Those styrofoam baffle vents that are installed in eaves of roofs, are they supposed to continue to the ridge vent or just one piece from the cavity to just above the top plate? Any opinions on this?
Thanks,
The Sicilian
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Replies
Just one piece.
To keep an open air flow from the eave into the attic space.
Basically to keep the blown-in insulation from closing up the air space.
Thats what I thought but in the cold winters are you just piping frigid air into your attic or open areas?
Yeah,
But I don't think it is as bad as you might think.
The old hot air goes up thing, and all. I think it is simply more important to get that superhot air out in the summer.View Image
Remember that the baffles are used in "cold roof" construction, that is, you depend on the insulation adjacent to your heated space to retain the heat. The cold roof, if done properly, prevents ice damming along the eaves by keeping that area cold enough that snow on the roof doesn't melt as a result of heat escaping from the interior. You are building in a cold air layer between your heated, insulated space and your roof.Formerly BEMW at The High Desert Group LLC
What the heck do you do when the roof is hipped?
Not much changes; you still have soffits and rafter bays. A baffle on any side of the hip would still allow air to drift from a soffit inlet up toward a ridge vent, if you have a ridge between the hips, or a roof turtle (passive vent cut into the the roof near the top). Or you could install a turbine vent. Point is, you need to vent a cold roof, or it won't stay cold. The venting solution just might not be as unobtrusive as a continuous ridge vent.Formerly BEMW at The High Desert Group LLC
On the hipped part of the roof, if you have sheetrock on the underside of the rafters, then the hot air coming up every baffle hits a dead end in every rafter bay. A roof turtle every 16" sounds ugly.
A sheet metal roof or metal shingles on sleepers is the best solution. State Farm gives a 27% discount on the homeowner's insurance, which is about $400 per year for the next 50 years. (Denver is a high risk area for hail) So I'm spending an extra $4000 to save $20,000. It should work in all climates, from hot to snowy.
So should baffles be used in every cavity between the rafters or every other one? In this case the referred to addition has empty space between top plate or ceiling height to the peak of the gable which has a ridge vent.
Also in this case would a continuous run from the top plate to the ridge be less efficient?
Thanks,
The Sicilian
If you look at Vince's reply, he's used to seeing the baffle run continuously up yhe bay. His theory makes sense; just something I've not seen done. My personal opinion ... I'd put a baffle, just one length of it, in every bay. I think more inlet area promotes more active flow. Doesn't make my opinion more right or better than another, though.
Another opinion is that corrugated ridge vents just don't offer enough free vent area, particularly as they get bunged up with spider webs over time. I think cold roofs need larger vents, and I like the idea of turbine vents, if you can put one or more in an acceptable area visually. I put one in my mother's attic, and it's very effective both in summer and in winter. If not a turbine, then some other vent(s) high on an unobtrusive exposure of the roof.Formerly BEMW at The High Desert Group LLC
You need one baffle for every rafter bay and a continuous soffit vent. Your attic space is coupled to the exterior in this system so yes you are piping in fridgid air but your insulation should keep the house warm.
Bob,
Its been a couple of years since I first posted this but I'm glad you replied because I am revisiting this issue this month. A little more details: this space is a 12' x 16' wide addition with a gable roof and the gable space is about 6' above the ceiling of the kitchen addition which about 90". The space is unused just open area.
So I have been told by different contractors that one way would be to put one baffle in each bay and someone else said that each baffle in each bay should be continuous up to the ridge vent. So which is it?
The Sicilian
My feeling on this, and I'm no expert just interested in this field, is that the baffles only exist to keep insulation from blocking the soffit vent. So I'd say use a continuous soffit vent, one baffle per rafter bay which is only tall enough to keep back your insulation. You shouldn't need to go all the way to the ridge unless you've got a cathedral ceiling, in which case there is some debate. If you go to the library and find Gene Leger's "Complete Building Construction" it has a lot of details about appropriate soffit/ridge ventilation design.
One thing I discovered with attic venting is the incredible amount of dust that it brings in. Baffles might alleviate that.
In a cathedral ceiling, the rafter bays have insulation from the top plate to the ridge since it is open to the living spaces and usually either drywalled or covered with wood, and as such keeping the heat in the living spaces. With an open cavity, I believe that the rafter space would not be insulated as the ceiling below would be. I am not absolutely sure on this but it would appear to be the proper way to do it.
Otherwise in that open cavity air enters through a soffit vent outside and travels along the deck of the roof through the baffles and out the ridge vent. In the winter this is very cold air. If only one baffle, then all cold air would disperse throughout the cavity above the ceiling and eventually would have a pull to the ridge vent. However the deck would not appear to be as cooled as with a continuous baffle vent to the ridge. Does this make sense?
Regardless of baffle configuration, unless you have insulation above the roof deck, then the roof deck will always about as cold as the outside. The wood roof deck has little to no insulative value and a very large cold mass on the ouside (the rest of the world).The baffle is only to provide an air channel. If that is done by not having insulation near the roof deck in an open cavity situation, then you only need baffles near the soffits.
Not much blown in insulation up here (Vermont). We use the "baffles" continuously from the top plate to the ridge so that convection is created via the continuous air space over the insulation between the soffit vent and the ridge vent to isolate the roof deck from heat loss and prevent ice damning which we do have a lot of up here. Also on a recent job the energy consultant referred to the "baffles" (proper vent) as a rain shield for whatever moisture migrates through the roof in the way of condensation or just plain bad roofing. For this purpose the vent is started at the ridge so each section overlaps and moisture is transferred from the cavity to the soffit vent.
Through the mid-nineties many builders here installed metal over strapping with no solid sheathing under it, the resulting condensation did some damage. It's also not unknown for cedar shingles on skip sheathing to create moisture problems in the rafter cavity. With the advent of cedar breather that's not much of a problem these days although an occasional customer requests skip sheathing.
One of the main reasons to vent an attic is to remove moisture from the air in the attic so it does not condense on the underside of the roof sheathing (in a heating climate). If you have a continuous baffle from soffit to ridge it limits the amount of air that can get into the channel, which is a bad idea. More likely the moisture would condense on the rafters. I suppose its unlikely though that your channel would be perfectly air sealed.No amount of venting is going to compensate for a leak. If the roof leaks, you've got bigger problems. Condensation should not happen in a well designed roof/attic.All that said, in a cathedral ceiling you don't have much choice but to run baffles from top plate to ridge.
If you must vent a cathedral ceiling, an excellent approach is to space 1 in. rigid foam 1 in. down from the roof deck, so that the fg is completely isolated from the outside air. Unless fg is completely enclosed in a cavity, and not exposed to frigid outside air, its r-value drops by a huge percentage as the temperature drops. That's because of convective loops that develop within the batts. They're still a problem in enclosed cavities, but not to the same degree.
Neither dense-pack cellulose nor spray foam suffers from that issue. In fact, their effective r-value increases slightly as the temperature decreases. Andy Engel
Senior editor, Fine Woodworking magazine
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Andy, a 1" thick piece of RFBI is going to act as a vapor barrier/retarder on the wrong side of the insulation. Condensation will gather on the RFBI-FG interface -it'd be like having a kraft faced FG bat installed backwards. 1" of RFBI is not likely thick enough to control condensation. Thinking about it now, though, baffling top plate to rafter isn't much better. Though the baffle is likely more vapor permeable because its so thin. Really the baffle should be totally vapor permeable - but I don't think such a baffle exists. The best solution is to just use RFBI the whole way and foam the seams so that there's no air movement. And toss the FG.
Edited 9/26/2005 7:34 pm ET by BobS
I'm OK with that, particularly if there's no internal VB. In the first place, if you use eps, 1 in. isn't a VB. Second, I'd be surprised if the inside of such a foam assembly in all but the coldest climates would cool enough to reach the dew point given lower interior humidity during the winter. In fact, lacking an interior VB, put in as many inches of any type of foam as you want. The assembly can dry to the inside come summer, and isn't likely to become wet anyway, with good air sealing. It's very unlikely to ever reach a dew point. And otherwise, fiberglass insulation installed in a vented cathedral assembly is one step above worthless in terms of keeping the heat in.Andy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
None of this matters in geological time.