Foam board on outside of sheathing
Hi,
I’m new here….was just referred over here to get “an experts” opinion. 🙂
I’m building a cottage. No experience, just lots of reading and willingness to try anything once. My question involves insulation.
I’d like to get as much insulation in as I can (northern Ohio…..cold!). I’m using 2X6 construction, with a layer of sheeting, then tyvec or something for a windbreak, and then a board and batten siding. I’d like to use the foil-faced foam board (polyiso…) between the two layers (sheeting and board and batten) but have been warned that it may cause excessive moisture retention since it is on the “cold” side instead of the warm side. Thoughts????
If I don’t use it on the outside, I could use it on the inside. I was thinking of using fiberglass, since I have enough for free, but might consider blown-in dense pack cellulose, or foam, instead. If I THEN put a layer of the foil-faced polyiso…..how do I get the required 1/4″ gap on both sides???
Thanks again for any advice offered! I’ve got lots of people watching me (ready to say “I told you so” when I fail) so I’m determined to do this right! <grin>
Christine
Replies
Yes, you can add foam to the exterior of a building. The theory is that the foam raises the temperature of the stud cavity and the exterior sheathing enough that condensation will not occur.
How much foam you add and whether or not you also need a vapor barrier on the inside is climate dependent. You can read up on it on the building science website.
Christine
Welcome to BT.
Perhaps these two FHB articles will be helpful.
If you click on a poster's name then a profile will pop up.
If you fill out your profile (click on your name a dialog box will appear), then we will have at least some geographic information to help with answering your question.
Rich
Christine
Bump.
By bumping your thread it should pop up again and maybe you will get some more responses.
Rich
It's off topic, but I was wondering if it's ok to post articles from the magazine on a public forum.
I'm also an online subscriber, but I hesitate to "copy and paste" because I assume that they wouldn't want us doing that since it sort of defeats the purpose of requiring a subscription. Theoretically, it's no different than taking a digital picture of an article and posting it.
If I'm way off base here, don't hesitate to let me know.
Don
I am not smart enough to give good advice without some backup.
I posted the article about super insulated homes and we had a long lively thread discussing it. That in my opinion was a good use of cut and paste, magazine and internet and subscriber working together to really flesh out an article.
I just want to steer people towards good information, not just my opinion.
And I figured they would let me know if they didn't like it. Or they would block cut and paste some how.
Rich
Welcome, If you have 2x6 and spray in closed cell you will get about R-35 in the walls. This should be enough. The situation is $. Closed cell is going to run you about $1.00/bf or about $5.50/sqft of wall cavity.
Ok, I'm struggling with the math. How does $1 a bf come out to $5.50 a square foot? How "wide" is a board foot?
Gosh, if I can't handle this...I'll be REALLY sunk when it comes to the rest of the house, huh?
Is it possible to rent a machine and do the sprayed foam yourself?
I wonder how many thousands of cans it would take, otherwise....... JUST KIDDING!
Thanks for the responses!
CC
Wall cavity is 5.5" thick, so a square foot of wall filled up with spray foam is 5.5 board feet.
Don't use polyiso on the outside because it's not vapor permeable. Use blue or pink board instead.
Spray foam is not DIY. You might have it priced out though, you probably don't need 5.5" of it (2" plus some fiberglass to fill up the cavity is one way to do it) just to see how it compares to rigid foam.
I get so confused. I just read that polyiso was GOOD on the outside as long as you didn't use a vapor barrier on the INSIDE, too. (And as long as you taped the seams and bottom edge). I STILL don't know what I'm doing.....
Don't worry about being confused. People who are smarter than I am feel that it's ok to put PIR (polyiso) on the exterior. I don't like that it's a vapor barrier; for too much of the year in our climate, warm, moist air is trying to get out. I want it to at least have a chance at making it through the wall without getting trapped against the sheathing. Maybe if I was better at making sure the inside and outside surfaces were airtight I would be more comfortable with it.
I'll give it a shot.
The polyiso has a higher R-value than other foam boards, so people like to use it. The foil facing is super vapor impermeable. It's aluminum foil, so liquid or vapor won't get through. The blue or pink stuff, extruded polystyrene, allows vapor to pass through, but the thicker it gets, the more impermeable it gets.
Make sense so far? If you cover the outside of your house with foil-faced polyiso, then moisture won't sneak in from the outside, any vapor will come from the inside. If it comes from the inside (showers, boiling water, dogs panting) then the vapor will condense to liquid when it hits a cold surface inside the wall cavity, right?. But with foam on the outside of the wall, the cold surface will be somewhere in the middle of the foam, so the vapor won't condense. It'll just be a humid wall cavity, like the humid un-wall cavity. And that's fine as long as moisture doesn't get trapped in there with a sheet of plastic.
Still good?Because the foil faced polyiso is impermeable and thick XPS is nearly impermeable, it's probably no big deal if you use the polyiso or the XPS. As I said, polyiso has a higher R-value. We just published an article by Betsy Pettit about upgrading old houses and on the last one, they used four inches of foil-faced polyiso on the outside. I asked her about the vapor barrier effect on the outside and she said that it really isn't a problem for some of the above reasons... (no interior vapor will ever make it to the condensing surface, four inches of XPS would have the same effect, and probably some other reason). So what's the bottom line?Cover the outside with foam, fill the cavities with cellulose, skip the spray foam in this case and spend your savings on beer.Any help?Dan Morrison
Senior editorEdited 4/11/2008 9:10 pm ET by DanMorrisonEdited 4/11/2008 9:14 pm ET by DanMorrison
Edited 4/11/2008 9:15 pm ET by DanMorrison
Dan's reply was an excellent summary. The only missing info for putting the foam on the outside is how thick should it be. It's generally available in 1/2", 1", 1.5 and 2 inches. If it's to thin (based on the local climate), the dewpoint may be well inside the wall cavity which might cause condensation where you don't want it. I'm working on a house right now that has 2 layers of 2" thick polyiso outside of the sheathing (walls and roof). That's plenty thick, but creates other challenges.
I am still trying to figure out the optimal thickness for exterior foam to be effective. Does anyone have any rules of thumb for determining ideal thickness for exterior foam?
Ok....'nother question. Isn't air and moisture resistant the same thing? Can something let in air, but not moisture, and visa versa?? Maybe that's my confusion.
As for more than 1/2" or so of foam on the exterior..... doesn't that cause problems putting the final finish on? I was told that the exterior wood (board and batten in my case) would be hard to put on if I put too much foam on the outside. No way to fasten the wood down.
Adding foam thicker than 1/2" generally requires installing nailing strips to which the siding will be fastened (horizontal strips if the siding will be vertical). For example, if you wanted to put on 1.5" foam, you would install 2x2 vertical nailing strips on the sheathing every 2'. If the siding was vertical, you could probably get away with horizontal strips every 4'. Of course you need to design adequate roof overhangs on both gables and eaves to accomdate thicker walls. Also would need to fir out corners, the tops and bottoms of the walls, and around windows/doors to have have nailing for trim. The foam panels would sit right on the sheathing and should be snug the strips.
If I understand that correctly, then there isn't any thermal break in the areas where the furring strips are? You would have outside wood/sheathing/whatever, and then the furring strips with foam BETWEEN them, then inside sheathing, studs, etc.
Is that correct? Could you put 1/4" or 1/2" foil-faced polyiso OVER the whole kit-n-caboodle before the final wood siding? This would be the thermal break?
CC
There are three things that pass through walls and cause problems.
First there's liquid water. If you or your builder can't keep those molecules out you're in big trouble. It's not too hard to do.
Air is a little trickier. You don't want air washing through your walls because it carries heat with it, and most insulations don't work when air is washing over them. Housewraps are designed to keep air from washing through the wall, as is the "airtight drywall system." But, housewraps have tiny holes that do allow air to eventually pass through, just not fast enough to cause air-washing. Sort of like a tire with a slow leak. Nothing catastrophic happens, but eventually the air leaks out.
The "airtight drywall system" is a way to keep inside air from washing through the wall in the other direction.
The trickiest bugger is water vapor. It's water dissolved or carried in air, so any leaks in your air barrier, inside or out, will allow it into the walls eventually. Your walls are not hermetically sealed. When it's humid outside, or if you steam up your bathroom, some of that warm, moist air is going to find its way into the wall.
What happens to that moisture once it's in the wall is the sticky wicket. It can do three things, assuming there are air barriers inside and out. It can slowly pass through to the exterior, if the exterior air is drier than the indoor air. It can pass slowly back indoors, if the interior air dries out (like if you turn on the bathroom fan or a/c). Or, and this is where the problems occur, it can stay right in the wall.
If, like Dan Morrison said, there is sufficient R-value (insulating capacity) in the exterior foam, the water vapor will just stay in its form as humid air and it will eventually equalize with either the indoor or outdoor air through the housewrap or drywall.
If, however, there is not sufficient R-value in the exterior foam, the water vapor will condense and turn into liquid water. You really don't want liquid water inside your walls.
Very thin XPS and any thickness EPS foam don't restrict the water vapor from continuing on happily to the outside, but do give you a thermal break. Polyisocynurate with a foil facing DOES restrict the flow of water vapor, so it's CRITICAL that you use it in sufficient thickness to elimate the possibility of water vapor condensing inside the wall.
Under no circumstances (in a northern US climate) should you put 1/4" or 1/2" polyiso on the outside of your house. You could put 1/4" or 1/2" XPS on the outside, but the 1/4" isn't really thick enough to act as a thermal break.
IF you really want polyiso on the outside, you MUST install it in sufficient thickness to avoid water vapor condensing in the wall. That makes installing everything else on the outside a pain in the arse, and like you noted there are still weak spots where you will get thermal bridging.
Sorry for the long answer. That's why earlier I said just spray the inside with spray-foam. It's comparitively foolproof.
Hi. I have had good success using regular fiberglass in the studs and putting 1/2" polyiso on the INSIDE. I tacked it with nails, taped the jounts with Tyvek tape, and then hing sheetrock with 2" screws. Benefit from my standpoint:
- Vapor barrier is on the inside where you want it (in NJ).
- Get some more R plus thermal break from cold studs
- Easy to do yourself. Lets you close in faster.
- No fussing with window attachment to sheathing. Everything ourside is normal. It is easier to extend window jambs inside.
I would not go with more than 1" because electric boxes would get hard to anchor firmly (not much on the stud) and worry that drywall might sag as screws bent within the thickness of the foam.
Good luck!
Was that the foil-faced polyiso?
I'm having so much trouble with the basement guys that I'm ready to chuck the whole thing! Grrr. I just went out today and found that they didn't call me when they striped the forms (like they were supposed to!). They waterproofed it fine, but drilled the sump pump hole on the wrong wall! The idiots can't read blueprints. I found that out the other day when I kept questioning them about the window placement. "It's only 3" off", they said! I made them make it right.
Also, I see that they put in at least 1/2 of the windows upside down (the frames.... I can't knock out the headers on some of them....or if I do, I'll have a 1-1/2 gap in the frame. I have no clue why they didn't frame to the RO size, rather than figuring on knocking out one side. ) Also, one looks more like a parallelogram than a square. It's an inch off. I'm not sure I can even get the window in without some major shaving and shimming. Is this normal????? I've never dealt with a basement company before!
There were 6 guys there (the owner wasn't there...a four-year employee/foreman was in charge. Some guys were BRAND new...didn't even know how to handle a form or the names of the tools they were asked to tote). They all treated me like I didn't exist and absolutely ignored everything I said, unless I practically DEMANDED that they answer me. And when they DID, and they double-checked what I asked of them, they found they were wrong 100% of the time. I wasn't being a pain, I was making them do things according to the blueprints. If I hadn't been there, the sewer line would have been 4 foot out of the ground! The windows would have been 3" off on two different sides of the house. As it is, I have one window still off, another crooked, 3 upside down, and a sump pump line on the wrong wall --- and 2 foot above ground... and the wrong size. Idiots.
Oh! And the guy running the front end loader (the owner of the company), ran into and smashed my well casing and bent/broke a hydrant.....even after I pointed it out, put yellow flags all around it, and was assured that it wasn't a problem. Same guy took stone from MY stone pile on the other side of the barn and used it to make a driveway in my yard from the driveway to the house. After I told him I didn't want a driveway. Turn your back for a little bit....
Let's see.... I don't want to bitch too much at them just yet because they still need to backfill and grade, and pour and finish the basement floor. I shudder to think what still can happen.
Sorry to rant....but boy that feels good to get it off my chest! Which reminds me....
My son gave me a hard hat. I'm going to spraypaint it pink and letter it with "Boobs AND Brains. I'm allowed to have both!"
Your ranting is well deserved. I don't think I would pay for a foundation with that much wrong.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
This is precisely the reason I don't like poured basement walls.
All of the problems could be easier to fix, as well as having blocks with holes you could foam.
To late now.
I have run into problems of this sort with all, I repeat all of the contractors that pour walls. Tape measures and blueprint reading is something these people around here can't or won't master.
Now having said that, the contractors that pour commercial walls are very good.
Nothing like framing on a parallelogram.
Hi. Yes, the foam was foil covered. (Since there are no air spaces in this application, I am not making any use of the foil -- that's just the way it came. Makes the room very bright before the drywall goes up!)
BTW, if you do this, hang the rock on the ceiling first. Otherwise you have a gap (caused by the foam board) between the ceiling rock and the wall top plates. I'm sure that's not good for firestopping and other reasons.
The method I was describing would be for the installation of foam panels on the outside of your exterior sheathing. Frame and sheath conventially, then firring strips on the outside of the sheathing with foam panels inbetween. Your siding then goes directly onto firring strips. Yes, there would be thermal bridging at the firring strips, but far less than what you get with a conventionally framed wall. You could further reduce the amount of thermal bridging by using two layers of foam, one set of panels oriented vertically, the other horizontally (dictated by your siding choice). You might want to check out wall systems for your climate at http://www.buildingscience.com. Their schematics are pretty good.
Hi, I just posted this same question about a month ago. I think someone referred you to that thread. There are links to building sciences web site I think would help you understand the issues you will be dealing with. I would send you the link but I am virtually web illiterate. Find that thread and also I've found RiversongHousewright(Robert) incredibly knowledgeable and helpful. Hope this helps.
One more thing, you can't install foam without drywall or it has to be fire proof.
Could you give more detail on the furring you propose, 2"X2" seem a little big. Aside from the 1 to 4 inches of rigid insulation adding another 1.75 inches seems extreme. In fine homebuilding the use 1"X3" furring. Would this be sufficient for fiber cement siding?
It sounds like the 2x2s are what I'd call "blocking", not "furring strips".
2x "blocking" goes on the roof decking and wall sheathing. The foam boards are set in the wells created by this perimeter blocking. Furring strips (1xs) would be placed across the top/outside of the foam board, then screwed/nailed to the blocking (over a second course of foam if you like), to create nailers for roof decking and siding (with air space in between them).
Tom21769 accurately described what I was suggesting. 2x (blocking, firring, whatever you want to call it) is attached to the sheathing every 24" creating frames into which the foam panels are placed. The plane of the foam (1.5" in this case) would be flush with the 2x strips. This method simply provides nailing for the siding.
In Fine Homebuilding up to 4 inches of rigid foam was attached to the sheathing and taped at the seams with 1X3 furring strips on the outside to attach the siding. Do you think this is strong enough or do you have a better method? Wouldn't it be best to have a continuous foam application as the 2X2's would be a thermal bridge?
Yes, I do think a continuous foam layer is best, and the 2x blocking (which I referred to as "firring" in an earlier post) would present a thermal bridge. If the goal is to get 3 (or 4) inches of foam, I'd alternate layers of 1.5 (or 2)". For example, I'd lay full sheets in horizontally, with blocking at top, bottom and at 4' intervals. Then I'd attach vertical blocking every 2' and lay a second layer of foam in between the vertical blocking. This minimizes bridging (only where the blocking intersects) and provides solid nailing every 24" for siding.
I am currently doing reno on a house that was skinned about 15 yrs ago with 4" (2 layers of 2") of Thermax without any blocking. Needless to say, the siding is falling off everywhere as it was just nailed to the foam with 16P galvy box nails.
Thank you for your reply, well said and easily understandable. I'm thinking of residing my house and since the sheathing will be exposed thought rigid foam here in Minnesota would be a good idea.
A board foot is a cubic measurement for lumber: 1"x12"x12" = 1 BF. (144 cubic inches or .083333 cubic feet)
So as Mike says 5.5"x1'x1'x $1 is $5.50.
CarpCoop
Edited 4/3/2008 6:24 pm ET by carpcoop
OK, old school wood sizes. Rough sawn lumber was sold by the board foot, and still is. 1" thick and 12" wide and one foot long. Spray foam is spec'd out as x number of board feet in a system. A 600FR kit will give you 600 sq ft 1" thick. You have 2x6 walls so you could put in 5 1/2" of foam which comes to 5 1/2 board feet/ square foot, and I am sure that you can do the rest of the math. I have been using 2 1/2" of closed cell and then using a layer of Astro-foil on the inside (tight to the foam) for R-24 in a 2x4 wall. The reason being is foam is $1 /bf for R-7 and Astro-Foil is R-7 and is $.40/sq ft. Now true there is no labor factor involved, and it takes 4 times as long to apply the Astro-Foil as it does the foam but it is what I do and have been pleased with the results.
Personally I'm a big fan of exterior foam (either kind) I feel that it has an effect on the insulating value of the wall far beyond its R rating , because it is continuous.
Having said that, its use can present problems, not just condensation, but installation details also such as windows and doors requiring extension jambs
The most important thing (IMO) is having a decent interior AIR barrier, Drywall usually performs this function but is compromised in many areas, like electrical boxes, which leave holes in it.
So, the tighter you make the exterior of the shell, the more attention you should take in preventing leaks in your interior air barrier.
So you are saying I SHOULD ALSO have an interior air barrier? Like plastic or something? (I'm not having drywall....doing wood planks on the walls and floors). I thought I SHOULDN'T have it if the outside was airtight?
I liked the previous response... save my money for the beer. I don't drink beer, but with all the responses and still not having some conclusive answer, I'm ready to start! <grin>
Note I said interior AIR barrier not moisture barrier. If you are going with a wood interior, I would put the foam on the inside to act as an air barrier. Most moisture gets in to the walls by air flow. Reduce air flow and you reduce the chances of condensation inside the walls.
Since you are investigating insulation techniques I'd like to provide you a reference for densepack celluous in that it may help your thinking on the cottage.
Being a new structure you have a wide range of options available to you.
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Cheers
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I had read about the Mooney wall before. Would it make sense to do that if my walls are already 2x6's? I thought a few of the messages said it wasn't cost effective if the walls are already that thick (although it would help with the thermal bridging).
I like the densepak idea, though. I just recieved a marketing DVD from some place (can't remember) with their new "spider" insulation. It looks like a cross between blown insulation and blown foam. I'd love to see it up close, because I can't tell.
Do you use the same blown cellulose for densepak as you do for just regular blowing? What makes it "densepak", if so??
Hopefully JohnT8 or MikeSmith will be along to expound on the idea.
It is standard celluous placed under greater density than than normal which takes a larger blower if I recall correctly.
Just wanted to expose you to the various options open to you.
How large will the cottage be?
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bump
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It is 24X36 with a 6X8 bump-out for the airlocked entry. 1-1/2 story with 4' walls upstairs -- 9/12 pitch roof. 12' balcony off the upstairs bedroom over top of a sunroom below. Lots of southern-facing glass and passive solar features.
Even the basement has 57 sq.ft. of glass in it. I'm hoping that I have enough thermal mass to temper all the glass. I think I do. Just in case, though, there is a huge sliding barn-type door that will close off the sunroom from the rest of the house in the hottest (and coldest) weather. Because the cabin is very rustic, it'll look fine.
Since this is my "last hurrah", I'm doing it the way *I* want it.
Can those bigger blowers be rented at the big box stores?
Woman, forget calling that a cottage, it's a house! :o)
I have no working knowledge of the dense packed cellulose as I've never done it.
Perhaps JohnT8 can help you with the equipment question.
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Edited 4/26/2008 10:05 am ET by rez
Yeah, well, it started out as a cottage. It kinda grew. :) I started out with the "countryliving" plans for the 14' wide cottage, but making it 24' long.
Decided it wasn't big enough (or sturdy enough) so went to 24' wide. And added 4 extra foot to the height so I could add a 2nd floor with decent headroom.
Then decided to add a basement. Heck, might as well pour the (future) sunroom floor, too, and use it as a deck in the meantime. But by the time I would have put in a footer at 4' deep for the sunroom, it didn't cost too much more to just make it 9' like the rest of the basement....so did that. Of course, that meant that the sunroom will have to be built on right away now.
Luckily I made all these plan changes before I even started breaking ground, so it's ok.
Sometimes I look at this bohemoth, though, and wonder where my little cottage went. It's not what I had originally planned, but I think it will be more practical in the long run. Definitely more marketable for whomever ends up with it when I'm gone.
Yes, planning it all now certainly beats having additions, lean-tos and bumpouts occurring every couple years hereafter.
Making the shell tight will increase your insulation potential whatever route you choose.
Congratulations on pursuing your dream.
So many dream and don't pursue.
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quoting..."so many dream and don't pursue"
You've got that right! It took me 50 years to figure out that dreams won't come true unless you attempt them. I like reading quotes.... one says, "If you shoot for the stars, and fall short, at least you've hit the moon". Or something like that.
I like one I just read recently (in fact, it might be someone's tag line on here....can't remember), but it said something like "if a man says something can't be done, he needs to stay out of the way of the man doing it".
Although trying to build a dream is a really scary thing. If it doesn't work, I've ruined the dream. A dream is safe.... no one can destroy it. CREATING the dream, however.....you're setting yourself up for disappointments. I can't tell you how many times I've cried at night (when I finally finish my "day job" and can spend time on my dream) because I'm so scared that I'm not doing something right and I don't know how to fix that. If I know about something, I can figure it out. I can ask questions, I can read books. Its the things that I don't even KNOW that I don't know yet that scare me. Maybe there is something stupid that I didn't even know to ask about that is critical and will be a major downfall. Do you know what I mean?
For instance.... I just had the hole drilled in the wall for the septic line. The basement guys had it way up high. I knew, from talking to the health department, that the line needed to be between 1/4 and 1/8" drop per foot. Too much or too little and it won't work. If I DIDN'T know that, I might have thought, "hey, a 6 foot drop ought to be great because I KNOW I don't have to worry about it running downhill with that kind of a drop". It makes sense to the uninformed, but it's dead wrong. Maybe there are other things out there that make perfect sense to the educated, professional framer/builder/GC, etc., but to me..... I don't even know to look for them.
Thanks for listening. Sometimes I tend to ramble.....
Even the dog hides. :)
Well Christine, I'm sure I can speak for most here and say we are glad you showed up on the BT doorstep. What part of northern Ohio are you in?
As for mistakes every house has some kind of boogers somewhere in it so the idea is to keep them as minimalized as possible.
Don't be afraid to ask any questions on here no matter how minute they might seem at the time. You might just save yerself a big headache down the line somewhere.
And take pics to post. The more the better. Speaks a thousand words kind of thing.
Folks here will help you and point out things that need attention.
You've already come a long way.
And if you feel like rambling have at it. That's what BT is for.
Cheers
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Edited 4/26/2008 11:27 pm ET by rez
>>I knew, from talking to the health department, that the line needed to be between 1/4 and 1/8" drop per foot. Too much or too little and it won't work.<<One thing you need to know is that there will always be someone who will contradict what you have heard from someone else and you need to evaluate the source of the information, and the reasons given for things. For instance, in reference to your 1/4" pitch rule, I just bought a highly recommended book on plumbing from the Taunton press that says, steeper is better, regardless. He goes on to explain why. But that contradicts what many other reasonable people will tell you.Nowhere will you find this happening more than with all thing related to energy and moisture issues. Good luck on your project, by the way. Don't loose too much sleep over it, but vigilance in oversight is a good thing.Steve
The way they explained it to me was that if the pitch was too steep, the liquids would run away too fast and leave the solids. (ugh. Don't want to think about it).
Even if that isn't the case, and extensive research has proved differently now ..... they are the ones that have to inspect it and approve it, so I better do as they say. <grin>
While we are on septic issues.... when I had the tank put in (3 years ago, so I could beat the regulations that were coming into effect the next spring... making a $6K project into a $20K project), they filled the tank with water to keep it from floating over winter. Should I pump that water out now, or at least before I start using it? I could just put a sump down there, I'd think, since it is only water. Seems to me that the septic tank would be more efficient if it wasn't already diluted with water.
Leave the water that's already in the tank and just start using it. It will work fine as is. It really just means that the effluent going to the absorption area is "cleaner" for a few days.
bump
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had read about the Mooney wall before. Would it make sense to do that if my walls are already 2x6's?
A regular 2x6 wall will be superior to a regular 2x4 wall. But I'd have to have someone do the numbers to see if a regular 2x6 wall was superior to a 2x4-based Mooney. If I had to guess, I'd guess the Mooney was better. It has almost the same thickness but less thermal bridging.
The Mooney really excels on renovation projects. I haven't waded through this whole thread, but it sounds like you're building new. If I were building new, I would look to see if there were any SIP builders in my area. The SIP gives you an OSB-foam-OSB sandwhich with which the exterior walls are built. As good as the Mooney is at improving on the standard 2x4 on 16" wall, the SIP is still superior. You can pick your R value (based on the thickness of the wall). There is little to no thermal bridging.
Most SIP's are made with EPS foam (white stuff), but a few SIP manufacturers use other types of foam. Features vary from manufacturer to manufacturer. Some do 8' panels, some really large ones (50'+?). Some offer add ons like having the windows pre-installed. Or electrical chases pre-installed. Or even in some cases you can get the drywall already on when it arrives.
When pricing SIP's, keep in mind that it is not only your exterior structure, but also your insulation. They can also do roof/floor panels.
By the by, Dan's summary was very good.
My general info on foamboard: EPS- Expanded polystyrene is the white stuff (like styrofoam coffee cups). XPS - Extruded polystyrene is the blue or pink stuff (depending on whether you're buying Owens or Dow). I've heard some builders refer to XPS as "dowboard". Poly-iso is polyisocynate -sp-. It is slightly yellowish in color and often has a foil face.
Generally speaking (actual values vary), the EPS will be around R3 per inch, the XPS around R5 per inch, and the poly-iso around R7 per inch. Prices generally reflect this.jt8
My advice to you is not to inquire why or whither, but just enjoy your ice cream while it's on your plate-- Thornton Wilder
Do you use the same blown cellulose for densepak as you do for just regular blowing? What makes it "densepak", if so??
Densepack is simply blowing cell in dense enough that it won't settle. In a typical attic insulation, you blow it into snow-like mounds, but you also blow in several more inches than you really want because it will settle. With the Mooney application, you don't want the cell to settle, so you blow it in as dense as you can.
On my previous reply, I mentioned SIP's. Here is an example of a SIP manufacturer:
http://www.fischersips.com/
I haven't used them, but their site should have some good info.
jt8
My advice to you is not to inquire why or whither, but just enjoy your ice cream while it's on your plate-- Thornton Wilder