can hit the underside of the roof sheathing and condensate due to moisture and temperature differentials. Full venting of the roof structure encourages a flow of air from the soffit area to travel the full height of the underside of the roof structure and vent at the ridge, keeping the attic space dry and mold free. In certain cases, improperly installed metal roofing can be a disaster, not an asset when it comes to attic moisture control.

[Nate R]

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation

There are many other relevant articles on this site. Joe seems to be THE guy who REALLY likes to figure out building SCIENCE, not "we do it this way because we always have." I love how much free information is available there.

[Greg]

Well, I've been building for 28 years and have seen shingled roofs leak due to improper installation (staples), poor slope, slipshod tar paper installation, wind driven rain, and all other manner of problems.  Like you said, if installed correctly and well, a shingled roof works well, usually. But there are always those "gotchas" that can't always be described by a post or covered in a response.

Many comp roofing manufacturers require "adequate roof ventilation" and so consulting the instructions is probalby a good idea.

[iamsoners]

Thanks all--Nate, that article was especially helpful. 

I'm leaning towards an unvented roof with closed cell spray foam on the bottom.  Creating a truly vented roof would mean cutting open all of our soffits and would also force us to extend the rafter length significantly--we currently just have 2x4s and I expect we'll have to do more than 4 inches of foam but I think the foam still saves us some space which will be important for the room we're creating.  As I understand Greg and the building science article, the risk with this is moisture intrusion from the topside creating a moisture problem in the sheathing--a risk I guess I'm willing to accept.

But that does have me thinking--how the hell will all these spray foam roofs be replaced eventually? If you wanted to take out the decking, it'd sure be a huge mess.  I honestly don't intend to own this house in 30 years so I plan to not find out...

I'm planning on having a few insulation contractors come out to do bids so I can compare the cost to DIY--I'll be interested to see what they say on the issue and will update ya'll.

Thanks again for all the knowledge!

[paddedhat]

Well, I've been building for 28 years and have seen shingled roofs leak due to improper installation (staples), poor slope, slipshod tar paper installation, wind driven rain, and all other manner of problems.  Like you said, if installed correctly and well, a shingled roof works well, usually. But there are always those "gotchas" that can't always be described by a post or covered in a response.

Many comp roofing manufacturers require "adequate roof ventilation" and so consulting the instructions is probalby a good idea.

Greg, you are still not making sense on this one, regardless of your experience. Attic ventilation is typically required for the reasons I spelled out. It has nothing to do with addressing potential roof leaks.

 As a builder for the last three decades, I can recall one roof leak on my new homes, and it was a chimney related issue. Thousands of squares, one leak. Due to an obsession with attic ventilation detailing, none of those homes ever suffered from ice damming either, which is pretty common here.

Shingle manufacturers specify the need for attic ventilation for two very specific reasons. First, it leads to lower temperatures in the attic, thus keeping the shingles cooler. Arguably, this will lead to a longer shingle life. The second reason is a little less noble. A manufacturer of building products will typically issue VERY detailed instructions, and make it very clear that all warranties are voided unless the product is installed "Per Manufacturer's Instructions". This gives the manufacturer an easy out when they are faced with owning up to addressing premature failures, and defective products. Don't know if you did much in the way of window installations, 20+ years ago, but one company was famous for boldly stating that the flanges of their product MUST be installed with 1-3/4" aluminum roofing nails. The fact that said nails are about as rare as unicorn sperm is not a coincidence.

[paddedhat]

Soners, Good choice. Closed cell poly rocks. I do a sealed, conditioned crawl space in my places, and use a few inches of sprayed poly on the foundation walls. Really makes for a water and air tight job.

Saw an interesting comparison of closed cell vs. poly in real world conditions recently. A relative bought a new, big, and stupid trophy house in Georgia. Place is probably 3500+ sq. ft.  There is a matching house next door. The house next door is done with fiberglass batts to whatever minimum standards are required.  The other place is done with full spray in the wall cavities and about 3-4" on the underside of the roof sheathing. The foam house has exactly half the hvac system tonnage rating as the other, and in the summer generates almost exactly half of the electric bill.

As for avoiding packing the rafter depth down to accommodate vent trays and fiberglass, and skipping all the hassle of hacking the place apart to install soffit vents, good call. I did it on one remodel, and I swore it would never happen again. Huge pain in the butt, and not cost effective compared to a "Hot" roof. Good luck.

[_JT]

Well, I've been building for 28 years and have seen shingled roofs leak due to improper installation (staples), poor slope, slipshod tar paper installation, wind driven rain, and all other manner of problems.  Like you said, if installed correctly and well, a shingled roof works well, usually. But there are always those "gotchas" that can't always be described by a post or covered in a response.

Many comp roofing manufacturers require "adequate roof ventilation" and so consulting the instructions is probalby a good idea.

Greg, you are still not making sense on this one, regardless of your experience. Attic ventilation is typically required for the reasons I spelled out. It has nothing to do with addressing potential roof leaks.

I dunno -- it seems to make sense the way I read it. Here it is again:

You need to carefully evaluate your roofing to see if it's 100% moisture proof and how non-venting construction may affect the roofing and any warranty.  With shingles, you have thousand of potential moisture intrusion points and some under-sheathing ventilation as is traditional helps with that.  If you have metal or some other less-permeable roofing you could probably just abandon the roof ventilation altogether,

So, to my read, he says that the OP should examine his roofing to make sure it's doing its job, since it is possible for roofs to leak. Specifically, he says you have 'thousands of potential moisture intrusion points', which I took to be penetrations from roofing nails. I think we can all agree that this is a fact. Roofs leak sometimes. And the nails are potential intrusion points. He then says to check and see what non-venting does to the warranty. I know nothing about that, but I can't see that looking into it is wrong.

He also says that attic ventilation can help with that, the 'that' being trace amounts of moisture that could come in from any of those possibilities mentioned.

Then, here's your response:

Not quite sure where you developed this concept, but it's completely wrong. Shingled roofs, are watertight when properly installed and maintained. This holds true for any modern roofing material. Ventilation is typically required to prevent the accumulation of moisture on the underside of the roof structure. This is typically the result of air introduced by leakage in the thermal envelope of the home, and air intentionally or incidentally introduced from the outside of the home via vents, attic windows and construction gaps. This air can hit the underside of the roof sheathing and condensate due to moisture and temperature differentials. Full venting of the roof structure encourages a flow of air from the soffit area to travel the full height of the underside of the roof structure and vent at the ridge, keeping the attic space dry and mold free.

So, your immediate gripe seems to be #1 that he said that moisture intrusion was possible in some cases with shingled roofs, either through improper installation, old age, or crazy weather conditions. And your response was that shingled roofs are watertight when properly installed and maintained. I don't see that those two opinions are mutually exclusive. I agree with you that shingled roofs are going to keep you dry most of the time, when they're done properly. But I also agree with Greg that it's well within the realm of possibility for there to be some small amount of moisture intrusion -- have you really never seen anything like this? I'm jealous of the quality roofers in your area, if so!

Gripe #2 appears to be that he said attic ventilation can help with that moisture intrusion. You then replied that that was not what it was for, but rather to alleviate moisture built up through the condensation effect, temperature differentials, and air leakage from the envelope. Again, I don't see why it HAS to be one way only. What he said doesn't (to me) contradict in the slightest what you said. He said (essentially, to my read) ventilation helps get rid of excess moisture, and you replied that that was wrong and went on to detail how it is designed to get rid of excess moisture. Now, that's a simplification, but just letting you know what it read like to me. I happen to agree with both of you completely.

[Greg]

I think it's ok, paddedhat is pointing out that roof ventilation is not and should not be used to mitigate any roof leakage.  It's a potential side benefit in my view, but it wouldn't take much of a leak to overcome any drying effect of the ventilation.

In remodeling I see all kinds of horrible roofing jobs, new roofing is often spot-on.  I think we're basically in agreement except for my optimism about side effects of roof ventilation.  Unvented roof designs are still fairly new, new enough many areas still won't allow them and some comp. shingle makers don't like them.