I give up. How do I insulate this?
I’ve asked questions on and off about the theory surrounding insulating attics here for a bit and gotten some great responses, but since the time’s come to decide (and I still haven’t) I figured I’d offer this more directly to the group.
How do I insulate the attic in this diagram & pic? The sloped area is currently filled to the sheathing with cellulose (not dense) & kraft faced FG. Cellulose on the rest of the attic floor. Only 1 gable vent.
We’re re-roofing in spring so we can add a ridge vent then. I haven’t found any roofer willing to put in soffit vents, except for a SmartVent from DCI which I don’t think will work in MA.
I’ve gotten quotes to icynene it and the cost feasible but painful and delays a lot of other projects. Just not sure I like the theory behind foaming the roof so much. And I’m not sure the foamers can do it well.
No mechanical ventilation yet and no a/c. Attic is a walkup.
For a cold roof, I’ve also thought about ripping sheets of XPS and sliding it down the sloped part but that seems like a lot of work – plus I still have to vent the soffit somehow.
Rigid panels on top of the roof is out for many reasons.
Ideas? Here’s a pic looking down the sloped part.
I actually had a nightmare about this last night, so its time to directly ask the professors what to do instead of just get the theory.
Thanks!
Replies
Around here insulation in a 2x6 ceiling would be inadequate - our code is for R40 (=your R32??) - with the exception of the area under debate. I don'tr know if the rest of the roof is of minimal ins'l value, but it might be worth raising that value - and you may be able to get grants of help from your utility co to do so. But it entails adding a new set of 2x, ins'l and sheathing over the existing. May not be the best way, but you may not have thought about it.
Failing that, then I think I'd be inclined to remove the drywall, put in trays or troughs, then stuff in the appropriate ins'l, and reboard. You appreciate that there has to be an air channel. Oh, it's dawned on me that you might rather place and staple those trays/troughs from the top down, then add foaming ins'l thru small holes in the ceiling...
All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.
Thanks for the response Piko. I agree, 2x6 is inadequate, no matter what I fill it with, but we don't have the money to strip the sheathing and sister on 2xs, and resheath. Its less than 3' of the roof, so I'm okay with it being inadequate there and healthy everywhere else. I should have mentioned this in the OP, but we can't rip down the sheetrock. There's a good 1/2" of plaster on the whole ceiling that is curved from the ceiling, down the sloped part, to the walls. It would be difficult and expensive to replace that kind of craftsmanship. Also we are living in the house. I don't think I could convice my wife to rip down the ceiling.
Edited 11/21/2005 11:48 am ET by BobS
OK. As I pointed out, our BC building code allows for reduced ins'l in exactly that area...yours may do too. So, that leaves you with installing trays to the u/s of sheathing , and stuffing some kind of ins'l down afterward. F/G gets bunched up, thereby losing effectiveness, but cellulose blown in should work well, as would foam. Except that only the former is something you might do yourself. Perhaps pushing 6" strips cut off a batt down piecemeal might be the way for you to do it - stuffing one batt at a time would leave it bunched at the top and void at the eave - use a pitchfork or rake so there's a longish handle to save you scrunching up in the corner.All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.
Thanks,Piko. Sounds like you are a cold roof proponent.As you can see in the picture, there's no backdam to keep the insulation out of the soffits. I supposed I can kindof shimmy one down there - but I'm not sure how to lock it in place. So maybe batts would be better. Do you have any recommendations for a soffit vent? The soffits are really narrow, covered with trim coil, and no one wants to work on them. So I'll have to do it myself on a ladder - looking for something easy to install that works well (aren't we all) since I'm not crazy about ladder work.
If you're going for foam or spray-in, the subbie will know what to do . Otherwise go with the tray and a dam of f/g batting, shoved in as per my prev note.All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.
Blow it full of dense-packed cellulose or foam and relax. If there aren't moisture issues there now, it's doubtful you'll create any. Venting a roof in New England, particularly if all you can provide by way of soffit vents are the drip edge vents, is truly of questionable value. If you have moisture issues in the attic now, the best fix is not likely to be roof venting. The best fix will be either air sealing the framing or drying the (likely) damp foundation.Andy Engel
Never try to teach a pig to sing. It wastes your time and annoys the pig.
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.
Andy, did I read you dense packed your house attic CT in the archives? I'd love to just do that, but the settling thing has me a bit uncertain so I looked at icynene. I assume you've done it without settling, or at least without problem? There's some black marks on the sheathing from mold so I assume there's been a some moisture problem. (We've only been in the house a year).2 questions with a hot roof :1. we are tightening the house slowly and getting the basement damp proof with XPS or whatever will probably not happen until we refinish it in a year or two. Problem until then?2. We have no central a/c (yet) to use in the summer to get rid of whatever moisture builds up in the foam or cellulose over the winter. How does all that moisture get out?
Yes, I dense packed my CT house. Love it. I haven't used a thermal imaging camera to check, but I'm not worried about settling. Cellulose's normal settled density is something like 2 psf. Densepack's density is at least 50% beyond that, and more like 100% in many cases. If you open up a densepacked cavity, the stuff bulges out. It's not going to settle.
If there are black stains, you have moisture issues, and probably air sealing issues as well. I'd tackle them before insulating. The moisture issues probably can wait until the spring, but air sealing should always be done before insulating. Get you to the archives and buy Fred Lugano's article, Fixing a Cold and Drafty House. Fixing a Damp and Moldy House is excellent, too.
Air sealing is critical because air that can enter a framing cavity through a given hole can carry hundreds of times more moisture than could enter by vapor diffusion through that same area of unsullied construction.
As to air conditioning, well, I don't have any either. I miss it about two weeks a year. If you control the moisture from your foundation, then there shouldn't be enough getting into your framing to require AC for drying. How will it dry? That's a good question. The moisture drive is from wet to dry and from warm to cold. In the summer, outside heat drives it inward, and it dries to the inside. In the winter, the drive reverses with the temperature. That said, I think that framing still dries to the inside during the winter because although the thermal component definitely pushes outward, the diffusion component will be to the dry side, which is inward. Moisture can dry to the outside in the winter, but slowly because the capacity of the air to carry moisture is lower. Still, those Canadian highs we get are quite dry, and we've all seen snow piles diminish through sublimation. I don't know, but I suspect that a lot of drying happens in the fall when the outside air is still warm, but fairly dry. At least in New England.
Normal inside air changes should handle normal moisture. The thing to remember is that we aren't talking about saturation unless you've got a leak, or there's a vapor barrier preventing drying. We're just talking about normal moisture equilibrium that's not much different from what happens to a piece of wood sitting in your wall. Andy Engel
Never try to teach a pig to sing. It wastes your time and annoys the pig.
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.
Andy I appreciate your experience in this. I'll take a look at the articles. Good company salesmanship - you should get a commision.So I guess its safe to say you don't buy some of the Building Science corp's theories about densepacking cellulose not being okay and that you really need to foam. I can't airseal the top plate as its 3' away from where I can reach from inside the attic- maybe a caulking gun with a tube on it - but I guess densepack or foam would do the job.About the drying... I don't think moisture will ever really dry to the outside in winter - it'll just condense. Maybe on the underside of my I&W shield which will cover the area in question. Perhaps I'm misunderstanding you...
If moisture can reach the outside in the winter, there are some days that it can dry. Not many, but some. Ice and Water negates that.
I buy most of what Building Science says. Joe Lstiburek is both smarter and better educated than I am. But I also know that I have an unvented dense-pack cellulose roof assembly, and that it has no moisture issues. I know it performs better than the other third of my attic, where the insulation is in the floor. I should take a photo on a frosty morning to demonstrate this.
Houses are complicated systems, and everything I say about energy issues is nuanced. The conservative approach is foam. The approach least likely to insulate well is batts with an open air channel above them. Cellulose will insulate better than batts, and dense packed, it prevents most air movement. Not as much as foam, which is the gold standard. In your case, the area you intend to insulate is fairly small. It's got an air barrier, the plaster. It's more likely, from what you've presented here, that any air leaks will happen at the flat ceiling plane as it will be the area of higher pressure. I doubt cellulose would be problematic. Foam would be bombproof. Batts would be a lot of work for a small result. Andy Engel
Never try to teach a pig to sing. It wastes your time and annoys the pig.
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.
I guess I did misunderstand before. I had assumed you were talking about insulating at the roof deck. But it sounds more like you are saying densepack in the bays and keep the rest of the insulation at the floor. If I foam, I'm gonna do the whole roof. It adds a volume of a 12x12x8 room to the house to heat, but I think maybe that's not so much and the better insulation would balance it out. Both densepak and foam scare me a bit because I don't understand the drying mechanism enough. Seems like you'd need and equal amount of summer drying to balance the winter wetting, but that's hardly the case in New England. Further the summers aren't 40 degrees above room temp whereas the winters for months at a time. The gradients don't balance. However, the fact that it works is maybe good enough.Can you tell I'm an engineer?
That explains a few things. <G>
Andy Engel
Never try to teach a pig to sing. It wastes your time and annoys the pig.
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.
bob... where does the "winter wetting " come from ?
if you control the moisture sources in the house and air seal the envelope .. there isn't going to be any winter wetting that can't be handled by the insulation..
sounds like the problems of fiberglass insulation ar being associated with cellulose.. and that is not the caseMike Smith Rhode Island : Design / Build / Repair / Restore
Mike,I was refering to any vapor permeable insulation, not just FG or cellulose. Drying is to the exterior in MA during the winter. So vapor diffusion will build up some moisture in whatever insulation is used in a hot roof since there's no vent channel. That's what I meant about "winter wetting". Maybe not the right term.I'm fine with the concept of a hot roof and I believe it could work, I'm just trying to understand the drying mechanism. From what I've read in various places, the concensus seems to be that removing air movement stops most of the "winter wetting", but there still is some from vapor diffusion - and that moisture has to go somewhere.
Edited 11/22/2005 7:32 am ET by BobS
"removing air movement stops most of the "winter wetting", but there still is some from vapor diffusion - and that moisture has to go somewhere"
Air movement is responsible for 98-99% of moisture movement in houses. The little that moves by diffusion is absorbed by the cellulose and wood framing. It may increase their moisture content by a couple or so % but they will not get wet as most people visualize (if the house has normal moisture levels and no leaks). This moisture then leaves by diffusion again during the summer as temperatures of the roof rises.
The US DOE did some research in the late 70's-early 80's on housing in Minneapolis with walls retrofitted with cellulose and found summer north facing stud moistures as low as 3%-exceedingly dry. They were afraid of winter moisture causing summer problems as you are but were really surprised with their findings. They then ascribed a possible extra feature of cellulose insulation as being able to absorb and release moisture as being a desirable thing. Another study by the NY State Energy Commission in the mid-late 80's states similar with cellulose in attics.
Hundreds of thousands of homes have the slopes insulated with no ventilation and no problems. It's just that the ones that do have problems are remembered and the insulation is blamed. Not very many professionals and tradepersons have even a good introductory seminar on building science and as such the problems that do occur are always misdiagnosed and the incomplete knowledge keeps being passed along at the street level. If we learned our sex ed at the street level today where'd we be ? Not here likely!!
There really aren't that many people who have a good grasp of the "House-as-a-System", the Building Science, Indoor Air Quality, Healthy Housing and energy efficiency issues that are inter-related in today's homes but .........everyone from lawyers, doctors, engineers architects to the postman and barber have opinions on such matters!!! I have in front of me, the June 2005 issue of Energy Design Update with a front page headline of "Teaching Architects Building Science". You should read this somewhat damning piece on the lack of technical training in the schools that really should have it. In many engineering schools also, there are no building science courses. The sad thing is if we're not even training the present students, how are we going to re-train the present practitioners? Are we doomed to have major lawsuits on this stuff for another generation or two?
Thanks for the info, Experienced. I'm getting a lot of good info here and its mostly calmed me down.You don't have a pointer to that study do you? I've got a university libraryian 100ft from me so I could maybe get it. I think I'd be a lot less concerned about it if I didn't already have cells in the sloped part with black stains on the roof. Does the dense packing make that much of a difference? Also the fact that Lstiburek is down on cells is a bit unsettling (no pun intended) - then again, he's not the gospel. Concensus would be nice, but I guess that's asking too much.I'm not sure the black stains are showing up in the photo so well, you can see them a bit by the rafter on the right and then in the middle further down. Maybe that's not as bad as some, but it still there.
Damn the word processor on this site!!! Just lost a full post and part of the second trying to import some text from BSC.!!! SO here goes:
The black stains may be there from the distant past. They are low in the cavity and may be from the days before eave protection materials (Ice & Water Shield) and metal drip edges to protect the first roof boards at the eaves and the top of the fascia board by directing water outward to gutters. The fascia/soffit is now "capped" in metal coil stock. I would be sure that no water is getting behind it from the roof edges as it will not allow drying.
Don't know just which article that BSC and Joe are putting down cellulose. Just checked one on MOLD and they are giving it good review for the minor water problems we may be concerned with here. See:
http://72.14.203.104/custom?q=cache:UM531ENI3G8J:www.buildingscience.com/resources/mold/mold_causes.pdf+cellulose&hl=en&ie=UTF-8
Others (I haven't checked every reference to it on their site) talk about new housing where you can get very high quality airsealing with other system/materials and they don't want the cellulose to be acting as part of the air barrier. So they say not to use it as air barrier here. I wouldn't worry about It.
Edited 11/22/2005 11:27 am ET by experienced
Sorry, I should have put in the quote since its not on their site. This comes from the Builder's Guide to Cold Climates 2000 edition:p201, Insulation chapter:"Just so we are all clear, blowing a cathedral celing "solid" with cellulose will not eliminate the need for an interior air flow retarder or the need for roof ventilation if the temperature of the first condensing surface (underside of the roof deck) is not controlled by the use of rigid insulation."All their cold weather roof designs with cellulose packed into the cathedral ceiling have a layer of rigid insulation above the roof deck, which is nice in a book, but presents several practical problems and is nearly impossible to find someone willing to install in that manner. That's why I figured they were down on it. Sorry, not trying to put words in their mouths.I agree the stains may be there from the past, but I can't be sure. When we reroof, I'll be sure the roofers detail the eaves appropriately to prevent water entry. Currently there's no I&W shield on the roof, but there will be come spring when the roof gets done.
This is the extreme treatment to prevent condensation and seeking perfection since we can't expect all trades (in 2000) to honour and protect the air/vapour retarder. And he's talking about the large and long vaulted ceilings with potlights and other penetrations where being wrong in one instance could cause locaslized roof sheathing rot.
Another point is that Joe had been in the US for about 13-14-15 years by this time where the whole topic of air leakage control was just being realized but not practiced widely. So he had to use the "belt and suspenders" approach for safety and avoid possible law suits for which you folks are noted.
(PS: In the last 2-3 years the "Lawyers" section of our Yellow Pages is starting to look like yours. Full 2 page ads with captions such as this: "If you have personal injury, contact us". The era of "ambulance chasers" may be upon us also)
Edited 11/22/2005 12:24 pm ET by experienced
Cold climates edition? Isn't Boston more mixed? And an air barrier is de rigeur anyway - that's just drywall or the like. You could slip some foam sheet down those cavities, and push it up against the roof deck before blowing cellulose.Andy Engel
Never try to teach a pig to sing. It wastes your time and annoys the pig.
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.
My feeling is boston is more mixed, but the book defines cold climates as having at 4500-8000 HDD. You and me both.
Looked at your X-section again and you really don't need th eave protection; drip edge yes. Our codes say you need protection until you hit 8/12 slope; after that, the slope is too steep to cause ice dams that would force water back up under shingles.
I am also trying to understand drying issues with hot roofs (NYC area, not that much different from Boston). I have 8/12 roof and wonder about I&W. Buildingscience.com appears to require I&W over the whole roof deck, which surely precludes moisture exchange between interior attic and exterior. I have a feeling there is more going on here than CYA. How critical is it? Roofers I talk to say the roof should "breathe" although they admit archies sometimes recommend I&W all the way up. Most roofers seem to just go with 3', I know one guy who recommends 6'.
I have a 1-yo roof, now considering just repairing it or ripping the whole thing off and starting again.... Bobs, get all the protection you can on your roof. Go with the highest estimate, not the roofer with the best rep according to clueless HOs......
Thanks for the advice. Are you ripping it off because you had a hot roof problem or just to jive with what the building science guys say? I give what they say a lot of weight, but they aren't gospel - they change their mind about what's right enough that it makes you think twice.I've learned a lot more about roofs and attic insulation that I ever wanted to know in the past 6 months. Lots of things seem to contradict each other. One thing has been fairly certain throughout - you don't want to listen to roofers about what's right to do with respect to insulation. This isn't their wheelhouse. I have a lot of respect for what they do and there's NO way I could ever do it. There are some fine craftsmen out there, but I have yet to have one come to my door that knew anything about this stuff - usually they have some recommendations that even I know are dangerous. The most expensive happened to have the worst ideas.I didn't see the I&W over the whole roof w/BSC, but usually that's to prevent inward vapor drive, or no possibility of drying to the interior (with closed cell foam) or in a different climate than ours. But the "Boston" building profile just uses roofing paper, which I think is just felt. At least, not I&W.
Oh ripping out just to insulate would be crazy....it's just that it looks like I got the keystone cops for my roof....no flashing, just slap on some roof cement....around here it's all illegal immigrants on the crews, price war driving decent roofers out of business or into the high end....wish I had gone high end, but everyone said how great the guy I went with it is....as a retired roofer said, they were mowing their lawn yesterday, they're putting a new roof on your house today.....
I think its a crap shoot to some degree. Almost no one on a referal list is gonna go into there attic and check for leaks. They're just gonna say the yard was cleaned every day. My MIL had the same problem as you. The guy I'm going to go with happens to be one of the cheaper ones, but he's doing things most others wouldn't in terms of detailing, he's got a good reputation, and most of all he seems very good to work with based on a roof patch his guys did. Still, I'm a bit nervous but that's my nature. I plan on staying home just to watch, on those days. At least I'll know how it went on.Sorry you've had such bad luck with this. Roofs are a real headache, huh?
Bobs, my principal concern with hot roof is moisture buildup from the rest of the house. Since I haven't yet put a VB down on the basement floor (next year's project) and since I have hydronic heat rather than FHA, I'm considering putting an ultra-efficient dehumidifier (Aprilaire or Santa Fe) in the attic. I've thought of draining to air handler drain pan but overflow drain for that would just ice over in winter, and winter nights are what I am principally concerned about (exhalation from sleepers, temperature down because of setback). So I would run 1" PVC drain line down to the basement to hook into DWV there. I guess if I don't just discharge into laundry sink I'll have to consider vent stack, oh boy.....This is aiming at building science (UBC?) targets of max 45% RH and min 45* surface temp in the attic.
Sorry to revive a moderatley old thread.I've read over this thread a few times, as well as the archives, and thought about it over the holiday weekend.My current thinking, base on all this, is to do nothing for now until the moisture sources are taken care of. In the spring, redo the roof and have the roofer take out the crappy box vents and NOT add a ridge or soffit vent system. I'll leave the one gable vent and seal it up later.Once the basement is taken care of and the attic is airsealed. I'll do one of 3 things depending on how much we need the walkup attic space at that point:1. denspak the sloped part and add insulation at the attic floor. Seal up the gable vent (or not - don't think it matters).2. Cathedralize the attic by putting up netting or more likely Tyvek and denspak behind it down to the top plate, including the sloped part. Then a few inches of EPS for a good airbarrier+insulation. Depth of each layer adjusted so there's no condensation at the interface in case any vapor gets through the EPS. The sloped part won't have EPS of course. The EPS will give me some drying potential in the summer if anything gets wet. I think I've seen Mongo say he does this a lot except with polyiso. Tyvek/EPS act as a drainage plane in case of a roof leak so I'd see a spot on the sloped part drywall (well, i can hope it'd work that way).3. Cathedralize the attic by spraying icynene on the underside of the roof deck. Maybe add some EPS.In all three I'd add a quality dehumidifier to keep the RH < 50%. I'll get into ventilation once the rest of the house is airsealed.How do those sound - number 2 in particular?The main part of the attic doesn't have mold, just a bit on the sloped parts so maybe I'm worrying about this too early. Right now, I just want to know if I should tell the roofer to cut in a ridge vent or not, and none of these 3 need that.Andy and Experienced thanks for calming me down about this. I'd be interested in anyone's feedback on #2 since its the cheapest and gets me what I'd like - a useable attic. My only real problem with all these is I&W shield being a vapor barrier on the wrong side. Otherwise its more or less like my walls.
Edited 12/1/2005 12:44 pm ET by BobS
ok - goofy idea here, but:
could you possibly take a 5' length of 3" PVC pipe (for each rafter bay), then "sharpen" the end with two cuts of a miter saw, to sit flat against the soffit and facia from the inside (and more importantly allow soffit vent air to flow into the attic space?
Strap the pipe to the underside of the roof decking (pipe hangers), then fill the cavity (around the PVC pipe) with spray foam? The cool air would flow up the pipe into the attic space, and with a 3" PVC pipe inside a 2x6 rafter you'd have 1.5 inches of foam against the drywall side.
I'll probably get ripped on for the idea, but who knows? Sort of an expensive version of a Cor-a-vent.
oops - relooked at the drawing - so -use a 2" hole saw into the unvented soffit, then line up the PVC-a-vent with the hole, foam, then use those round soffit louver covers over the new vent holes.
Edited 12/4/2005 12:43 pm ET by JasonD
Jason,Thanks for the suggestion. Its not that crazy. At one point I was thinking of ideas like this, but from what I've read, for attic vents to be useful they really need to run the length of the building and not just use those circular vents. That said its better than nothing.THe real problem with venting is that I can't find someone willing to put the soffit vents in, which means that it'd have to be me on a ladder with a hole saw or reciprocating saw cutting thru the soffit. Its made harder by metal that's wrapped around the soffits. If I thought I could do this successfully on a ladder, I'd probably try it, but my ladder skills are poor (and my wife likes the idea of me on a ladder less than I do).Maybe I'm not asking the right people - the roofers won't put in soffit vents. Should I just call a carpenter?
Call a carpenter!! Don't go there as they will bring along the classical theory of venting and say you can't do without it thus getting themselves some work. Classical theory like the earth is flat takes a long time to change even when the evidence is against it. I think the largest church in the world only wrote some thing in 1992 saying they were wrong with whatever actions (including house arrest) they took against Galileo for speaking the truth
Don't let moisture get to the area- venting will be unnecessary. There will be a bit of air movement in the horizontal board joints with winds. If this technique was so bad it would not be used by most retrofit insulation contractors as they'd all be getting sued!!
I think I'm pretty much sold on the no vent idea and my wife is starting to buy it. Mostly I was worried about the cost of doing it safely and leak detection but I'm getting my head around that. What do you mean air movement in the horizontal board joints? Do you mean the sheathing when the wind is blowing at the gable side?
Exactly! Our older house exteriors are not that airtight. Many times when I was in attics and a light bulb went, you could see light around the eaves fascia boards.
So then that would seem to preclude using any insulation that doesn't get into those little gaps between the boards if the attic is going to be unvented. So does that mean only foam? I can't see cellulose getting in there.Maybe I'll try and get a bead of caulk along the rakes into the gaps.At the same time, air coming in thru the board gaps isn't the air that's gonna condense, its the air coming from inside the house.
BobS, thanks for reviving this thread.I've thought about a dehumidifier as backup in a hot roof. Thermostor now makes a version of their Santa Fe dehumidifier that is designed to hang from rafters. Buildingscience.com has an article on whole-house dehumidification and an ultra-efficiency dehumidifier in the attic was the clear favorite of several alternatives that were evaluated, including just relying on the air handler for dehumidification. I might run some intake and outtake lines to bedrooms, since the period I'd be most concerned about is at night with setback. Also that would allow the dehumidifier to be used instead of AC when it's very muggy and but not particularly hot during summer nights.That being said, I think it would be necessary to run a drain line into the DWV system, requiring a vent stack. The Santa Fe can produce 100 gals/hour. I've thought of exhausting to the drain pan for the air handler, but that might not evaporate fast enough in winter, and the "backup exhaust" of the drain pan to outside would freeze over.Also dehumidifiers are not guaranteed to work I don't think below 55*, and code for hot roofs only requires that attic temperature stay at 45* or above. There may be a danger of freezing up and killing the dehumidifier (and those suckers are not cheap).If I am out to lunch on any of this, I'd welcome being corrected.If I do this, I reckon I'll go with foam, if I can find a foamer I trust to get a good seal against the roof deck and not leave pockets.Edit: I was figuring on closed cell foam for the rake walls, open cell for the roof deck.
Edited 12/5/2005 10:20 am ET by Taylor
Taylor,
I was gonna put a dehumidifier in the attic so its out of the way. I saw on "this old house" a pump system you can hook up to a dehumidifier, in case you forget to or dont' want to bother draining it. I'd feed the output of that pump out my gable vent before I seal up the gables. Its just a small hose. So the dehumidifier can drain directly outside. Then I'd put the dehumidifier in a drip pan (like for a washing machine) with a float sensor to kill the power in case the pump fails and the dehumidifier overflows. Haven't thought it all out yet but seems like it should work.
I'm with ya on the closed cell for R-value on the gable walls, but keep in mind a lot of leaks happen at transitions so you want to make sure the roof is well detailed at the rakes because you won't easily see a leak behind the closed cell. Judgement call.
Foamers don't do open and closed in one trip, I don't think. I think its a different mixture, different hoses, etc so you might pay a premium to get two different kinds of foam in one enclosuer. Better to pick one, maybe the open, and then cover the gable walls with some rigid insulation if you want closed cell there.
Can the drain line for the dehumidifier freeze up if it's in the 20s outside? Granted there should be no standing water (then I worry about heat leaking through that drain line.....).Do you have a pointer on that pump?Thanks for the headsup on foam. I have insulated stud walls with 3" XPS, quite a pain....
http://www.thisoldhouse.com/toh/tvprograms/asktoh/showresources/episode/0,16663,709282-832803,00.html
I suppose it could freeze up. It will take some more thinking...
Why are you putting dehumidifiers in the attic- summer or winter use.
A deh. can only bring RH down to 50-60%, much too high for the period. If you've got RH higher than that in the house in winter, you've got other moisture concerns that should be addressed!
From Thermastor's FAQ:
How large an area can one unit handle?
A single SANTA FE, SANTA FE Rx or ULTRA-AIRE APD will maintain relative humidity levels below 50 percent in air-conditioned structures up to 2,500 square feet, even if there is more than one air conditioning system. In larger homes, the ULTRA-AIRE 150-H can control up to 3,500 square feet or multiple units can easily be employed.
I've been thinking about it for an attic with hot roof in winter (steam heat), but it would also be useful for bedrooms on summer nights....
I know. I just meant for it to be a backup moisture removal mechanism, as an attic vent would be. Of course, primiarily I'd like to stop moisture from getting there in the first place by fixing and finishing the basement - which is already pretty dry. The dehumidifier is just for an extra layer of safety. I didn't know they could only get down to 50-60% though. Maybe its not much safety. Figured I'd leave it up there year round and have it kick in if the RH got too high.
Be careful of the foam on butt edge boards with good size gaps. Read a post somewhere this AM about the foam expanding out through these gaps and pushing up the singles as well as pushing up roof boards near the eaves!! Maybe the nails in boards near the eaves had rusted quite a bit and they were about to lift in a good wind.
Nothing like building up a bank of everyone's experiences!!
Brian:
Found some of the papers mentioned in a post. One is titled Attic Ventilation and Airsealing: A Technical Review of the Issues and may still be avilable from the NY State Energy Research and Development Authority, Technical Publications Dept, Two Rockefeller Plaza, Albany, NY, 12223. This is a mid-late 80's pub so much may have changed in the gov't departments since then. This is one of the better intros to the change in understanding of insulation, moisture, venting, etc by looking at buildings through the eyes of a building scientist rather than a tradesman. I only have the first of a two part series but it described the historical research done and how those working in the construction field did not pay attention!!
Here are a few choice statements:
From the background intro to the article-
"The traditional approach assumes the energy problem in attics is conductive heat loss and the solution is insulation. Further, diffusion of moisture into attics leads to rot and the solution to this problem is ventilation.
Unfortunately, the real world has proven to be more complex. Evaluations have indicated disappointing energy savings and ventilation standards/approaches are questionable. Some attics have no probelms even when "undervented" while others develop problems with presumably adequate venting. Some concerned practitioners have suggested that added venting may actually worsen attic moisture problems and increase energy use in some cases."
"In 1981, ASHRAE, a major building science organization, altered its description of venting standards from "Recommended Good Practice" to "Past Practice"......and has made no new recommendations since then."
From section titled Ventilation Effectiveness-
"In fact, there is no guarantee that providing a specified area of passive ventilation openings will yield some desired level of ventilation"
From section titled House-to-Attic air Leakage-
"The moisture and, to some extent, thermal implications of house-to-attic air leakage were first noted by FB Rowley of the U. of Minnesota in the late 1940's. In the early 1960's, other Canadian and American resarchers indicated that air leakage is indeed more important than vapour diffusion but few practitioners paid attention."
From section titled Seasonal Moisture Storage-
"Based on these observations, Harrje concluded that "avoiding condensation may be an unnecessarily severe design constraint""
"Some researchers and practitioners have reported finding attics with damp cellulose that may have averted moisture problems"
From Energy Design Update Jan/1993, page 7 titled:
The Mysterious Origins of the "1:300 Rule
"(Bill) Rose found it was hastily derived from a single lab experiment on a flat roof 45 years ago." (my comments: this experiment was set up to test vapour barriers not attic venting!!!!!!)
"The vent-to-roof ratio, 1:300, appears to be merely an incidental detail, says Rose, peculiar to the specific test setup"
"For want of a better number, the 1:300 ratio was subsequently adopted as good building practice and was eventually added to building codes"
My comments: sounds like "wierd science" to me. I'm glad we don't still do that today!! (LOL)
And finally, from Gus Handegord, one of the retired "old guard" of Canadian building science (and mentor to Joe Lstiburek), in a talk to a meeting of the Building Envelope Council of Alberta in 1998 (his final sentence):
"It was once suggested that one problem in transferring technology in the building industry is that nobody reads anything. It may be equally valid today since practitioners as well as many researchers in building and construction seem to have neglected to read or re-read the Canadian Building Digests, even those that fall into their respective areas of interest."