Hi,

We have a ranch house in Iowa with a walkout basement. We live in the semi-finished basement, and main floor is still unfinished and framed only. We're starting to look at re-starting our construction project again and finishing the main floor. I'd like help on figuring out the best way to maximize the insulation in the attic.

The plan was/is to:
1. drywall the ceiling
2. Use those things attached to roof sheathing from eaves and go as high as needed so insulation won't plug them. Is there a point that it get's too high and will mess up with attic ventilation? We have all vented soffits, hip roofs, and ridge vents.
3. Use cardboard, or something to keep insulation from spilling into the soffit and done such a way so that insulation covers the double top plates of the exterior wall framing. I do not have those trusses with the extra space for insulation, so above the exterior walls, insulation will be 8" tall at best before it touches the things for vent stapled to the roof sheathing.
4. 1-2" skim coat of closed cell foam insulation on attic side of sheetrock, over cardboard, over double top plates.
5. Blow in cellulose to achieve as much R as possible (R50+, R70+ or more if possible), though not as much around the exterior.

Will this create a problem if insulation gets too high? I would think it would end up touching the roof sheathing right by the truss top chord around the perimeter, though there would still be venting on the sheeting with those things stapled to roof sheathing between trusses ( I can't think of the name.)

Am I just asking for trouble and creating some sort of moisture/mold trap? There is no HVAC equipment in attic, so did not pursue a closed attic option.

How do I figure out if my ceiling can support such a setup? I have trusses spaced 2' on center. I found my copy of the truss specs that came with the trusses. Could someone help me figure out:
1) What thickness of drywall would I need to support the heavy load of cellulose so it won't sag?
2) Would I need blocking perpendicular and in between the bottom chord of the trusses for more places to screw drywall into so that drywall won't sag?
3) Figure out if my trusses can hold all the weight?

Any an all discussion will be greatly appreciated. Thanks!

BG

Your plan sounds good and you have the basic idea. The one issue that you should pay attention to is to get the air flow directed into the vent chutes (aka proper vent) to both keep the cellulose out of the soffit area and keep the ventilation air from blowing the loose cellulose around.

The solution I like best is to use rigid insulation with a two inch air channel under the roof sheating. You can cut 2x2 insulation spacers on the sides for the two inch gap. Go up 4 feet which is the width of the insulation panel and you should have plenty of space for insulation, or higher if you wish. Install a blocker f rigid insualation to fill this gap to the roof vent. When you have the ceiling air sealed with a thin layer of closed cell spray foam, seal these blockers at the top plate and build up the foam insulation at the top plate area.

My gut feeling is that you will not have an issue with the weight of the cellulose, but do your homework. Check with the cellulose manufacturer to see what the insulation weights/st. ft, check with the truss supplier to see suggested weight limitations. You may want to have a structural engineer look the application over and do the calculations.

If the ceiling is air sealed and the ventilation system is proper, you should not have moisture issues (if the roof doesn't leak!)
best wishes

I believe that 5/8" thick drywall is recommended for ceilings with higher R-values of cellulose. That is what was used with my R-60 cellulose attic insulation.

You can do a search on "soffitt vent baffles" to find information about those items, such as:
http://jobsite.buildiq.com/articles/risk-management/ask-expert-baffles.aspx

Boontucky-girl,

Drywall is not rated to support the dead load of that much cellulose over time. A layer of OSB screwed in will eliminate the weight issue. You are supposed to leave 1/8th inch gap between the sheets for expansion. You should spray caulk all these joints. It will also make the drywall lay flatter when you apply it. Most areas require 5/8s Drywall for fire protection. 5/8s also maintains flatness better and looks better finished. especially on 24 centers.

Fiber glass netting can be purchases by role cheap. It can be stapled to the bottom of the 2X4 that tops the truss. You need to leave 2 inches next to the top for air flow. If the roofing is hot, for example asphalt shingles you will benefit from a radiant barrier and a 2 inch space then netting. All the space under the netting can be filled with cellulose. 30 inches of cellulose will act as air barrier and R-113 insulation.

Additional insulation will not make a difference that matters. You still have to deal with the thermal bridging issue. 1 inches of polyisofoam between the OSB and drywall can also assist in air tightness, insulation and as a thermal block to the wood framing. This can be significant. It will require hanging the drywall with 3 and half inch screws.

Brian

Thanks Brian.

I'm beginning to think that my goal to pile insulation in the attic may not be realistic for us with our budget. I just got a quote from the drywall labor, and I am not sure I could pay the labor to hang OSB and to hang ISO on top paying to hang the drywall. And DIY this isn't an option at this time.

I also looked at the cost of drywall sheets, and they've gone up since we did the basement two years ago.
And it would be another grand just on cost of OSB. How do I evaluate that the extra expense is worth the investment? Granted, I will get my higher R-values, which I had counted on for sizing HVAC equipment, but the cash and time to do it does not really seem feasible.

I am to meet with a spray foam guy and see what his bid is. I would doubt that increasing inches of closed cell spray foam in the ceiling would be any cheaper either. I'm already assuming to spray foam will be over 10 grand based on quotes we got two years ago, unless prices have gone down in the last two years. And that's not counting cellulose install. In the attic we can probably DIY the blown-in, but having a hard time finding a contractor to wet spray or dense pack the wall.

Am I being silly for wanting to spend money on what locals consider a "premium" insulation system when around here a normal house is typically built with fiberglass? It seems that I have a hard time finding contractors who don't do normal construction but go above and beyond that without charging an arm and a leg.

Sorry if I seem to rant.

Boontucky-girl,

Remember that a standard home built to code is the worst home your money can buy to meet the minimum required by law. Most new homes do not even get built to that standard.

Are you keeping the home or selling the home in the next few years? If keeping it, it will pay back in comfort, quiet, lower heating bills. If selling in short order do not bother.

40% of the total heating and cooling energy is lost through the roof of a to code building. That is what you can save 95% of by maxing out your effort there. That translates to a 40% derate of the HVAC system. Note that most HVAC contractors intentionally oversize after sizing for the cubic feet of home and do not derate for better envelope performance. You will need to tell them to derate and size to actual need.

You only need the 7/16ths OSB to hold up the cellulose load. Hire a temporary services laborer for $20 an hour, or an out of work neighbor that is a manual laborer. Have the material delivered from home depot. $8 per sheet plus $60 delivery, Rent a drywall jack for $50. 5 minutes to hang each sheet of OSB. If I recall correctly, if you keep payments to an individual laborer below $500 there is no reporting requirement. That all works out to $10 to $12 per sheet hung or about 34 cents a foot. That is only $350 for a 1000 square foot of costs. If you are asking for a contractor to do this you are running into his minimum billing. Charging $50 per hour would raise that to $450. Charging $100per hour would raise that to $600.

The raw chemicals for 2 # foam bought in 55 gallon drums are about 60 cents per board foot. Do it yourself spray foam in 300 board foot kits is about $1.20 per board foot. It can be messy but not difficult to DIY. You just need to work at being clean. You can take your time or have a friend that needs a job help. DIY does not mean you need to do the labor.

This information is so you know what they are making on the job. After the minimum quantity for the job they should be willing to charge less as you buy more. They are profiting at anything $1 or more per board foot. $250 or $300 setup and $1 per board foot for 2# and $.50 per foot for .75# foam. Changing to .75# after first 2 inches is fine. It is often hard to find reasonable foam installers. We often do it for our clients because of that. DIY equipment is slower but works fine. Here again we work to cut the cost of every step to keep the whole job at a lower costs.

See if they will work on a time and materials basis. By the job bidding has to have contingencies added. Good contractors are lower cost by time and materials. It will also show you how much they are charging. We charge $50 per hour for construction jobs and provide materials at cost. We find this arrangement makes everyone a winner. This will lower risk and provide fair and reasonable profits for the contractor and a best price for the home builder.

Brian

Foam installers in my neighborhood are a lot hungrier than they were two years ago, but nobody can afford to work for free. It rarely makes sense to do a truly high-R attic all in foam.

Beyond a couple inches the net greenhouse effect of the HFC245 blowing agent for closed-cell foam is greater than the carbon footprint of the energy sources it's offsetting. Lower-damage blowing agents are just now showing up on the market, but they're still nowhere near as benign as cellulose. Open cell foams are usually much more enviro-friendly, but still nothing like cellulose.

OSB+ deep low-density cellulose is usually the value-leader, even if it needs to be supported with OSB (as Brian rightly recommends.) Air-sealing it with closed-cell foam isn't always a bad idea- 1" of ccSPF on the OSB is also a class-II vapor retarder on the "right" side of the assembly for IA, but it's an expensive vapor retarder compared to paint or poly.

You would need to look at the specs for the trusses to see how deep you can go without exceeding the dead-weight loading of the truss chords. "Energy Heel" trusses designed for deep insulation all the way out to the outer edge of the top plate of the studwalls usually have sufficient dead-load ratings. If these were site-built DIY trusses you may need an small bit of engineering analysis to come up with the limits, but that shouldn't be more than a one engineering-hour to get the answer.

Boontucky-girl,

I agree with everything Dana has communicated here. I do think a contractor needs to charge a minimum of $50 per hour to maintain healthy financials. Anyone charging below that amount is not likely to stay in business long term. You also have to be detailed and quality oriented if you are charging in that cost range because you are not allowing for a lot of room to make mistakes.

If your chord strength is low it is usually easy to correct with proper metal strapping at the center spans. You will need to have the fix documented by a PE in your area if needed.

Brian

I really appreciate the detailed responses. I hope I didn't mean to imply that I want a contractor to barely make a profit. It was late in the day and I might have been off kilter.

Thanks Brian for such an insightful post. I am building this house as if it will be the last one, so not planning to sell anytime soon.

Good tip forwhen we re-visit HVAC. We did the rounds two years ago and my first screening question was if they do heat loads. I had an idea that there is a method to proper sizing HVAC, so I'll be sure to remember to ask for derating based on envelope performance. I've read a lot researching this topic to believe that higher R is the way to go. So I am very grateful for the help in figuring out.

Your paragraph describing how to calculate costs brings up tons of questions. When you say 5 minutes per OSB board, does that include having to cut it to fit rooms? Interior walls have already been framed in. We have about 2K sf. Most 8 ft, one area 10ft, and one room with a vaulted ceiling, so a couple of areas where height differs. So would that take like an 8 hr day for someone not going super fast? Or does the lift make it so it takes less than that?

How is the OSB oriented? Long side perpendicular to trusses? I have hip roofs and in some rooms trusses go both ways, or it doesn't matter? Would the drywall then be hung perpendicular to the OSB? Drywall screws good enough or wood screws? Minimum depth for the screw to handle weight?

I would assume that the OSB goes up after all rough ins are finished, and just before drywall. How precise do cuts for electrical boxes, can lights, etc. etc. need to be?
I know we need insulation contact rated boxes since they will be buried under foam, and I read somewhere about adding like a sonutube with a cover over IC can lights, and spray over this. Is that prudent? Necessary? Or a cadillac when a chevy will do?

We are meeting with a spray foam guy that did our rim joist a couple of years ago. So we'll see how much his estimate will be.

I thought that the first thing we should do as soon as the bank gives us the green light, was to get get vent baffles on, put something to cover the space between baffle and double top plate, then get the spray foam guy in to do the skim coat on the walls, spray the vaulted ceiling, and do the spray foam above the exterior wall.
I would think it would be easier to get at this space before the ceiling goes up. Our roof pitch is only 6/12, so i can't imagine the guy would want to crawl on hands and knees in the attic to reach this space after drywall was up. We will need two trips anyway since we have to spray the vault part before drywall. Then the trades can get all rough ins finished, and after OSB/ceiling drywall is in, come back and do the attic side of spray foam.

At this point is when I'd like to do a blower door test before the rest of the walls get insulated with whatever and before drywall.

You say profit at anything over $1 per board foot. The bids I got in the past are done by sq ft of wall. How does that compare? I remember the quotes were around $15K for the closed cell skim job.
I will ask if they work on time and materials. Right now I need an estimate so the bank can run an appraisal on the proposed finished house and see if we can build with the limited budget that we can afford.

You're not in IA by any chance so you can give ma bid, are you?

Dana,

Yes, I didn't mean to imply that I want free labor. My frustration has been that in the ten years that we have been dreaming and researching about building a house, I see all these new things that seem very popular in places like the east and west coast, but I can't find anyone local that knows what I'm talking about, and the one or two that do it, are very expensive vs. "normal".

Thanks to this forum I've found answers to my endless questions because of people like you and Brian and many others willing to donate your time and expertise. So I want to make sure you know I'm very grateful. I've learned enough to ask questions, and I'm sure I'm very annoying to contractors.

I am making a lot of phone calls trying to find someone who does wet spray or dense pack around here. There is one that probably does, but I disliked the way they never returned my calls or answered my e-mail, so I am not quite willing to give them my business.

I would really like the spray foam skim job.To me it seems like a good way to max R by combining spray foam with cellulose in my 2x6 walls, since I no longer have the option to add more insulation on the exterior of my walls unless I tear off the siding. I've seen houses done with poly, and the level of care I would expect to ensure a good poly job, I think the spray foam might have better luck at actually sealing leaks. I really want to avoid fiberglass if I could help it, but may have to resort to blown in fiberglass to fill the rest of the walls. I do think we could probably do the cellulose in the attic ourselves with a rental from the big box. But dense pack or wet spray seemed to be more of a job for a pro (if I can find one)

I took a peek at my truss documents and they each truss drawing says BCDL 10 psf max. I assume that means bottom chord dead load is at 10 psf. So what would that translate in height of cellulose fill + spray foam + OSB + drywall that I can do?

Again, thanks for all the insights and help!

Boontucky-girl,

“Very expensive vs. "normal"” this is always the case during early adoption. It bothers me when lower competition raises the price of a service from reasonable to what the market will bear. It may make a few extra dollars for a few individuals but it slows adoption of better solutions. Cellulose is the cheapest standard products insulation alternative when applicable. It also provides an air barrier at 20 inches of depth for 1.6 pound loose fill. It is the greenest alternative of standard products. It has the lowest embodied energy of standard products. Note that there are greener and lower cost alternatives if you design for them from the start. In your case the options that have been described are what you have.

People that do not know enough get taken advantage of by many actors. Knowledge is your shield making sure you get the best deal possible. Never apologize for taking the time to learn.

Skim and cellulose is a good thought. Note that it will not stop the thermal bridging that goes through the studs. An R-2 insulation strip on the inside of the studs will improve the overall performance of the wall by 30 to 40 %. I have used strips of EPS and contained Aerogel strips Thermablok. NASA participated with a company to develop and test Thermablok product, http://www.thermablok.com/. Thermablok strips are only ¼ of an inch thick and do not take much space up. They are the highest R value per inch. Cellulose dense pack bats are available. They are recycled blue jeans. They are none toxic, not itchy, no dangerous fibers. http://www.bondedlogic.com/ for example. These are easier to install than fiberglass. I do not know your local availability.

BCDL 10 psf max. Is a typical rating for a no storage attic. 1 # for OSB, 2.2#s for dry wall, 4 #s for 30 inches the cellulose, and ½# for the foam that is 7.7#s. That is pushing it for a 10 # rated dead load but within rating. If concerned you could use 3000 # rated metal strapping to support center of chord spans.

The OSB can be above or below the chord. It needs to reduce the loading on the back of the drywall. Drywall is not strong enough to have a dead load beyond the rating of the board. Note that it depends on chord spacing also 12, 16, 19.2, 24 etc. The OSB does not need perfect coverage of fit. A ¾ inch gap will be bridged by the cellulose and therefore it will not transfer the load below. Cutting perfection is not paramount. You are supposed to leave 1/8 inch gaps for thermal expansion at a minimum. You are going to finish with drywall below. That is where the cutting and finishing needs to be better.

Screws used, depends on desired load characteristics. Screws ideally will penetrate a minimum of 1” into the final member as a standard. I usually use screws that are ¼ inch shy of the total penetration unless the final member is more than 2 inches. If holding down as in floor covering of the attic space, screw selection is of less concern. Attaching sheets from below is the most critical application. In the case of drywall it is number and location of the screws that matters. The drywall will fail before the screw is tested.

Brian

The dead load rating has some presumptive truss spacing as well- trusses set 24" on center will have substantially less load than trusses 16" o.c., etc., but the standard spacing for trusses is 24" so I'd assume that the marked rating would be for 24" o.c.. A call to the manufacturer would confirm that.  (It's sometimes worth the extra couple of trusses to hit 16" o.c. if you're going to be hanging a lot of

At a settled density of 1.4lbs/ft³ at ~R3.4/inch, a depth of 18" would add about 2.1 lbs/ft² to the dead load, and deliver ~R60 performance. The specs for some cellulose products are somewhat lighter and higher-R than that, some denser, but it's safe to assume that even at ~R75 (24" @ 1.5lbs/ft³ and R3.2/inch goods) you'd only be at 3lbs/ft² dead loading from the cellulose itself.  To that you'd have to add the weight per square foot of the OSB + gypsum + lighting fixtures + mechanicals, etc that might be supported by the truss chords, but it looks like you would be in good shape.  A layer of 7/16" OSB runs ~ 1.5lbs/ft², standard density half-inch gypsum runs 1.7lbs/ft² (and there are lower density half-inch products out there too) so at R75 you'd still be at only ~6.2lbs/ft² nominal, so I expect you have some margin. Even at 2lbs density (unlikely in an open blow unless the installer had a truly defective blower) and 24"  the cellulose would be only 4lbs/ft², and the total dead weight would be 7.2lbs/ft².  

Anything beyond R75 would have dubious economic value  in your climate zone unless you're going for a PassiveHouse cert or NetZero, but R60-65 will usually have a reasonable long-term net present value.  In northern IA code min is R49, in southern IA code min is R38.

On the walls, assuming 2x6 construction,  a 1" flash-foam with 4.5" of cellulose would be protective of the sheathing, but with 16" o.c. you get 20-25% framing fraction, and with a  center-R of  ~ R22-R23 the whole-wall R runs ~R13.5-R15.  If you can give up an inch on the interior slipping  1" of XPS between the wallboard & studs brings that up to  the R19-20 range, and is a minimal class-II vapor retarder (~0.8 perms @ 1") which should satisfy any code-inspector.  In a room with 18' of running exterior wall you're only giving up 1.5 square feet of living space for that performance boost.  If you want to cheap-out on the XPS you could cut'n'cobble 1.5" stud-edge strips, which would reduce your center-cavity R by about R1.5, but the net reduction in whole-wall R is only about R0.5- you're still over R19.

Brian- 2.2lbs drywall=3/4" goods, but with the structural OSB supporting the load, half-inch is fine.

I doubt they have the clearance for 30" of cellulose all the way out to the edge, unless I missed something. R100 makes economic sense for net-zero in that climate, but probably not here.

It's not spelled out in any spec and would need engineering analysis if you were bumping over the chord spec) but GLUING and screwing the OSB to the bottom of the truss chords with construction adhesive will increase the dead-load rating of the whole structure, giving you a little bit more margin than the nominal rating of the truss.

The aerogel strips for stud edges are pretty kewl, but pretty pricey compared to XPS. They can make economic sense for steel-stud systems at low-to moderate R in commercial construction, but probably not here.

Thanks Brian.

I forgot to mention that I do have 1" of ISO on the exterior already. It's cement board siding+tyvek+1" ISO+1/2"OSB covering the exterior walls, so I do have a little bit of thermal bridging protection. Not a lot but it sure will help some. I'll have to look at the blue jean insulation, but maybe I can ship to my home and have to do it myself if I can't find someone to spray the cellulose on the walls.

So how much R is 30" of cellulose?

Yes, there's no storage up there. So you are saying that I can add some metal strapping between the trusses at the midspan of trusses to help hold the load? I did see that there are a lot of 2x4's going across from truss to truss and I don't see those called for in the truss specs. Maybe my framer thought they are needed? Would that help any for "bracing" Where does one find 3K# rated strapping? If 's only needed at midspan, and not too expensive, I might add those myself for peace of mind. I found someone who does handyman work and charges very reasonable $18 per hr. So I'm hopefull I'm back on course!

OSB above the chord would involve a lot of cutting to fit between the webs, but would it need to be screwed, or just laid down? Might have to overlap at joints if no screws. I think that would be difficult since our roof is not a simple roof. It has gables, hips, mixed heights, so the trusses are very complicated, not just simple spans across. So I think OSB between drywall and truss will have to be my option. My trusses are 24" o.c.
So does drywall have to go perpendicualt to OSB joints? Does it matter?

I'd like screws that will hold the 7.7# and not pull out. My truss bottom chords look to be at least 2x4" for the smallest one. The bigger trusses have deeper bottom chords. Drywaller said he'd do extra screws on the drywall already.

So once we meet with the spray foamer and get his quote, we'll see how the budget is doing. Now next is to find the cellulose solution.

I had a person tell me that for the venting in the attic, that they use 2 layers of tar paper nailed 2" away from the roof sheathing, stapled to each truss in each truss bay, and covering the double top plate on exterior walls, then they spray the foam against this to fill that cavity between double top plate and vent "baffle." Does that seem like it would work? I had assumed I would need to buy vent baffles (or do the 2" XPS mentioned earlier in this thread) and do some sort of blocking with cardboard or more XPS for the spray foam. But tar paper seems cheaper.
I will have the foam guy quote as an option to fill that 5"-8" above with spray foam, vs. the skim coat. Not sure that the extra cost will be worth it, but I do know that there is no way I'm getting high R around the perimeter because of such small space.

V

(A lot of simultaneous cross posting here!)

30" cellulose is ~R100.

Screw & glue the OSB on the underside of the chords.

I'm not sure why they need to double-up the roofing felt on the rafter channels, but it would work.

How much vertical space DO you have between the top plate of the studs and the underside of the roof deck at the outer-most point?

I have see where "tar paper" or building felt was used to spray foam against and it did not adhere well in many areas. Also the building felt is a roll/sheet product that works best on a flat surface like sheathing, not trying to form it into a vent baffle. You are asking for additional insulation above the top plate area, if you use a rigid foam product, XPS or foil faced polyisocynurate you will get some additional R value as well as an air chute that is easily sealed. A couple of sheet of rigid insulation is not a major expense and compares closely to the cost of a good quality air chute.

Boontucky-Girl,

“1" of ISO on the exterior” That is adequate for blocking of thermal bridging for most homes.

“So how much R is 30" of cellulose? It depends on the exact product used, but 3.7 per inch would be 111.

Strapping, here it depends on the design of the trusses. Additional strapping will not do much if the the truss is completely cross braced as you seem to be describing.

I use rated metal pallet banding strapping. It is cheaper and readily available.

A good handyman can be a God send for little things your contractor does not really want to do.

OSB above the chord needs to be screwed but not as well. Screwing adds to strength of total system. Since this is beyond code it is not required and depends on the truss spacing. You might want to rip all of them to for example 24 or 16 inches on a table saw if they would then fit into the web better to get most of the cover. Overlapping is fine where easier. Smaller pieces can be set to fill holes. Again this is to protect the drywall and is not a finished surface.

Drywall perpendicular to OSB joints is slightly stronger but this is above code already.

Tar paper changes with heat. I would not see it as a good solution. There are other rolled materials that could be used that are stable.

The Screw & glue of the OSB on the underside of the chords is probably your best option generally.

I used slightly elevated numbers for weight values to be safe. Styrofoam products are not to be used in applications were the temperature exceeds 130 degrees. Polyurethane and Polyisocyanurate have higher temperature range in application.

I agree with Dana that EPS is lower cost than Thermablok. That is why I listed it as an option. Thermablok takes up only a ¼ inch. That is its advantage. If the extra inch matters then it is a better option. Since you already have an inch of Polyisocyanurate foam as a thermal block the additional expenditure on the walls is not worth it. Did you treat the roof the same? Thermal bridging through the ceiling is a much bigger problem. Heat likes to go up. By the way Thermablok is not that expensive because you only use it on the studs.

Brian

Dana1,

R100! Wow! I am assuming this would be in the middle, where the insulation will be at 30". As we go out towards the exterior walls, room gets less and less.

I have supposedly 8" heel trusses, where truss actually sits on the bottom chord, and the top cord sits on top of that, and there's a wedge in between the two. But measuring with a tape measure at the outer edge of the double top plate to the roof sheathing is more like 7", not 8". Then on the inside of the double top plate to roof sheathing is about 10".
Thanks for mentioning that I should glue OSB to rafters.

I am not too sure about roofing paper for vent baffle. I saw the styrofoam egg carton looking ones at the store, and they seemed very flimsy. I think this double plate area should be paid attention to, and I'd like to do it right since I can't fit much for insulation there. I am curious to see what the foam guy says about this space and how much he charges per inch of foam.

Thermo - I like the idea of using foam board for a baffle, so I will look into that. Perhaps the handyman can put that up if it's our responsibility to get those up and not the foam guy.

Brian - I didn't do anything to the roof other than roof paper, ice and water shield, and shingles. Or do you mean to put some sort of foam between OSB and drywall? Or between OSB and truss chords?

V

V,

With thermal bridging not addressed in the standard to code home. 60% of the heat is lost through that bridging. If you do not address this it is significant. Average Code home loses 40% of its total heat through the ceiling. So 40% of 60% means ceiling thermal bridging accounts for 28% of the total heat loss of the home that is built to code. R-2 will cut this value in half saving 14% of ongoing utilities related to heating and cooling. Average 2000 square foot home utilities are about $200 a month. 2/3rd are heating and cooling. So $200 times 2/3 times ½ is $66 dollars a month for life. These numbers are obviously not exact but are how you should decide what things you are going based on ROI. Comfort, noise reduction, drafts, etc. should all be considered.

The insulation beyond 60 will have a smaller pay back. It only affects the 40% of losses that are left. At best 20 to 30 dollars per month of utility savings are to be had here.

Getting every membrane assembly up in R-value has the biggest payback. If the home is not well sealed you cannot fix the home until you address the sealing. If your windows are marginal you cannot make up for it by fixing everything else. In the roof it is most important to seal completely first. Once that is under control it is then time to max the insulation in short areas with spray foam. Spend money on the weakest points first. Paybacks for the extra insulation in the middle are reduced if you have any weak links in the basics. I would consider using a radiant barrier instead of the tar paper for vent ends. You will get the additional benefit from the radiant affect. They can be found for 3 to 30 cents per foot in rolls 3 or 4 feet across. Make sure to get one of the heavier gauge ones. They will be tough enough, they are thermally stable and the foam will stick to them. Note that heavy gauge aluminum foil is just as effective a barrier though not as tough requiring you to be more careful when installing, but the least expensive per foot.

Insulation board is a good option depending on cost. ROI, ROI, ROI, The best return is designing to be a zero energy home from the start because you can eliminate standard HVAC equipment and build with low cost high performance systems that deliver at no additional cost.

Brian

Thanks Brian.

So you strongly would encourage me to do something to reduce thermal bridging in the ceiling. What would be appropriate? Is this where you're saying to use thermabloc?

Have you used it? About how much does it cost per ft? I went to the website and see there aren't any dealers available. Since I have the 1" on the outside, I wouldn't also need it on the walls would I?

My windows at the time I bought them seemed to have good U and R values when I compared between brands. Just have to make sure the foam seal is good around them.

I'm a little bit past the design stage, and I don't have the funds to make too many changes or additions. But I do want to make sure I get the things that can't be upgraded easily later, like insulation for example.