Insulating a shed ceiling or cathedral ceiling. What is the proper way?

[Don2222]

With good insulation it doesn't suck heat out. I have R60 blown in fiberglass on my ceilings. The walls are "only" R21.

I have R4 foil, Faced R19 Fiberglass crisscrossed with R30 unfaced fiberglass for a total of R53 in my ceilings and when I blocked the gable ends with insulated plywood doors I made, the whole house felt warmer on a windy day. That windy cold air that whips into the attic and displaces warm air! Then what happens to the warm air in your house? It has to displace that cold air that came in! My wife could really feel the difference!

The cold air coming in the soffit everyday is also displacing warm air unless it is enclosed in the rafter vents. You really do not know until you try it!

I bet in Alaska it would save you some real good money in your heating bill ! ! !

 

[Don2222]

I don't give a chit what that diagram says. If you put batt insulation against the bottom of the roof deck, the deck will rot and turn moldy on the bottom side. Foam or no foam, the decking will rot.
And ice damns are not exclusively due to warm air leaks in the attic. Melting surface snow refreezing also causes ice dams and leaks. This is one reason the shingle manufacturers used to require 4" exposure on low slope roofs, and now they no longer even accept that.

Look enough, and you'll find a webpage, article, photo or pdf from someone that will say it how you want to read it. Many years as a roofer is real experience and knowledge.

X2 I agree!

 

[woodgeek]

I have R4 foil, Faced R19 Fiberglass crisscrossed with R30 unfaced fiberglass for a total of R53 in my ceilings and when I blocked the gable ends with insulated plywood doors I made, the whole house felt warmer on a windy day. That windy cold air that whips into the attic and displaces warm air! Then what happens to the warm air in your house? It has to displace that cold air that came in! My wife could really feel the difference!

The cold air coming in the soffit everyday is also displacing warm air unless it is enclosed in the rafter vents. You really do not know until you try it!

I bet in Alaska it would save you some real good money in your heating bill ! ! !

You, the previous owner of my house and about 10 million folks in the 70s made the same mistake---insulating your house attic without airsealing first. Your observations are correct, stack effect and wind pulls warm air right through that nominal R60 insulation, so you lose heat as if it weren't there. Rather than the ideal solution, pulling up the insulation, airsealing, and putting the ins back down, you have realized a complicated but workable alternative--airsealing the rafter/gable side above the insulation. You did this in a way that ensures ventilation of the decking (good), but still creates a condensation risk in your attic, because your air/vapor barrier (the reflectix) is on the 'cold side' above the R60.

Of course, if you have no visible condensation under your foil, you're good to go. But your solution is not preferable to the current retrofit best practice of v carefully airsealing the attic floor and then dumping R-60 on top. And when that has been done correctly, sealing the gable and other vents has no effect on the heat loss.

 

[Don2222]

You, the previous owner of my house and about 10 million folks in the 70s made the same mistake---insulating your house attic without airsealing first. Your observations are correct, stack effect and wind pulls warm air right through that nominal R60 insulation, so you lose heat as if it weren't there. Rather than the ideal solution, pulling up the insulation, airsealing, and putting the ins back down, you have realized a complicated but workable alternative--airsealing the rafter/gable side above the insulation. You did this in a way that ensures ventilation of the decking (good), but still creates a condensation risk in your attic, because your air/vapor barrier (the reflectix) is on the 'cold side' above the R60.

Of course, if you have no visible condensation under your foil, you're good to go. But your solution is not preferable to the current retrofit best practice of v carefully airsealing the attic floor and then dumping R-60 on top. And when that has been done correctly, sealing the gable and other vents has no effect on the heat loss.

Not Quite.

I have 2 layers of foil encased with polypropelyne. See > http://www.reflectixinc.com/
See (broken link removed to http://www.homedepot.com/h_d1/N-5yc1v/R-100020855/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=reflectix&storeId=10051)

The first layer is under the insulation in the attic floor right against the sheet rock right up tight to the floor joists. Before I put this foil in, I filled all the wire holes with a can of spray foam insulation. This layer keeps the heat in the house in the winter. This is the best air sealing I can do in the old house without pulling up all the sheetrock ceilings.

The second layer of foil is in the rafters stapled over the rafter vents. This layer keeps the heat out in the summer.

This has been in there 3 years now and no moisture at all. I can easily lift up the insulation and foil to work on wiring or inspect it! ! ! ! !

Since it is not a perfect world, shutting the gable vents in the winter will always stop some heat loss! ! !

 

[woodgeek]

Divide the square footage of your attic floor by 60 (R-60) and multiply by a temperature difference, say 60 (for a nice cold night), and you will get the BTU/hr that should be going through that attic insulation. For a typical house, this will be 1500 sqft and 1500 BTU/hr = 440W. Not a lot. Now you close up your gables, and say your rafters are R-5 (reflectix+sheathing), versus R-0 with the wind whipping through there beforehand. You now have R-65 up there, and your loss is 400W. Net energy loss decrease for an airsealed R-60 attic on a 10°F, windy night from closing the gables: 40W.

Since you are certainly seeing some significant energy savings closing the gables, and get a warmer attic when you do, something doesn't add up. The most likely explanation: the poly laid 'tight' across the drywall is not airsealing, and you are not getting R-60.

Of course, I'm an idiot and figured I had finished air-sealing my own attic three times (over three years), only to then discover other huge leaks. Net savings from ~25 tubes of caulk and some foam board: equivalent to ~200 gallons oil/yr, and a **cold** attic with the roughly R-25 FG job the previous owner did. And about half the A/C days.

Not trying to bust you Don, love your posts...but IMO your house attic job is non-standard, and could cause others problems if they tried to replicate it.

 

[Don2222]

Divide the square footage of your attic floor by 60 (R-60) and multiply by a temperature difference, say 60 (for a nice cold night), and you will get the BTU/hr that should be going through that attic insulation. For a typical house, this will be 1500 sqft and 1500 BTU/hr = 440W. Not a lot. Now you close up your gables, and say your rafters are R-5 (reflectix+sheathing), versus R-0 with the wind whipping through there beforehand. You now have R-65 up there, and your loss is 400W. Net energy loss decrease for an airsealed R-60 attic on a 10°F, windy night from closing the gables: 40W.

Since you are certainly seeing some significant energy savings closing the gables, and get a warmer attic when you do, something doesn't add up. The most likely explanation: the poly laid 'tight' across the drywall is not airsealing, and you are not getting R-60.

Of course, I'm an idiot and figured I had finished air-sealing my own attic three times (over three years), only to then discover other huge leaks. Net savings from ~25 tubes of caulk and some foam board: equivalent to ~200 gallons oil/yr, and a **cold** attic with the roughly R-25 FG job the previous owner did. And about half the A/C days.

Not trying to bust you Don, love your posts...but IMO your house attic job is non-standard, and could cause others problems if they tried to replicate it.

Hi Woodgeek

I agree that air sealing is very important. It was 93 Deg F outside in the Sun today and the Attic was boiling hot!

So I opened up the gable vents and WOW withing seconds it felt 20 Deg F cooler!

So in the real world those gable vent doors I made with foam insulation on the inside REALLY hold the heat in the attic!

 

[Don2222]

Hey WoodGeek

How does this look for air sealing the walls of the shed?

I got J&M R15 Fiberglass covered by Reflectix Foil and taped with Foil Tape!
Foil Tape is for Class 1 Flexible Ducts, Fiberglass Ducts and air connectors.
All Weather Performance, Aggressive Tack, Operating Temperatures -25 to Deg F to 325 Deg F ! ! !
See pic below.

The foil is stapled to the wall studs but the foil tape is over the staples and the foil joints!

Like Mike Holmes would say 100% Thermal Break! ! ! !

Then the paneling will go on top!

 

[woodgeek]

I was never too worried about the shed. Hot tub !!

 

[StihlHead]

Of course, I'm an idiot and figured I had finished air-sealing my own attic three times (over three years), only to then discover other huge leaks. Net savings from ~25 tubes of caulk and some foam board: equivalent to ~200 gallons oil/yr, and a **cold** attic with the roughly R-25 FG job the previous owner did. And about half the A/C days.

Been there, done that. On both air and water leaks. Many tubes of calk later here (three cases I think) and I have the water leaks sealed. Air leaks are another beast altogether though. Drafts from... everywhere! And the condensation forms seemingly from nowhere. Condensation is a big issue, and this is a great thread, really guys. Good points from all sides, and variety of options and opinions.

 

[semipro]

X2 I agree!

More on this from the experts. An article in the most recent Fine Homebuilding and featured on the Green Building Advisor site.
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

 

[Don2222]

More on this from the experts. An article in the most recent Fine Homebuilding and featured on the Green Building Advisor site.
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

Excellent article. It seems that the spray foam is the way the pros go nowadays for cathedral ceilings. That is the expensive way and not a DIY method for sure!

Here is a quote from the article

"The code restrictions on the use of air-permeable insulation between rafters were developed to prevent the roof sheathing from rotting. When fiberglass batts are installed in unvented rafter bays, the batts allow moist indoor air to reach the cold roof sheathing. That leads to condensation or moisture accumulation in the sheathing, followed eventually by sheathing rot"

Now my system of creating a vented channel with the rafter vents and to make these channels sealed with the polypropylene encased foil allows the use of air permeable fiberglass underneath with no condensation moisture and rot! I know my system is not a currently well know or an accepted method but it sure works great and the article explains why! LOL

My system is a good DIY method so it costs less!