Cathedral ceiling insulation/ventilation


Old 09-27-01, 01:43 PM
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What problems can arise by not allowing for ventilation between the roof sheathing and the insulation on a cathedral ceiling (other than ice dams)? Thanks in advance.

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Old 09-27-01, 03:40 PM
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how is it framed? convential or truss? How large is it?

most common problems will be condensation occurring between the insulation and underside of roof deck which can cause mold and rotting... also roof deck will very likely become to hot and curl shingles

If it is convential frames you will need a a ridge vent and airflow between every joist from soffit to ridge...

If it is a truss a few roof vents should do it with a few soffit vents

Good luck

Old 09-29-01, 04:25 AM
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Thanks Insulman. Incidentally, the construction is frame and each "cavity" is about 6 ft. sq. (it's a long story) and has a metal roof. Installing the ventilation will be a major complication but it sounds like I'll need to do it.

Thanks again,
Old 09-30-01, 07:30 PM
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Is this new constuction?
If existing, are the rafters exposed?
I ask because if you are going to insulate with fiber glass you have know that the heat gain is going to be outragous.
Let me know if you are going to insulate and I'll tell you how to super insulate it and have no moisture problems.

Thank you for considering my opinion.
Old 10-01-01, 02:48 AM
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What I'm doing is insulating an old industrial building that was built with large post and beams with a ridge pole. The roof is 2x6 t&g decking over 4x8 rafters spaced 6ft apart and a metal roof.
What is superinsulating?

Thanks for your time.

Old 10-01-01, 04:16 AM
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Thank you for your response.

Super insulation is 97% efficient radiant barriers(RB). I'm guessing you're planning fiber glass(FG) which is less than 10% efficient. You're now aware of the moisture and rotting problems you can have with fg, not so with RB.

The fiber glass transmits radiant energy at a very high rate and if the insulaion is on top of the 2x6's they will warm up act as a radiant heat radiator. The RB reflects the heat so there is less heat enery to absorbed by the 2x6's. This, assuming you're going to put sleepers on top of the roof, insulate and then the metal roof. In combo with galv-alum roofing this would be unbeatable.

If, however you are going to insulated under the 2x6's then you have two options.

If you want the beams to show you can run 2x2's 24"c/c between the beams and install two layers of radiant barrier material and then drywall. (accord'g to 45 yr old govern't test this will be about 30% more efficient than a 6" FG batt, and will far exceed anything you would want to do with FG) The negative to this is that the beams will transmit heat energy from the deck to the interior. If you plan to install new metal roof you can over come most of this by installing galv-alum roofing. (use the 80,000 psi material) Install 1x4' strips to top of deck and this roofing will also act as an insulator, reducing the 2x6's heat energy absorbtion.

If you plan to cover the beams then you run your 2x2's across the bottom of the beams and do as above.

If you have any ?'s about method or material, let me know.

For info on RB enter into your search engine, "radiant barriers" or "reflective insulations".

Thank you for considering my opinion.
Old 11-02-01, 06:15 AM
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rb insulation

Hello Rbisys
Sorry for the delay in getting back to you to avail myself of your expertise, I've been swamped.

Anyway, here's my problem.

I've decided that I need to remove the metal roof that covers my 2x6 t&g decking upon which I need to install my insulation - rb or otherwise. Here are my questions:
I live in a cold climate and thus heat retention is my main objective. How does rb compare to other types of insulation for both radiant and conductive heat loss through the ceiling? Do I need to use a combination of rb and other? Will the air gap on the inside of the rb significantly reduce conductive heat loss?

Thanks again


Old 11-03-01, 01:37 AM
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You may wish to look into the spray applied polyurethane foam products for a high performance insulation alternative to your building.
Not only are high R-values easily obtained (between R-6 to R-7/inch, depending on manufacturer), its spray application allows the structure to be effectively sealed providing an air infiltration barrier. Additionally, it will provide an effective sound deadening barrier to the structure.
Just a brief synopsis for now,,,let me know if you would like more info....
Old 11-03-01, 05:11 AM
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polyurethane foam

Hi Mark,
Thanks for your input. This also sounds like an interesting alternative. Yes, I would be interested in more information. If I insulated beneath the roof deck and between my joists, do I still have to use a vapor barrier and allow for a ventilation space above the insulation.

Old 11-03-01, 02:05 PM
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The problem with foam is;

The gases that help give it the "R" values dissapate after awhile and the efficiency goes down.
I live in a panelized foam house and the noise transmission is terrible. Your 2x6's will dampen most of the sound.
The Radiant barrier is a better insulator and will not lose is efficiency. In fact I'm in the process of installing a radiant barrier system on the inside of my ceiling now.

When you check to see how much it costs to foam the ceiling I think you'll be shocked.

Thanks for considering my opinion.

Old 11-03-01, 04:20 PM
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First let me state,,I am not trying to sell you something,,,just trying to inform you about another alternative.

Understand that when polyurethane foam is processed it is a closed cell plastic that contains no gases or formaldehydes.
Once manufactured it is stable and does not loose its R value over time. It should not be confused with expanded polystyrene foam of the past.
Secondly, indeed if you fill a cavity completely with foam, or use a structural panel with a solid foam core, you indeed will notice noise transmission. Kinda like a drum,,,the idea is to apply 1-2" to one wall surface, leaving an air space between the foam and the other wall surface. This has been very effective in interior walls.

Yes this is a premium product, but like many things in this world,,,you get what you pay for. Its saveings in energy dollars will recover the extra expense in a relatively short period of time. Realize that when a structure is exposed to wind, foam is about 50% more efficient than traditional fiberglass when compareing equal r-values.
Another alternative to help control costs would be to have 1-2" of foam applied and then follow with an R-38 batt. The foam would seal the ceiling provideing an effective air infiltration barrier and provide an r-value of around r-12.5 to r-14 (depending on whose foam is applied). Following with the r-38 batt would boost the r-value to around an r-50.

About your specific questions:

vapor barrier: many of the foam manufacturers and distributors promote foam as an effective vapor barrier. But considering the relatively low cost of plastic sheeting,,,I would suggest useing ounce of prevention is worth a pound of cure.

Ventilation: again this is an area of some controversy....
many of the foam manufacturers and distributers claim that a cathedral ceiling insulated with foam need not be vented...But, many of the shingle manufacturers require an airspace beneath the deck to keep the warranty intact. I recomend that plastic baffles be placed from the eve's to the ridge vent with blocking at the eve's.

There is a wealth of information on the internet about polyurethane foam. May I suggest a search under "polyurethane foam insulation" in your favorite search engine. You may want to visit the, or sites in particular as these are informative, and will substantiate some of the above information. Realize that I am not an agent or represenative of any of these companies.

let me know if i can provide you with any other info:
Old 11-04-01, 05:44 AM
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Greetings, Wayne20

The ceiling rafters will still be a source of a large amount of heat energy passing through. Filling the cavities with foam will not stop this process, in fact, it will probably have little impact.

I love it when "R" values start beimg thrown around as if more is better. If you look in an engineering handbook you'll see that foam is only about 20% efficient in rejecting heat energy, aluminum is 97%. I notice that homefoamy recommended that a 1x2, or similar be installed on the interior side. If you install a radiant barrier along with this furring strip you will prbably double the insulation effectiveness. But I sure wouldn't claim an "R" 60 or better. You can also do this from the exterior side.

I have a 30,00sf, 2 story, icf building with a type 2 RB and galv alum steel roofing. The owner installed ceiling fans every 10 feet to circulate the air. Problem is, that he had to turn the fans off because the interior temperature only reaches about 75 degs (summer) and it's too chilly with the fans on. For excess humidity he runs a 5 ton a/c, but, has to be carefull because the temp will drop below 70 degs. The cost of the RB system was about $.45 / sf installed. Foam was quoted at $1.00 + / sf. Considering the out come, the owner is very pleased with results.

As to air infiltration, the RB is a soild sheet and air infiltration is very, very low. Granted foam will stop air infiltration, you also have to consider that during construction a qiuality job will minimize or eliminate air infiltration. Air infiltration is a major factor at construction seams and utility out lets. Since drywall is solid most of any problem is stopped there. Air infiltration has become more and more important because construction quality has fallen to an all time low. I'm turning down more and more jobs because of extreme low quality construction. I know that the owner is going to have problems regardless of the type of insulation and I want no part of it.

I will check with an engineer I know at a company that manufactures 1 and 2 component foam for the insulation market and see if the law of physics has changed concerning off-gassing.

Bottom line, the method I outlined will isolate and munimize the rafter effects and provide you with better results at a lower cost.

By the way, you don't always get what you pay for. Look at fiberglass and what you get that you don't want, and what you don't get that you do want.

As to selling products and methods, if your are not selling something, directly or indirectly, what are you doing on this site? We all have something to sell whether money exchanges hands or not. I have been selling, designing and installing RB system for about 30 years. If I haven't made it by now, I'm in deep do do. But I enjoy sharing my knowledge and experiences, AND, also alerting the public to how tha US Government, insulation manufacturers and utility companies are riping us off, BIG TIME. The sorry fact of the matter is, so many people have bought the BIG LIE and defend it vigorously. So what else is new?

Wanye, I think yo're getting more than you bargained for and more than your monies worth.

If nothing else, have fun.

Thank you for considering my opinion.

George H.

George H.
Old 11-04-01, 08:21 AM
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As far as I new this site was designed to help the do it yourselfer get free advice from all sources willing to help with their expertise...

It sure is a shame that rbisys, seems to think he is the only person in the know on how to handle insulation problems and offer good advice for the do it yourselfer...

As for blasting fiberglass cellulose and foam continuously, seems more like sour grapes than reality...

As for the ability of RB type insulations being far superior
what a joke...

Read what the EPA reports on effectiveness of RB type insulation products before taking rbisys word for actual truth..

Their are many applications for properly installing insulation. Consider all options and go with what works best for your particular situation...

Here are some web sites I encourage all to read for the benefit of getting some varying opinions...[/url][/url]

The site listed below uses graphs and calculators for figureing if radiant barriers or other methods are more useful to the consumer are beneficial in your area..[/url]

In addition R- values in fiberglass are based on a 75 degree mean temperature. Effective R-values fluctuate with actual temperature and do lose some of their insulating properties based on R-value.. However the recommended R-values in your area of the country take this into consideration.

I live in the Detroit Metro area and you won't find anyone doing large scale insulating using the rb system exclusively let alone in conjuntion with other products.. Seems to me with over 100 Insulation contractors in our area that if this RB system was good for this market area that someone wouldnt take the ball and be the only guy on the block in the know..

Don't believe everything you read posted by so called experts on this site...

By the way as I have stated before our outlet sells over 1/2 million dollars a month in fiberglass insulating materials.. But I do not exclusively promote fiberglass because there are other methods of insulating, a do it yourselfer can utilize that may be more cost effective and do just as good as a job depending upon the situation.

Good Luck to the do it yourselfers



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