HI. I have a roof ventilation design issue. The project is a low slope earth roof. The construction is mostly wood and steel framing. Wood TJL's are laid flat and the low slope is overbuilt up to sheathing and a waterproof membrane. Drainage is through parapet gutters and roof drains. The roof rafters are typically bound by masssive wood beams and steel girders. Glass typically runs to the underside of these beams. In most other cases concrete bounds the rafters in. Local building code does not allow for packing the rafter space with BIB insulation and a non-ventilated roof without a special approval from local building official. Local building official will not give special approval. I am thinking of blocking the rafters and leaving a reveal in the ceiling of the interior running perpendicular to the direction of the rafters, thus ventilating the rafter space into the interior. Do any of you have any experience or suggestions with/on this design issue. Thank you.
We've done a bunch with both sod and planted roofs, we alwaysput the insulation on the top side and avoid the ventilation issue entirely. depending where you are in the world the ventilation thing will be the cause of most condensation weather issues and can be difficult. I would relly think it through and I think to avoid bigger problems you will see that rigid insulation on the top side will allow for much cleaner detailling.
Gotta vent to the outside not inside. Would not work the other way around being as the whole idea is to prevent ice from building up on the roof memebrane i.e. making sure that the roof doesn't heat up (because the insulation on the inside is warmer that the air on the outside) and then freeze. What about the good old roof vents penetrating through the earth roof (a standard on commercial roofs). Can be a good way to break up the earth roof if you choose the right vents. Venting is important though especially if you earth roof is not that deep and there is a risk of freezing. If its three feet deep or more you might be able to convince the building department that it will never freeze depending on where you are.
Whistler- Don't you still have to vent unless your rafters are exposed to the inside such as most commercial flat roofs with rigid insulation over the membrane?
I have had a similar comment from a city on a built-up roof with steel trusses & 2x6 sub-purlins. We were allowed to place our batt directly to the underside of the roof deck so long as we provided a 1.0 max. perm vapor barrier to the underside of the batt insulation (in lieu of the 1†required vent space). Now this will only apply if you are under the 1992 Model Energy Code (sec 502.1.4), but it may display evidence of a proven system if you file an appeal. Good luck.
In any case roof ventilation only makes sense between an outer moisture barrier and an inner thermal barrier.
in most flat roofs the insulation is between the deck and the roof membrane. You are supposed to vent the membrane, but there is no air space per se. The membrane is laying semi loose to allow a little air to move.
In some cases the insulation is under the deck. You would vent this type just like a pitched roof with an air space between the deck and insulation.
In your case, I believe you have the insulation above the membrane. You should use the earth as insulation or you could throw in some rigid, but venting it would only undermine the insulating value.
whistler - change of topic but we're planning to do a planted roof in Los Angeles and having some difficulty finding detailed systems which will work. No one seems to have done it on a residential project in SoCal. Any suggestions on where to look for details or companies that will sell their products to do it yourselfers? (we do design/build but its for our own house...)
Thanks!
ive used a shipping container as a conditioned space, using the corugated metal roof as the ceiling w/ tapered inso and a membrane-
on top of that we used 2x8 to frame a platform for the soil roof. no plywood on top of the joists, just green tek soil trays that filter the rain on to the membrane, then the membrane sheds the rain off the container to another garden below
(interior)fir it out - inso - drywall interior - interior is sealed tighter than a virgin. (no condensation)
R.A.......Soprema has a complete system called "sopremanature" or something like that been used in europe for years and a good starting point. I have some details that may apply that I used for my own house but given our climate may be very different. The soprema stuff is good and speak with the sales rep. they want you to buy their soil which is way over priced ( we made up our own soil mix, that worked well). I think the other stuff to look for is the drainage, filter cloth fabrics and a good membrane system ie. siplast has some good products as well
Wyoming81 ... The purpose of the venting and air flow is important and if you trace through its role in the building cavitiy it will inform as to whether you need it or not. Generally I like to over vent when I have the chance but the outside air needs to stay on the outside of the vapor barrier and inside air can move about on the inside. although I have been asked to vent a cavity space in an old house. I did it but I can't seem to understand why they wanted me too do it ( ie would you vent an internal wall ????)
you could use an "inverted" roof, where the build is deck-membrane-rigid insulation-breather membrane-turf. There would be no insulation on the inside of the waterproof membrane, and therefore it would not need to be vented...but it depends on the u-value you're after, 0.35?
This is a standard roof build-up from Hydrotech.
Mar 9, 05 1:04 pm ·
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Low Slope Roof Ventilation
HI. I have a roof ventilation design issue. The project is a low slope earth roof. The construction is mostly wood and steel framing. Wood TJL's are laid flat and the low slope is overbuilt up to sheathing and a waterproof membrane. Drainage is through parapet gutters and roof drains. The roof rafters are typically bound by masssive wood beams and steel girders. Glass typically runs to the underside of these beams. In most other cases concrete bounds the rafters in. Local building code does not allow for packing the rafter space with BIB insulation and a non-ventilated roof without a special approval from local building official. Local building official will not give special approval. I am thinking of blocking the rafters and leaving a reveal in the ceiling of the interior running perpendicular to the direction of the rafters, thus ventilating the rafter space into the interior. Do any of you have any experience or suggestions with/on this design issue. Thank you.
We've done a bunch with both sod and planted roofs, we alwaysput the insulation on the top side and avoid the ventilation issue entirely. depending where you are in the world the ventilation thing will be the cause of most condensation weather issues and can be difficult. I would relly think it through and I think to avoid bigger problems you will see that rigid insulation on the top side will allow for much cleaner detailling.
Gotta vent to the outside not inside. Would not work the other way around being as the whole idea is to prevent ice from building up on the roof memebrane i.e. making sure that the roof doesn't heat up (because the insulation on the inside is warmer that the air on the outside) and then freeze. What about the good old roof vents penetrating through the earth roof (a standard on commercial roofs). Can be a good way to break up the earth roof if you choose the right vents. Venting is important though especially if you earth roof is not that deep and there is a risk of freezing. If its three feet deep or more you might be able to convince the building department that it will never freeze depending on where you are.
Whistler- Don't you still have to vent unless your rafters are exposed to the inside such as most commercial flat roofs with rigid insulation over the membrane?
I have had a similar comment from a city on a built-up roof with steel trusses & 2x6 sub-purlins. We were allowed to place our batt directly to the underside of the roof deck so long as we provided a 1.0 max. perm vapor barrier to the underside of the batt insulation (in lieu of the 1†required vent space). Now this will only apply if you are under the 1992 Model Energy Code (sec 502.1.4), but it may display evidence of a proven system if you file an appeal. Good luck.
Can you vent at the inside of the parapet walls?
thackray,
Basically I want to back up Whistler.
In any case roof ventilation only makes sense between an outer moisture barrier and an inner thermal barrier.
in most flat roofs the insulation is between the deck and the roof membrane. You are supposed to vent the membrane, but there is no air space per se. The membrane is laying semi loose to allow a little air to move.
In some cases the insulation is under the deck. You would vent this type just like a pitched roof with an air space between the deck and insulation.
In your case, I believe you have the insulation above the membrane. You should use the earth as insulation or you could throw in some rigid, but venting it would only undermine the insulating value.
whistler - change of topic but we're planning to do a planted roof in Los Angeles and having some difficulty finding detailed systems which will work. No one seems to have done it on a residential project in SoCal. Any suggestions on where to look for details or companies that will sell their products to do it yourselfers? (we do design/build but its for our own house...)
Thanks!
ive used a shipping container as a conditioned space, using the corugated metal roof as the ceiling w/ tapered inso and a membrane-
on top of that we used 2x8 to frame a platform for the soil roof. no plywood on top of the joists, just green tek soil trays that filter the rain on to the membrane, then the membrane sheds the rain off the container to another garden below
(interior)fir it out - inso - drywall interior - interior is sealed tighter than a virgin. (no condensation)
the 2x8 are obviously not insulated - exposed to the weather
R.A.......Soprema has a complete system called "sopremanature" or something like that been used in europe for years and a good starting point. I have some details that may apply that I used for my own house but given our climate may be very different. The soprema stuff is good and speak with the sales rep. they want you to buy their soil which is way over priced ( we made up our own soil mix, that worked well). I think the other stuff to look for is the drainage, filter cloth fabrics and a good membrane system ie. siplast has some good products as well
Wyoming81 ... The purpose of the venting and air flow is important and if you trace through its role in the building cavitiy it will inform as to whether you need it or not. Generally I like to over vent when I have the chance but the outside air needs to stay on the outside of the vapor barrier and inside air can move about on the inside. although I have been asked to vent a cavity space in an old house. I did it but I can't seem to understand why they wanted me too do it ( ie would you vent an internal wall ????)
you could use an "inverted" roof, where the build is deck-membrane-rigid insulation-breather membrane-turf. There would be no insulation on the inside of the waterproof membrane, and therefore it would not need to be vented...but it depends on the u-value you're after, 0.35?
This is a standard roof build-up from Hydrotech.
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