[django11]

FWIW here is what I did.  The original owner of my Cape Cod style house made a second floor in the unused space under the roof giving him a 1 1/2 story house.  I have eaves and a flat portion of ceiling on the second floor.  The "rafters" are only 2x4.  I was able to get sheets of foil faced  2 3/8 inch polyisocyanurate ( about the same r value as the sprayed on stuff) at a discount (apparently the sheets are use in containers to ship sensitive product to North America from Europe. Once here they sell the sheets at a discount instead of shipping them back).  So I just cut the sheets on the table saw 1/2 inch smaller than the space between the rafters and used canned foam ( which I happened to get on sale as well) to seal them in space flush with the bottom of the rafter.  This gave me 1 5/8 airspace for venting above the insulation and a tight seal.  I put the foil face out to radiate heat away.  Below the "rafters" I installed 3/4  foil faced polyisocyanurate in between 1" thick furring strips .  The 1" is to add a bit of rigidity and to leave a minimum airspace to allow the foil face to have some effectiveness radiating heat back in ( an airspace is required for foil to radiate heat).   It would have been possible to do multiple layers of the 2 3/8 inch stuff if I had the space and one can buy small foam kits to fill the gaps.

This is the solution I decided on to keep costs down and to minimize the loss of space on the inside.  The room used to be cold and now is quite comfortable.  Also, I had no icicles on the eaves this winter .  I understand that the r-value is middling.  I have about 20 on the knee walls, approx 25 on the sloped part and around 37 on the flat part of my ceiling.  High r values are great but at a certain point there are diminishing returns.  And one has to take into account how long you will be in the house to justify the expense...

One last point.  Surprisingly I can hear more noise from the outside with the new system ( compared to the old one with maybe 2 1/2 inches of fiberglass.

[jriggy]

Hey all,

If i am going to use rigid foam to create the air channel what type should I use? and could I use foil backed, with the foil side out, facing out towards the underside of the roof decking to radiate heat away?

Also, would it be ok to just snug-fit and glue the rigid foam in-between the rafters? or will I have to nail in a ledge of sorts for it securely sit on?

Getting there!

Thanks so much!

Jason

[thunderbrick]

Don't use foil faced.  If used behind vinyl siding (no body does that anymore) it can cook the stuff.  I'd use Dow or OC rigid foam. 

[jriggy]

Thanks tb! I will look into 1" rigid foam from those brands...

Another question before I have all my 'basic info' and proper language in place to feel comfortable asking pros on another forum...

Really how important is temperature transfer from the rafters???
Im trying to decide if rigid foam and then the spray foam is enough ($$$) or is it really going to be worth it to go the extra bit and hat-channel the drywall off of the rafters, so we could cover the remaining part of the rafter with, say an inch of spray foam --after the hat-channel is on, of course, so the spray foam coats around them...

~J

[Atlplasma]

Thanks tb! I will look into 1" rigid foam from those brands...

Another question before I have all my 'basic info' and proper language in place to feel comfortable asking pros on another forum...

Really how important is temperature transfer from the rafters???
Im trying to decide if rigid foam and then the spray foam is enough ($$$) or is it really going to be worth it to go the extra bit and hat-channel the drywall off of the rafters, so we could cover the remaining part of the rafter with, say an inch of spray foam --after the hat-channel is on, of course, so the spray foam coats around them...

~J

There are several flavors of rigid foam. Check here http://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation to learn more.

You should talk to a good insulation contractor to figure what will work best for your situation. Thermal transfer is a big issue but has to be addressed by adding a thermal break to the exterior.

[jriggy]

I signed up for the free 10 day trial at Green Building Advisor but I still can not see any of the info from these links. Ill get that figured out...

[django11]

Hey all,

If i am going to use rigid foam to create the air channel what type should I use? and could I use foil backed, with the foil side out, facing out towards the underside of the roof decking to radiate heat away?

Also, would it be ok to just snug-fit and glue the rigid foam in-between the rafters? or will I have to nail in a ledge of sorts for it securely sit on?

Getting there!

Thanks so much!

Jason

Here is a positive opinion on using foil faced under the roof deck and is basically what I did as outlined above.  http://energy.gov/energysaver/articles/radiant-barriers

Having done it a number of times it is much easier and faster cutting the rigid insulation 1/2 smaller than the rafter bay (on a table saw), use small triangular shims of insulation to hold it in place while you adhere it by applying spray foam in the 1/4 inch space that you created on each side.  The foam is a very strong adhesive and you are now airtight as well.

I put a panel of insulation over the rafters and between furring strips 24 inches on center.  So I do have a bit of thermal bridging (where the furring strip crosses the rafter) but only about 3 square inches every 24 inches.

Notice that if the drywall is screwed directly to the rafters the drywall screw heads will be cold (through thermal bridging) and humidity will condense on them.  When this happens you see little black spots on the wall or ceiling where the screws are.  In my room this was so severe that the drywall around the screw heads would swell up.

[django11]

Here is a decent primer on insulating and types of insulation :http://www.cmhc-schl.gc.ca/en/co/maho/enefcosa/enefcosa_002.cfm

[thunderbrick]

Notice that if the drywall is screwed directly to the rafters the drywall screw heads will be cold (through thermal bridging) and humidity will condense on them.  When this happens you see little black spots on the wall or ceiling where the screws are.  In my room this was so severe that the drywall around the screw heads would swell up.

That's asking for rusty screws and mold.

[Peter J]

Thanks tb! I will look into 1" rigid foam from those brands...

Another question before I have all my 'basic info' and proper language in place to feel comfortable asking pros on another forum...

Really how important is temperature transfer from the rafters???
Im trying to decide if rigid foam and then the spray foam is enough ($$$) or is it really going to be worth it to go the extra bit and hat-channel the drywall off of the rafters, so we could cover the remaining part of the rafter with, say an inch of spray foam --after the hat-channel is on, of course, so the spray foam coats around them...

~J

The 1x2 strips to attach channel bottoms just make life a little easier as you are installing foam board. You can't accidentally push it in too far It doesn't have to be pretty lumber, or even continuous... an intermittant stop is adequate, particularly when spraying foam over the top which will lock it place. Around here I can buy a bundle of 8' furring strips for about $10, I think that's a dozen or maybe sixteen.

Thermal bridging is a real concern, but the reality is it wasn't really considered much before the last 10 or 15 years. Since you're there anyway, we're only talking about a bit of material and time and the loss of 3/4" of headroom...almost no downside.  Did you look at the Mooney Wall? Unless I already had the hat channel, I'd strip it in wood on 16" centers. My guess is there's significant out-of-plane rafter to rafter, with furring you can shim the low ones  and get a flatter ceiling to boot.

Have you thought any about dense packing cellulose? You could even do it yourself if you're adventurous. A "flash" coat of closed cell foam to seal things up, strip the bottoms of rafters, install insulmesh and blow it in. This might be helpful;

http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm

[jriggy]

The 1x2 strips to attach channel bottoms just make life a little easier as you are installing foam board. You can't accidentally push it in too far It doesn't have to be pretty lumber, or even continuous... an intermittant stop is adequate, particularly when spraying foam over the top which will lock it place. Around here I can buy a bundle of 8' furring strips for about $10, I think that's a dozen or maybe sixteen.

Thermal bridging is a real concern, but the reality is it wasn't really considered much before the last 10 or 15 years. Since you're there anyway, we're only talking about a bit of material and time and the loss of 3/4" of headroom...almost no downside.  Did you look at the Mooney Wall? Unless I already had the hat channel, I'd strip it in wood on 16" centers. My guess is there's significant out-of-plane rafter to rafter, with furring you can shim the low ones  and get a flatter ceiling to boot.

Have you thought any about dense packing cellulose? You could even do it yourself if you're adventurous. A "flash" coat of closed cell foam to seal things up, strip the bottoms of rafters, install insulmesh and blow it in. This might be helpful;

http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm

Peter J,

I see what you are saying about the furring spacers... I talked to my guy that will be helping me with the rigid foam portion and he agrees with the wood strips, so thank you. I think I am just trying to 'pare down' where I can but I see this is not the stage to do so.

I did look at the Mooney Wall and am very intrigued with this approach now...

If I could use 1x2's or 1x3's instead of loosing 2 inches for the "Mooney strips" that would be a plus. IF I could still get the dense-pack around the end of the rafters in that 1 inch gap between rafter and drywall. ???

I did blow in cellulose under the floor myself. It was actually kinda fun! Since there was no explanation of difference in cellulose product, I assume this is the same stuff I used for the floor, just packed in real tight.

So, to recap  --we will have a 1" air gap, 1" rigid foam layer, then...
Is there an R-value gain with 4 to 4 &1/2 inches (depending on how thick the 'flash' coat of spray is) of dense-pack cellulose over the 3 inches of R-21 spray-in that has been quoted by an installer???

I am going to think hard on this approach... But the two things holding me back is the extra time I would have to put in with my hired helper (as opposed to having it all sprayed in a day by an installer) and we would then be in for three different types of insulation in the end... I get that it would probably work out to maybe $1500. or so less invested but.... Hmmm... The time and the added hours paid to my helper for us to do this ourself may just even it out in the end. But if there is a nice R-value gain with dense-pack, I may do it.

One more question:

We have two skylight installed...  Should I seal these two bays top to bottom, since they cannot be vented? Hoping this is OK and not another issue.

~J

[Atlplasma]

Peter J,

I see what you are saying about the furring spacers... I talked to my guy that will be helping me with the rigid foam portion and he agrees with the wood strips, so thank you. I think I am just trying to 'pare down' where I can but I see this is not the stage to do so.

I did look at the Mooney Wall and am very intrigued with this approach now...

If I could use 1x2's or 1x3's instead of loosing 2 inches for the "Mooney strips" that would be a plus. IF I could still get the dense-pack around the end of the rafters in that 1 inch gap between rafter and drywall. ???

I did blow in cellulose under the floor myself. It was actually kinda fun! Since there was no explanation of difference in cellulose product, I assume this is the same stuff I used for the floor, just packed in real tight.

So, to recap  --we will have a 1" air gap, 1" rigid foam layer, then...
Is there an R-value gain with 4 to 4 &1/2 inches (depending on how thick the 'flash' coat of spray is) of dense-pack cellulose over the 3 inches of R-21 spray-in that has been quoted by an installer???

I am going to think hard on this approach... But the two things holding me back is the extra time I would have to put in with my hired helper (as opposed to having it all sprayed in a day by an installer) and we would then be in for three different types of insulation in the end... I get that it would probably work out to maybe $1500. or so less invested but.... Hmmm... The time and the added hours paid to my helper for us to do this ourself may just even it out in the end. But if there is a nice R-value gain with dense-pack, I may do it.

One more question:

We have two skylight installed...  Should I seal these two bays top to bottom, since they cannot be vented? Hoping this is OK and not another issue.

~J

Just to clarify, with flash and batt, the spray foam goes in first and helps to air seal the structure. The batts or cellulose then go on top to provide extra insulation.

If you decide to spray, it's critical to ventilate the area (and probably your entire house) for several days afterward. Some posters I've read recommend moving out for three days to avoid chemical exposure, but that may be overkill. Although it seems to be rare, some owners develop chemical sensitivity when exposed to foam.

You might want to find some new construction with fresh foam and walk through the structure just to verify that you're not bothered by any of the agents used to expand the foam. As I said, it's not an issue for most people, but it would be a shame if you had to sell your house because you couldn't tolerate the insulation.

[MaxCast]

What about sealing around them real good.  You could then drill vent holes in the truss/gable above and below the skylight to vent to the adjacent vent next to it.
Sounds like you are well insulated for the summer months  but you want keep you new 1" "attic" as cool as possible in the winter.  A warm attic will melt snow which runs down and can make an ice dam at the eve.

[jriggy]

Atlplasma,

I am aware of the order the products go in. We are doing a vent constructed of rigid foam instead of those pre-made vent channels like Accuvent or whatever, then will spray foam OR flash foam to seal, and then dense pack cellulose.
 
Thanks for the info on the chemical situation. The attic has a door we can seal it from the rest of the house and open the skylights with some big fans running for a few days. Thanks for that!

MaxCast,

That is a great idea to vent the two skylight bays. Ill look into that further...

Jason