Ceiling insulation for dummies, please!?
The house I’m building has cathedral ceilings. Rafters are 2X10s with OSB decking on top, then felt paper and shingles. There is no “attic space” or anything that will be enclosed space between the living area and the roof.
Can I pack the entire 2X10 full of insulation (foam, or densepack cellulose) without leaving an airspace?? I haven’t finished the roofing yet, so I can put a ridge vent in if necessary, but there won’t be anything to vent.
I guess I don’t understand the reason to have a vent (ridge vent, soffit vents, etc.) if there isn’t any space to vent….so why create a space between the roof decking and the insulation??
I have a “whole house” fan through the roof to help keep moisture out.
I understand that an air space/vent helps keep the roofing “cooler”, but that it only cools it about 6%. A lighter colored shingle cools it 10%, so I don’t see the advantage if that is the only reason to have it vented.
Please enlighten me. My instinct is just to pack it full and close off the ridge vent and eaves.
Thanks!
CC
Replies
In general, cathedral cielings where the roof and ceiling rafter are one in the same is something only a first year archi would do. If you must do this then the roof needs to be vented to let moisture and heat out in the summer, and help prevent ice damming in the winter. The best way to do this is to vent the rafter bays with strapping and 1/4 ply and spray foaming the hole thing.
This has been discussed here many times before. You may be able to bring up past discussions by using the "search" feature above the list of discussions. Wayne's way isn't the only way. I also think if you go to http://www.buildingscience.com they talk about insulating cathedral ceilings. I believe that if you use closed cell foam, it is enough of a vapor barrier that you do not need ventilation. You can Google "hot roofs" too and that should give you more info.
I resent the implication and the broad generalization that a cathedral ceiling is a 'bad idea' that "only a first year architect would do". While you are entitled to your opinions (and reasons), this is a pretty strong statement considering this type of construction is common place and is 'designed' by contractors, homeowners, and seasoned architects.
A roof is generally vented to let moisture out ... that comes from INSIDE the house. If the ceiling is properly sealed, then there is no moisture to let out. There should be no moisture issues from outside, either (e.g. in the humid summer). Ice damming is caused by a 'heat leak' allowing excess heat to warm the roof enough to create a thaw/freeze cycle to occur. If your insulation is continuous and otherwise installed properly, you should really have no ice damming. Recessed lighting is a common culprit of ice damming problems.
Admitedly there has been a lot of discussion about cathedral ceilings and how to handle them. Your method is only one way. Many other ways have been successful as well as some of the other are pointing out.
yes, it is a strong statement, in a temperate climate cathedral ceilings with a shared ceiling/roof rafter is, I'll be even stronger, stupid!
A roof is generally vented to let moisture out ... that comes from INSIDE the house. If the ceiling is properly sealed, then there is no moisture to let out
A completely sealed ceiling is fiction, especially when said designers punctuate it with pot lights, not to mention all the standard ceiling penetrations, plumbing, electrical, etc.
Ice damming is caused by a 'heat leak' allowing excess heat to warm the roof enough to create a thaw/freeze cycle to occur.
Heat is transferred by thermal bridging of the common rafter, not to mention that hot air rises and all that hot air just sits there above the people space where it could have been used. The absolute best way to insulate a cathedral ceiling is not to build one with common ceiling/roof rafters in the first place, we have enough problems trying to resolve issues with existing ones!!!
Woaah!! Hang on here. otnay otay ightbray are we? Having an opinion, even a strong one, is fine, but calling others stupid because they simply don't agree w/ you is absolute idiocy.
There are successful cathedrals, unvented and even done w/ fiberglass. You're right about recessed can lights ... common problem and often I think they are 'stupid'. Can't think of to many plumbing penetrations in a ceiling. Handled right electrical boxes can be sealed to virtually eliminate vapor passing through.
Dude, there is a time and place for everything ... and that isn't always doing it the way you think.
wane...you don't seem to be talking from experience
you can certainly design and execute a hot roof...
and control internal moisture
and make sure the roof doesn't leak....
and blower door test the assembly to make sure there are no air leaks
it's simply a matter of planning & executionMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Your advice here sounds like it comes from a book from twenty years ago, or from a first year architect student.There are a lot of ways to deal with this without following your prescription.
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the warrantee on roof shingles is void on an improperly vented roof
typically here in the northeast you would staple up styrofoam baffles to allow continous airflow from the soffit to the ridge then pack the bay with insulation.
not knowing what climate zone you are in I can tell in our zone you would require r-30 minimum and you would not achieve that in a 2x10 bay with styro baffles (with fiberglas insulation) you might get that with closed cell
in this case you would need to attach appropriately sized furring to the underside of the rafter.
I recently framed a similar roof with 2x12 when my load calcs said I only needed 2x10 for the same reason.
"the warrantee on roof shingles is void on an improperly vented roof"Many an IMPROPERLY vented one. But not for a NON-VENT roof.Many manufactures now cover installation on a hot roof assembly..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
"he warrantee on roof shingles is void on an improperly vented roof"only for a couple of manufacturers.testing done to measure the decreased life expectancy of shingles on a hot roof as opposed to a vented one boils down to a few months at most.
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Can I pack the entire 2X10 full of insulation (foam, or densepack cellulose) without leaving an airspace
I'd dense pack it and seal it.
You're right, skip the vents.
Skip Wane too, off his meds I guess.
Joe H
RE: Aneway - I was going to say something, too ... Thought MAYBE it was me ... I was about ready to check into Betty Ford ... You obviously know him better than I.
It has been a while since I have seen this much mythology in a thread so early.
Your challenge is to eliminate dewpoint condensation with whatever method you use. with thick enough denspak cellulose or BIBBs, you eliminate infiltration and dewpoint contact.
Then you want to add additional insurance by placing a good tight VB on the interior surface. Theoretically a vapour retarder with good insulation is sufficient in many climates.
You best choice is sprayed in polyurethene foam which provides R-7 per inch and is a VB itself. The Corbond company has done controlled scientific studies to demonstrate to building officials that venting these spaces in not necessary, and has had venting requirements overturned in many states and localities. Much of that is presented in their book, "Walls that Work"
Another choice that I use for economy jobs is to place FG batts in the rafter space, then skin across the bottom of the rafters with Thermax with taped seams( foil faced polyiso R-7/inch) for added thermal break and VB, then strapping for SR.
The idea is #1 to get a good R30 o4r thereabouts, and to have a Vb to prevent excess moisture laden air from entering spaces that could allow contact with cool surfaces where it can condense.
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Piffin,
Please don't confuse a "vapor barrier" (usually referred to as a vapor retarder these days) with an air barrier.
You wrote, "...to have a VB [vapor barrier] to prevent excess moisture-laden air from entering spaces that could allow contact with cool surfaces where it can condense." But you mean an air barrier, not a vapor barrier. If your intent is to prevent air movement, install an air barrier -- for example, well detailed drywall, or, as you propose, Thermax with taped seams. You detail is OK -- you just call it by the wrong name.
Old habits die hard
Thanks
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Sorry that I'm clueless. What is "SR" from a couple posts back...
and why did you fur out the rafters after blowing cellulose?
Some day this will all make sense to me. :(
CC
i think you're referring to this...
<<<<<
Another choice that I use for economy jobs is to place FG batts in the rafter space, then skin across the bottom of the rafters with Thermax with taped seams( foil faced polyiso R-7/inch) for added thermal break and VB, then strapping for SR.>>>>
the furring is so he will have something to fasten the SR to ( Sheet Rock ) without using extra long screwsMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Mike did the verbal, noiw I'll draw the picture. If you crane your neck and look up, you can see a hint of what I am saying.
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...
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My folks had a cottage with cathedral ceilings...no ridge vent...no baffling...no soffit vents....terrible moisure problems...falling sheetrock etc. Ended up doing a raze and re-build.
My folks had a cottage with cathedral ceilings...no ridge vent...no baffling...no soffit vents....terrible moisure problems...falling sheetrock etc. Ended up doing a raze and re-build
Assume you eliminated the moisture source also?
DW doesn't fall off ceilings unless there's a hell of a lot of water coming from somewhere.
Joe H
I don't think there was any VB in the ceiling either. The slab had no VB and was a moisture wick. The whole place was a complete demo and ground up rebuild. Excellent builder did the work...nice place now!
I am certainly not arguing that it can't be done wrong and screw a house up! I have dealt with lot of problems where somebody did it wrong.What I am saying is that there are lots of ways of doing it right, and that anyone who paints with such a broad brush as to state emphatically that any and all cathedral ceilings will have those problems is making an his ignorance known.
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Please don't confuse a "vapor barrier" (usually referred to as a vapor retarder these days) with an air barrier.
For my own understanding of this, could you elaborate? If I'm getting it right, an air barrier would also be a vapor retarder, but a vapor retarder not necessarily an air barrier?
Just trying to get a handle on the lingo and function.
Everything will be okay in the end. If it's not okay, it's not the end.
Peter J,
1. Is every air barrier also a vapor retarder? No. For example, air-tight drywall (drywall installed with gaskets or caulking in such a way as to pay close attention to air sealing) is an excellent air barrier, but it is not a vapor retarder -- at least not until it is painted.
2. Is every vapor retarder also an air barrier? No. In an older house, with old lath and plaster on the walls, the walls may have no air barrier. But because such walls usually have several layers of paint, they have a perfectly adequate vapor retarder.
3. Can the same material act as a vapor retarder and an air barrier? Yes. The classic example is interior polyethyelene. Poly is always a vapor retarder and sometimes an air barrier. To be a good air barrier, the poly needs to be carefully installed (without tears or rips), to have the seams sealed (usually with Tremco acoustical sealant), and to have good detailing at electrical boxes, partition intersections, and rim joists.
my BI allowed me to make a hot roof with des-pak cellulose
we put insulweb across the joists ,stapled & glued, then we furred it and blew dens-pak cells in each bay
As you may be starting to see, this particular question is a very contentious one. And it has been thrashed out here on BT many, many times before. Use the "advanced search" gizmo in the message list to search for messages containing one of these phrases: 'cold roof', 'ventilated roof', or 'hot roof'. You will get an eyeful.
Basically, there are two camps of opinion: Those who favour keeping the roof deck cold by ventilating it (I am among them); and those who believe a so-called 'hot roof' (an unventilated roof is by definition 'hot') can perform well if it is sufficiently well sealed and insulated.
In fact, it is possible to build an unventilated roof and get it to perform satisfactorily...but the cost to do so in time, money, and trouble is extremely high, so the question becomes not, 'Will it work?' but rather, 'Is it worth it?' (It will be especially difficult to get good performance out of a 'hot' roof in a climate such as yours, where you get reasonably large amounts of warm, heavy snow each winter, but few spells of extreme cold.)
Roof leaks due to ice damming occur mostly when the outside temperature is above -9ºC. When it is colder than that, the small amount of snow-melt that occurs under the snow pack on a 'hot' roof does not build up pooled water behind the ice dam; it freezes as it hits the dam and merely makes the dam itself larger.
On the other side of the argument are the ventilated, or 'cold' roofs, which also provide satisfactory perrformance...but usually at a much lower cost. The key point is that 'cold-roof' design is much simpler to build on most houses, can be accomplished using hand-placed insulation rather than expensive sprayed-in foams, and as a system has performed very well over a very long period time--virtually since the invention of artificial insulation. In my opinion--and that of many others--these attributes make vented roof the 'default' choice, and one should go with that unless there is a damned good reason not to.
In other words: 'If it ain't broke, don't fix it.' Cold roofs work; this is well proven. Why go looking for something else if you don't need to?
There can be legitimate reasons to consider something else, but how compelling they are depends upon the individual case. Ventilated roofs become difficult to do correctly when a house has no eaves, as was common on some Cape-Cod bungalow designs. OTOH, while venting the roof in a cathedral-ceiling structure also requires some planning (a clear air channel is required in every rafter bay) there are a variety of ways to do that, and none of them are particularly difficult.
I suggest you take a look at some of the other threads on this subject for the actual construction details and theoretical physics behind the whole thing.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
ahem... IMNSHO...... (bs)cold roofs ( ventilated roofs ) require just as much work to build them correctly..
and done incorrectly they lead to rotted roofs & ceilingssame is true of hot roofs neither requires more work than the other.... although... i can build a hot roof a lot faster, cheaper & easier than a ventilated roofwe build both... depending on the circumstancesand in cathedral ceilings ... vented roofs cost more and perform less in terms of insulationMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
cold roofs ... done incorrectly they lead to rotted roofs & ceilings
same is true of hot roofs
No argument there at all. However (heh, heh):
neither requires more work than the other.... although... i can build a hot roof a lot faster, cheaper & easier than a ventilated roof
I shouldn't assume what specific techniques you'd use to do that, even though I think I might have a pretty good idea.
As far as I'm concerned, best practice for a ventilated roof in something like a cathedral-ceiling chalet is to insulate the roof on the outside with polyiso panels; run strapping over that, and a second roof deck (or metal roofing) over that. Then you can stuff 8" or 10" of FG batts on the inside and wind up with a super-insulated roof that's perfectly vented. That's pretty quick and easy; nothing a competent DIYer couldn't do all by hisself.
I'll let you elaborate on best practise for a 'hot roof'....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
i've done the poly-iso panels on the exterior..
and it's a lot more work and a lot more money... not to mention you have to detail the rakes & eavesMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Maybe...but spraying with two-component P-U foam would cost even more, whether on the inside or outside. (At least here it would; your mileage may vary.) And wouldn't buy much (if any) improvement in R-factor.
In any event, I said that's 'best practise'. For some, double-insulating the roof would be considered overkill. 10" of FG under a 1½" ventway in a 12" nominal rafter bay would provide R-40+, which is considered quite acceptable even here. Adding another 4" of polyiso on the outside takes the place beyond 'acceptable' and on into the 'super-insulated' category. And if your client is going there, construction costs are expected to be higher.
As for detailing the rakes...
View Image
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
What you describe is cheap but not a very good way to do this, and could cause problems unless you had good integrity in your interior air barrier. Your details provide for cold roof for th e protection of the roof surface, but no way to keep moisture from condensing in the rafter spaces. I've had to make repairs to places with just that detail more than once. Not here in this moderate climate, but in CO
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In old cottages we've both seen vent channels made of untarred homosote board because it was the cheapest stuff one could buy. Add some paper-backed wool, a few staples, and nail the gyprock (or more press-board) over it all and slap on some paint. Voilà ! Done deal....
As you said, this is bad news.
I build vent chutes with XPS panels or tarred homosote, and I now seal the panel edges to the rafters with 1-part gun foam instead of tape (learned that trick from Mike Smith). Then the wool goes in, the VB goes over it and gets taped. Then I put on strapping and screw the gyprock to that.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
After doing it both ways more times than I can count, I am really scratching my head trying to figure out what you are talking about. There is not a significant difference cost wise and there are times it is less expensive to do a hot roof.
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While everybody goes back and forth on this, read the PDF. BTW I agree with others that a properly built hot roof like you want to do is the way to go. Just avoid can lights.