Cathedral ceiling and lack of insulation space

[Marc]

I've seen many roofs that doubled as cathedral ceiling on only one side of the house with the other side having attic space. No baffles on the cathedral ceilings but the shingles on both sides aged at the same rate. Never seen otherwise and I'm closer to the equator than most of the country.

Marc

[kurt]

Nope. Vents don't prolong shingle life. It's about condensation and moisture, period.

I'll defer to local custom; if folks are nervous with foam and no vents, and the local belief says it's bad, OK. Don't do it. It's ignoring hard building science and all engineering that says it's OK, but as we all know, superstition rules in the building trades.

But, the arguments against foam are based in something else being totally screwed up, not some inherent incorrectness of foam and no vents.

One only uses foam when they are confident the other components are satisfactory. If you think the roof is bad, don't use foam. Which leads to the question "Do you really want to do all this work without having a good roof?"

[Stephen Lagueux]

It's about condensation and moisture, period.

could not have said it better myself, short and straight to the point.

Here is my clumsy way of explaining it.

Dew point will occur when the cold air meets the warmer air somewhere mid insulation depending on temperature differential from outside to inside, RH and GPP. If a proper vapor barrier has been installed on the warm side, that water has nowhere to go if the insulation is tight against the sub roof. That's where ventilation is important.

The air space needed between the roof and the insulation provides a passage way for the warm air to rise out through the vent. That air movement creates a chimney effect that sucks air in from the ventilated soffit. The movement of that air on the insulation surface creates an evaporation effect and lowers the vapor pressure at the surface. The water located in the insulation at the dew point level will be attracted to the lower pressure area which is the surface of the insulation.

The more the water goes to the surface, the more evaporation takes place and excess humidity now in the air goes out the vent depending on proper ventilation and the insulation remains dry.

If no air space was there, water would still be attracted to the surface as long as the sub roof is dry. That water now in the wood has nowhere to go and that's when problems begin.

[Fredartic]

In the house, I had pulverised all the area that present black stains, the attic, the walls, the floor with a specific product used with a machine for that that I had rented at Home depot. Here's the link: http://www.homedepot.com/p/Concrobium-1 ... XDpgbWQVmM

Is is safe that way?

[Fredartic]

Don't vent the cathedral ceiling, insulate it. A down and dirty foam insulation for tight spaces that works well and is cheap to apply is:

-cut rigid foam insulation to fit tightly between the rafters. Press it tight to the roof deck. Shoot for R-10.

-install your 5" Roxul to the interior.

-apply your vapor barrier.

You get an R-34 assembly.

If you can afford it, 7" of open cell spray foam will yield R-22 that will out perform R-38 fiberglass. Blow cellulose into the attic areas to at least R-50.

Tom, from what I see on the website of Dow company they recommand to do the opposite, first, install the matt and after the styrofoam...

Installing SM/CLADMATE/WALLMATE in Cathedral/Vaulted Ceiling (Gypsum Board Exposed Inside Ceiling w/STYROFOAM Between Gypsum and Wood Framing)

1. Install batts between rafters/beams in the conventional fashion, leaving at least 2" ventilated air space between the surface of the batt insulation and the underside of the roof sheathing or roof deck.

2. Ensure that venting from the soffit to the ridge vent is continuous and not blocked. If needed, install baffles at the wall/roof junction to prevent blocking of the airspace at that location.

3. Prior to installing the STYROFOAM? extruded polystyrene insulation, install a continous 0.006 inch polyethylene vapour retarder on the underside of the rafters. Install STYROFOAM SM, STYROFOAM CLADMATE? or STYROFOAM CLADMATE XL directly over the vapour retarder, fastening it to the underside of the rafters/beams. Install the boards perpendicular to the rafters/beams.

4. If using STYROFOAM WALLMATE? extruded polystyrene insulation install perpendicular to the rafters/beams by srewing or nailing the 1x3 strapping to the underside of the rafters/beams. Install a continuous 0.006 inch polyethylene vapour retarder to the surface of the WALLMATE insulation by stapling it to the face of the 1x3 nailers.

5. Install 1/2" drywall finish over the foam insulation. For ceilings insulated with STYROFOAM SM, STYROFOAM CLADMATE or STYROFOAM CLADMATE XL, drywall screws must be long enough to go through the foam thickness and penetrate at least 3/4" into the rafters/beams. If using STYROFOAM WALLMATE insulation, fasten the drywall directly into the 1x3 strapping using minimum 1-1/4" drywall screws.

6. Avoid penetrating the vapour retarder when installing light fixtures.

I'm confused... thanks

[Fredartic]

...and from again Dow company...

Cathedral/Vaulted Ceilings ? Installing STYROFOAM Square Edge, RS, Tongue & Groove, DURAMATE? Plus, TUFF-R?, Super TUFF-R, or THERMAX

(Gypsum board exposed on inside of ceiling with rigid insulation between gypsum and wood framing)

1. Install batts between rafters in the conventional fashion, leaving at least an inch of ventilated air space below the plywood/OSB roof deck.

2. Install vapor retarder on the underside of the rafters. This could be 4 mil poly or kraft facer on batt insulation.

3. Nail insulation to the underside of the rafters over vapor retarder.

4. Install drywall finish over the foam insulation. Drywall screws will need to be long enough to compensate for the thickness of the foam insulation.

5. Avoid penetrating the vapor retarder when installing light fixtures.

???

[Tom Raymond]

Dow is covering their corporate arses. Code says vent it so they say vent it. The ideal solution is to spray it, but gutting it, spraying it with closed cell, and refinishing it will cost more than towing it out and pulling in a new unit.

Google flash and batt, then replace the spray foam with at least R-10 rigid foam. I like 1" isocyanurate. As long as you fit the foam carefully it works really well.

If you want maximum insulation for minimum dollars then find a reputable cellulose company. They can install mesh and dense pack the cathedral area, then blow in the attic area. You should be able to acheive an average R-50 for less than $2000 and not have to remove much more (if any) of the interior.

[Fredartic]

Now I understand. Thank ou Tom.

If it cost $2,000. it's an option to consider seriously considering time (my vacation), the cost of material and the trouble to do the job by myself, considering that I'm not a pro also...

But last year, we had an estimate for spray foam the cathdral ceiling and blowing cellulose in others rooms... for $4,000. So, it was too expensive and that's why we were looking for alternatives.

We will check for others companies and have more estimates.

The cathedral ceiling area is approximatively 12 by 12 for one room and the other part is 12 by 24.

[Fredartic]

What do you think about that polyurethane product? It is 2 inch and 3/8 thick for a R16,3 value that cost 0,75 square inch. If I use 2 pieces I will get R-32 for 4 inches and 3/4 thick that would let more than 2 inches free space. Should I vented that free space or not in that way?

thanks

[Fredartic]

...or if I use 3 pieces deep, it will take 7 inches and 1/4 and letting 1/4 inch on my 7 and 1/2 total space available, so no vent but R-48,9 value for just an additional $370.

it seems a good idea, isn't it?

...and than I will put my t and g pine panel over it and no more worry about my energy billing!!!

[Fredartic]

...or if I use 3 pieces deep, it will take 7 inches and 1/4 and letting 1/4 inch on my 7 and 1/2 total space available, so no vent but R-48,9 value for just an additional $370.

it seems a good idea, isn't it?

...and than I will put my t and g pine panel over it and no more worry about my energy billing!!!

oups! here the link:

http://www.prokit.ca/liquidation/materi ... lyurethane

[Tom Raymond]

I can't read French and the English site is under construction, but that looks like what I was talking about. You may find it cheaper to buy something similar here though rather than deal with customs and duty.

Don't insulate the whole thing in rigid foam. It is far too labor intensive. You need one layer of foam to build R-value in as little space as possible, then fill the rest with batts. Cheap and easy. And it works.

[Fredartic]

I can't read French and the English site is under construction, but that looks like what I was talking about. You may find it cheaper to buy something similar here though rather than deal with customs and duty.

Don't insulate the whole thing in rigid foam. It is far too labor intensive. You need one layer of foam to build R-value in as little space as possible, then fill the rest with batts. Cheap and easy. And it works.

Tom, I don't know in NY state but here that kind of product is so difficult to find.

Also, regarding the customs boarder, we usually pass the custom easily with our building stuff for the house. Of course, it will need few trips because of the size of the stuff...

If the cost of the batts (for 5 inches and half deep, it cost $1,06 per sf, plus the price of the vapor barrier vs $0,75 per sf for polyurethane) is not cheaper than the cost of the polyurethane, which solution is better?

Considering that I'm not supposed to compact the batts???

thanks

[Fredartic]

Any other comment? Everyone agree with this method?

[Stephen Lagueux]

You can see that there is no vapor barrier. Also, there are no ventilation, blocked by 2 x 8...

And you have microbial growth too. But what do I know, I'm also french canadian...[:-monkeyd

[Fredartic]

on the third picture, you can see the wall, there is a board that I don't know what it is. It's a light stuff, not thick and it seems that the aluminium siding is right nailed on it.

My question is: do I need to add OSB or plywood on that board before installing 1 x 3 and canexel LP siding?

If it needs plywood or OSB, which depth is necessary?

[Fredartic]

And you have microbial growth too. But what do I know, I'm also french canadian...[:-monkeyd

Stephen, the microbial problem has been resolved. I said that it was treated with an appropriated produtc. I don't worry about it anymore.