I thought I'd put this in a separate post so it didn't get lost.  I'm working with my builder on some alternatives to SIPs to see if we can get the same performance for less money.   He's giving me some good options that he is confident will come in cheaper, but it raised a question.  Part of the equation has to be the foam board on the exterior of the sheathing - that seems to be a key to the plan.  BUT I don't know how much I need.  Is R5 on the exterior enough, or would I need to go higher.

To give you an idea, here's what he gave me as an option -

3" spray foam in the cavities (R18), 3" batts in the cavity (R11), and 1" foam on the exterior (R5.6)

Using the same calculations as my other options, assuming insulation is 85% of the wall space, that gave me a total of a real R31 in the walls.  Is that 1"/R5.6 on the outside enough to prevent the heat radiation through the studs?

I'm wondering if, for the money, I'd be better off to either skip or go lighter on the spray foam in the cavities & go thicker on the outside??

I think the answer is that it depends on other things.  What you want in a house is the total package being as insulated as possible  It is always a trade off between walls, ceiling, and doors and windows. 

 

A few quick points.  1.  in a typical  stud wall the framing factor is more then 15%.  Sometimes it can be 30% if the framers go for it. Lets assume 25%.  Corners can be a big thermal break if not treated carefully

U = 1/r   so what you have with 1” of xps in a 2X6 wall is U = 0.75 * 1/(5+22+1) + 0.25* 1/(5+5.5+1) =.048528  take the inverse of that and you get 20.61.

If you use 2” xps =  R26.4 whole wall   so this wall should perform 28% better (not counting  infiltration)

For the record, no xps = U = 0.75 * 1/(22+1) + 0.25* 1/(5.5+1) =.0711  take the inverse of that and you get 14.02.  not so good in a cold climate  1” xps increased R value by 47%

On the other hand, a 12” deep Larson truss  with densepack cellulose will be u =  0.75* 1/(12*4) * 0.25*1/(6*4) = 0.0260

R 38.4  Now that is good bang for the buck.  I am negotiating  with someone to help them with a Larsen trust home.  I don’t know if it will pan out.

 

On the other hand a window ranges from U0.40- U0.18 typically.  IF you assume windows are 10% of the wall surface and you were comparing  U 0.18 windows + 1”xps vs  U 0.33 windows and 2” xps

0.9*(1/20.61) +0.1* 0.18 =  16.2  whole wall+ windows R value  VS

0.9*(1/26.4) +0.1* 0.33 =  14.91  whole wall + windows  R value 

There is your tradeoff

 

Basically I make a big spreadsheet with all the possibilities on it and sum up floors, walls, windows until I can calculate total heat loss  then I play around with various upgrades and subtractions to get the lowest total heat loss for the least amount of money.   Then you can make rational decisions about how various improvements make sense or not.    No two houses are alike so there is no one right answer as to what is worth it or not.

Generally speaking you always work on the biggest source of loss first and gradually move to things that are less of a problem.

wow, awesome reply.  Thanks!  I also have a couple spreadsheets of different options, sounds like we're on the same page.  My initial budget included sips and geothermal.  It's starting to look like (shockingly) I can't have everything I want.  Didn't someone write a song about that?  Anyway, if I shift some of the "sip money", I think I will be able to swing the serious windows - the 925 series, with the high solar gain glass package on the south side & the low solar gain package on the rest.  For the most part, that should give me R6-R9 for all of the windows.  I think that will help too.

""On the other hand, a 12” deep Larson truss with densepack cellulose will be u = 0.75* 1/(12*4) * 0.25*1/(6*4) = 0.0260 R 38.4 Now that is good bang for the buck. I am negotiating with someone to help them with a Larsen trust home. I don’t know if it will pan out."" In the process of having this priced out to wrap the outside of my timberframe. Larsen on both walls and roof with dense packed cellulose. My first thought was SIPS but the pricing was too far out for my wallet. Mark

Another "Alternative to SIPs" thread? It's starting to sound like SIPS are accepted as the "Gold Standard" and everybody's just trying to figure out a way get the same results for less $$. I hope that everyone who's reading this thread will check out the other threads on this, and other GBT forums, that are relative to this topic.

Im not sure that it isnt the Gold Standard for insulation/thermal bridging....it the cost and the fact many subs dont like dealing with them that have people looking for the best of both worlds. I wont apologize for that. Moneys tight. Mark

Mark, no one would expect you to apologize for your comments. And, I'm thinking now that "gold standard" was not a good choice of terms for the message that I was trying to convey. My contention is that, "In MOST cases, you won't get equal, or better, results, for less $$, with any of the framing systems that are being discussed on these forums, than you will get with SIPs. I've even listed several examples of projects where SIPs are likely not the most viable choice, and I don't think that I should rehash all of that. Most serious participants, and visitors, to these forums will read through the various threads, and can draw their own conclusions.

In regard to Timber frame homes, we've "skinned" quite a few timber frame homes with SIPs here in Northern California, and some owner builders made it work for them. Also, many, not all, timber frame companies include SIPs in their packages. Having said that, I think that a diyer, can probably save some $$ over SIPs with other types of insulation, and end up with a very energy efficient structure. One very common practice in timber framing is to install structural decking over the roof trusses, or structural rafters, and then screw down OSB/foam nailbase over the decking. The nailbase is quite a bit less expensive than SIPs, and all of the SIP manufactures offer this product. (I'm well aware that you, as most of the other participating "pros" likely know all of this, its those newcomers and "silent visitors" that we're providing ideas for, right?

Parting shot, for now: SIPs offer many benefits that meet "green building" criteria other than just superior energy efficiency, benefits that have been listed, and discussed many times in these forums and are certainly available on many more sites on the internet.

Hello all, Great info as I've been wondering about the same issues for an upcoming build in south-central PA. The info above compares 1" vs 2" rigid foam exterior with 3" spary foam + 3" fiberglass batts in the cavities (if I understood it correctly). Due to budget reasons, I was wondering if I could get decent performance with 1" rigid exterior foam, and then silcone calk like crazy in the cavities and finally fiberglass batts to fill in the 2x6. I know it would not be as good as with the spray foam + batts ... but I was wondering if it would be acceptable, given budget contraints. I can buy a case (or several) of the caulk and DIY for both the calk and the batts, where as I would not DIY spray foam.

Also, as a side note: I was wondering if there are any concerns with spay foam becoming slightly more brittle over the years ... or somehow losening up from the wood so that it doesn't seal as well. Do you have any ideas on that? I'm not saying it does this, just wondering about it from a novice looking at it. Seems to me like a very high quality calk would have better long term sealing - if one were careful to apply it everywhere.

I'm just starting out with this stuff and trying to figure my way (on a budget), so any comments and feedback is greatly appreciated.
Thanks!

I'd be interested in the $ per R that you come up with. My understanding is that fiberglass is least expensive but under-performs the specs and rigid foam and spray cellulose are similar in price. Spray foam is significantly more expensive.  So 2" rigid foam (least bridging) sounds attractive.

as little as 3/4" xps on the exterior is a good addition. there is some issue with condensation with that amount on the exterior depending on your climate. I am building 2x6 with 7/16 osb then 1.5" xps and dense pack cellulose in the cavities. with this system I will use 2x8s around the windows and doors. the whole wall r is over 20 where it would be around 13 without the addition of the foam. the foam is around fifteen dollars a sheet currently so the expense is about 2500 dollars for my 2000 sq ft ranch. don't forget to put foam under your slab as well.
good luck

since I posted this, we have finalized our "system". In the walls, we are doing 1" of spray foam + 5.5" batts. On the outside, we will use 1" of foam. The builder is using the foam board instead of OSB in non-structural areas, and in areas where structural sheating is required, those will get 1/2" OSB and 1/2" foam board. He did a lot of research on this for me in terms of building codes, rigidity of walls, etc. It looks like this is going to be a really good system for the money. Using foam board instead of OSB in some areas keeps the cost down - less than $500 more than just using OSB on the whole thing. The spray foam will add some rigidity and help with air sealing. The combo of spray foam & batts will give me about R21-R22 in the cavities, factoring in a little bit of compression of the fiberglass, plus R3-R6 in the foam board. The whole setup is costing about $2500 more than just doing R19 batts in the wall & nothing else..

Jerkylips, did you look at DOW SIS (structural insulated sheathing)? It is available in 1/2" and 1" thick. You could use 1" SIS where you need structural wind bracing in the corners and standard 1" of XPS or EPS over the rest of the house. It may end up costing the same as 1/2" OSB w/ 1/2" foam for material & labor... something to ask your builder or have him look into. This will keep your R value of the exterior foam consistent.

Posted By Jere on 15 Mar 2010 09:30 AM
Jerkylips, did you look at DOW SIS (structural insulated sheathing)? It is available in 1/2" and 1" thick. You could use 1" SIS where you need structural wind bracing in the corners and standard 1" of XPS or EPS over the rest of the house. It may end up costing the same as 1/2" OSB w/ 1/2" foam for material & labor... something to ask your builder or have him look into. This will keep your R value of the exterior foam consistent.

We looked at this stuff a while ago, but it was UNGODLY expensive.  I don't remember the actual numbers, but I want to say it was something like $30 for a 4x8 sheet.  At that time we were looking at using the 1" for the whole house, so it added several thousand dollars.  Now that I'm thinking about it, it may be worth looking into the 1/2" for the structural areas with 1/2" non structural foam over it - it would give me an extra R3 in those areas.  I'm not sure how the price of the 1/2" compares to the 1", though..

thanks for the feedback!

I'm not sure the cost difference between the 1/2" & 1" SIS... something to look into.

Also have the foam board extend down over the floor joist... basically from top of foundation to the trusses. You can get the foam board 8', 9', & 10' tall... so if you have 9' ceilings you could use 4'x10' sheets and that would cover the exterior part of the floor joists and framing.

Posted By Jere on 15 Mar 2010 11:50 AM
I'm not sure the cost difference between the 1/2" & 1" SIS... something to look into.

Also have the foam board extend down over the floor joist... basically from top of foundation to the trusses. You can get the foam board 8', 9', & 10' tall... so if you have 9' ceilings you could use 4'x10' sheets and that would cover the exterior part of the floor joists and framing.

I'll definitely talk to the builder about it.  It's probably a lot more do-able if we're not using nearly as much of it, too.

As far as the coverage, we're doing 2" of foam on the foundation walls as well, so we should have good coverage from footings to trusses.

Eric... great post on total insulation R value... via u math.

I have been kicking myself to get going on a cost per R per system spreadsheet..

We all should share such spreadsheets too.

aj

Babybldr, no need to lower your sights if you have access to storage and a trailer. I paid $5/bd for reclaimed 2x8x2" xps (Dow Blue Board at R10) back in Nov. Call Tom at 724-727-7325; he is in Greensburg in western Pa. He had lots of inventory then. If that doesn't work, insulationdepot.com has surplus and reclaimed insulation in a warehouse in Williamsport WV (at slightly higher prices.)

Also, insulation turns up regularly at monthly construction auctions in Leola and Quarryville. You'll find dates and directions at auctionzip.com searching on the keyword "construction." There is another monthly auction in Baltimore with good prices on windows doors and appliances. http://www.southernsalesservices.com/

Go here for 2x6 framing techniques that will save lumber and improve energy efficiency: http://www.toolbase.org/pdf/techinv/oveadvancedframingtechniques_techspec.pdf

I have considerable experience as a scrounger in so central pa. Drop me an email if you are looking for something specific. But give me enough lead time. The dirt is flying on my site.

Todd... my scrounge hero... I am with yaa on finding low cost foam! I need to get with the program and find more Upstate NY locations for surplus buiding
materials. I do have one guy who resells ripped off roof foam on Route 30 near the Sacandaga and that's it.
aj

this reminds me, I forgot to post it - this guy is in IL & has cheap "factory seconds" on foam board - no delivery, but if you're close enough to pick it up, it's really cheap..

http://www.insulationfactoryseconds.com/

Eric... why does the Larson truss come out so low?

On the other hand, a 12” deep Larson truss with densepack cellulose will be u = 0.75* 1/(12*4) * 0.25*1/(6*4) = 0.0260

R 38.4


tell me about the math... what is 6*4... and why is that low?  Somewhere I saw a thermal graph of  I joist walls 12" thick that looked like a great alternative to building the L truss

aj