I'm designing a home that will have a couple of rooms with a cathedral ceiling matching the roofline, and would like to have the rafters exposed. I'm wondering how to accomplish this and still get adequate R-value insulation for this space, as there would be no attic space in this area. Could rigid insulation be adequate on it's own, inserted b/w the exposed rafters? SIPs? I know that one option is to simply drop the ceiling and have exposed "rafters" that are decorative only, however this would not be the preferred option. Would prefer to work with the actual roof rafters and am looking for an innovative solution / product.
material is usually free in the states if you can find a space to cure it. Phragmites is an invasive reed in north america - there are a lot of wetland reservations that would be happy for you to take it off their hands.
Otherwise, you could do exposed purlins instead of rafters.
Span the purlins with rafters and struc sheathing or SIPS.
fill the cavity with batt, iso, rigid, or whatever...depending on the depths of your rafters.
lie shmie. sip on the roof can be a pain in the ass for the framers. better off over-framing with another set of sticks, and spray-foaming in between with corbond. done this many times. it sounds redundant, but using sips is redundant as well. be sure to check with your roofing manufacturer if you have to ventilate or not. if you are having exposed rafter tails, 9 times out of 10 you can set your upper rafters to the desired thickness of the tail, and then you don't have to scab anything on which is nice. if you are using an engineer, he can design the structure as a diaphragm so that both layers of rafters are working together; if you are a purist and "decoration" bothers you so much. if you are designing the structure, than you have to follow code so that one can take the load of the other. in one way or another though, everything is structure, and everything is decoration. i don't know why you guys get so hung up on decorative things. in the end, it's about the quality of the space.
structural sheathing above the rafters (t&g) then polyiso or EPS above that, then airspace/roofing.
Is this something that's done in residential roofing assembly? Where does waterproofing go? below the insulation or above (reverse roofing)? How is insulation attached? By being adhered? How do you make the airspace? With shims? Is there a limit for this type of assembly as far as pitch is concerned?
I ask because in a commercial project this would be hard to pull off. More specifically, rigid insulation over wood sheathing will not get your building insured if you are dealing with the likes of Factory Mutual. Specifically rigid insulation over wood structure. Their concern (FM) is that adhered insulation has a great chance of coming undone during a strong storm. If the insulation is mechanically fastened, then the roof has a much higher chance of leaking. They have studies to back this up.
Perhaps this is not such a big deal in residential architecture.
nope, it isn't. just lay some sleepers on top of your sheathing, insulate between them, lay another layer sheathing over that with air space for ventilation if you need it, then underlayment and roofing. water proofing always goes outside the roof sheathing. i think you are referencing more low-slope type roofs.
Detail from the first PDF on upper right page 6 (same detail appears in 2nd PDF):
"Nail base for shingles (plywood or OSB) screwed through rigid insulation to wood decking or timber frame"
That's a big problem no? you just screwed through your sheet membrane. Multiple times. That would be a horrible detail for Passive-House. Sure, the insulation itself will keep water away for the time being, but 5-10 years from now this becomes a leaky roof. Thus FM insurance not allowing for this particular detail.
lebossman, I still don't see a way of not puncturing the waterproofing while having a continuous insulation. This is not an issue encountered with flat (or low slope) concrete roofs.
for residential roofs, the shingles themselves combined with the underlayment are the water proofing. they are overlapping, self-healing and are punctured by the nails. metal roofs are much the same, except the screws are rubber gasketed, or if they are standing seam, the panels overlap. continuous insulation isn't normally called for in residential architecture.
le bossman yea, I get the basics of residential construction. At the same time, for residential construction to to become more energy efficient (like Passive-House standard for instance) some of these techniques will not be good enough. Continuous insulation should be a must. In a vaulted ceiling you are risking interior condensation if your insulation has gaps in it.
You say that shingle underlayment completes the waterproofing. I am familiar with roofing paper (vapor retarder) going underneath the shingles, meaning air barrier needs to be introduced as well elsewhere in the assembly. The concept of continuous water protection around the entire shell also gets lost with this approach.
Residential detailing is sloppy at best. meh. As far as vaulted residential ceilings are concerned, I've seen more of them that had serious issues (water leakage, mold) than the ones that actually worked.
holz, screwing through waterproofing works great for a year or two. Then your wrap starts shrinking (like most plastics based building materials). And then you have thousands of little holes leaking air every each way. I though you were on a war path with leaky houses :)
In the detail I mentioned previously (from your PDF) everything appears to be screwed through! Well, air barrier gets screwed through, and WGB gets nailed through. Intentionally driving a single screw through an industrial roofing product voids your warranty.
I was thinking specifically in the terms of the original question posted in this thread. Like I said, vaulted ceilings in residential construction are pretty awful even for an industry with lax construction standards.
and yet no one provided a sketch. Yea I thought it was rather obvious, insulation on the roof. Steelstuds if you were looking to retain the R-value for the insulation you could always go with a continuous membrane (bedding, sprayed, etc). Not withstanding, and holz.box can correct me isn't the concern with passive-haus to have a building isn't leaking air (and the limits for annual energy)?
air barrier gets screwed, that's ok. it's not air-tight, but it shouldn't leak a ton.
on the roof, you a roofing underlayment goes down before shingles which are nailed. if you are doing a rainscreen, you'd screw that down to battens that are screwed through your WRB.
ATP,
i think steel wasn't sure that penetrating an air barrier would allow for a passivhaus qualified construction. as long as those penetrations are screws, taped or caulked, it's generally going to be pretty tight.
Insulation for exposed roof rafter cathedral ceiling?
I'm designing a home that will have a couple of rooms with a cathedral ceiling matching the roofline, and would like to have the rafters exposed. I'm wondering how to accomplish this and still get adequate R-value insulation for this space, as there would be no attic space in this area. Could rigid insulation be adequate on it's own, inserted b/w the exposed rafters? SIPs? I know that one option is to simply drop the ceiling and have exposed "rafters" that are decorative only, however this would not be the preferred option. Would prefer to work with the actual roof rafters and am looking for an innovative solution / product.
structural sheathing above the rafters (t&g) then polyiso or EPS above that, then airspace/roofing.
you can do SIPs, but they tend to be more expensive and not look so great, so you'll have to adhere a finish to the underside between the rafters.
be sure that it is properly airsealed as well...
decorative rafters would be a lie.
thatched roof.
material is usually free in the states if you can find a space to cure it. Phragmites is an invasive reed in north america - there are a lot of wetland reservations that would be happy for you to take it off their hands.
labor is not cheap, though...
... and unless you live in hawaii or europe, you'll have to go through an extensive code review process and testing... also not cheap...
good news is that thatch has an R value of about 40.
I think Holz. got it.
Otherwise, you could do exposed purlins instead of rafters.
Span the purlins with rafters and struc sheathing or SIPS.
fill the cavity with batt, iso, rigid, or whatever...depending on the depths of your rafters.
lie shmie. sip on the roof can be a pain in the ass for the framers. better off over-framing with another set of sticks, and spray-foaming in between with corbond. done this many times. it sounds redundant, but using sips is redundant as well. be sure to check with your roofing manufacturer if you have to ventilate or not. if you are having exposed rafter tails, 9 times out of 10 you can set your upper rafters to the desired thickness of the tail, and then you don't have to scab anything on which is nice. if you are using an engineer, he can design the structure as a diaphragm so that both layers of rafters are working together; if you are a purist and "decoration" bothers you so much. if you are designing the structure, than you have to follow code so that one can take the load of the other. in one way or another though, everything is structure, and everything is decoration. i don't know why you guys get so hung up on decorative things. in the end, it's about the quality of the space.
Is this something that's done in residential roofing assembly? Where does waterproofing go? below the insulation or above (reverse roofing)? How is insulation attached? By being adhered? How do you make the airspace? With shims? Is there a limit for this type of assembly as far as pitch is concerned?
I ask because in a commercial project this would be hard to pull off. More specifically, rigid insulation over wood sheathing will not get your building insured if you are dealing with the likes of Factory Mutual. Specifically rigid insulation over wood structure. Their concern (FM) is that adhered insulation has a great chance of coming undone during a strong storm. If the insulation is mechanically fastened, then the roof has a much higher chance of leaking. They have studies to back this up.
Perhaps this is not such a big deal in residential architecture.
not a big deal.
http://www.buildingscience.com/documents/reports/resolveuid/9211ad8bef39246eacf9a86539214cdf
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks/files/bsi-036_complex_3d_airflow_networks.pdf
http://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulation-out-your-walls-and-ceilings
http://sustainable.cchrc.org/docs/REMOTE_Manual.pdf
http://www.rootdesignbuild.com/blog/passive-house/prepping-the-roof/
nope, it isn't. just lay some sleepers on top of your sheathing, insulate between them, lay another layer sheathing over that with air space for ventilation if you need it, then underlayment and roofing. water proofing always goes outside the roof sheathing. i think you are referencing more low-slope type roofs.
thanks holz!
Detail from the first PDF on upper right page 6 (same detail appears in 2nd PDF):
"Nail base for shingles (plywood or OSB) screwed through rigid insulation to wood decking or timber frame"
That's a big problem no? you just screwed through your sheet membrane. Multiple times. That would be a horrible detail for Passive-House. Sure, the insulation itself will keep water away for the time being, but 5-10 years from now this becomes a leaky roof. Thus FM insurance not allowing for this particular detail.
lebossman, I still don't see a way of not puncturing the waterproofing while having a continuous insulation. This is not an issue encountered with flat (or low slope) concrete roofs.
for residential roofs, the shingles themselves combined with the underlayment are the water proofing. they are overlapping, self-healing and are punctured by the nails. metal roofs are much the same, except the screws are rubber gasketed, or if they are standing seam, the panels overlap. continuous insulation isn't normally called for in residential architecture.
generally, screws/tapes/caulks are weaker joints than an unpenetrated air barrier, but for all intents and purposes, screwing is fairly airtight.
nails, not so much.
at least someone's paying attention to the passivhaus stuff!
le bossman yea, I get the basics of residential construction. At the same time, for residential construction to to become more energy efficient (like Passive-House standard for instance) some of these techniques will not be good enough. Continuous insulation should be a must. In a vaulted ceiling you are risking interior condensation if your insulation has gaps in it.
You say that shingle underlayment completes the waterproofing. I am familiar with roofing paper (vapor retarder) going underneath the shingles, meaning air barrier needs to be introduced as well elsewhere in the assembly. The concept of continuous water protection around the entire shell also gets lost with this approach.
Residential detailing is sloppy at best. meh. As far as vaulted residential ceilings are concerned, I've seen more of them that had serious issues (water leakage, mold) than the ones that actually worked.
holz, screwing through waterproofing works great for a year or two. Then your wrap starts shrinking (like most plastics based building materials). And then you have thousands of little holes leaking air every each way. I though you were on a war path with leaky houses :)
steel,
WRB's get penetrated all the time by fasteners (nails and screws).
but air barrier is usu. not the WRB.
the use of screws over nails leads to less building leakage/moisture movement through wall assembly.
and yes, one should generally be aware of the number of penetrations through your barriers. more holes = more leakage.
holz,
In the detail I mentioned previously (from your PDF) everything appears to be screwed through! Well, air barrier gets screwed through, and WGB gets nailed through. Intentionally driving a single screw through an industrial roofing product voids your warranty.
I was thinking specifically in the terms of the original question posted in this thread. Like I said, vaulted ceilings in residential construction are pretty awful even for an industry with lax construction standards.
and yet no one provided a sketch. Yea I thought it was rather obvious, insulation on the roof. Steelstuds if you were looking to retain the R-value for the insulation you could always go with a continuous membrane (bedding, sprayed, etc). Not withstanding, and holz.box can correct me isn't the concern with passive-haus to have a building isn't leaking air (and the limits for annual energy)?
steel,
air barrier gets screwed, that's ok. it's not air-tight, but it shouldn't leak a ton.
on the roof, you a roofing underlayment goes down before shingles which are nailed. if you are doing a rainscreen, you'd screw that down to battens that are screwed through your WRB.
ATP,
i think steel wasn't sure that penetrating an air barrier would allow for a passivhaus qualified construction. as long as those penetrations are screws, taped or caulked, it's generally going to be pretty tight.
geesh you guys are boring...
thatch is awesome.
I know its probably impractical for this specific instance, but...
it's just so soft and thick... how can thatch not make you happy?
toaster:
modern variant, code approved
Seriously!
[url=http://farm5.static.flickr.com/4085/5016319669_6d5e57bb20_b.jpg]bigger[url]
Wait! What?
Disclaimer. I know nothing about architecture.
who farted?
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