I am in the planning stages of building a new 2 story 3000 square foot house in northern Illinois. The temperature extremes are about 0-30 in the winter and it gets up to 90+ in the summer with moderate to high humidity.

I have been researching online for the past week looking for the best and most cost effective way to insulate my new home. I really like the benefits of sprayed closed cell foam, but at $10,000+ to do the 2x4 walls and 10" in the trusses this might not be an option for us. And i came up with a few different options that might work and come in at a little bit lower price that would be easier to manage. So i guess i'm looking for some suggestions and recommendations from people that know what they are doing.

1.) 3" of closed cell in the 2x4 walls, 10" of closed cell in the trusses. Obviously this is going to be the most expensive option.

2.) 3" of closed cell in the walls, and 2" of closed cell in the trusses with an additional 8" of open cell sprayed on top of that. Can you even spray open cell on top of closed cell foam?

3.) 3" of closed cell in the walls, and 10" of open cell foam in the trusses. But would these 2 different types of spray foam kind of fight each other? And how would i address the vapor barrier on the second floor?

4.) 3" of closed cell in the walls, and 15-20" of blown in cellulose insulation blown in the 2nd floor ceiling in the attic. Would this kind of defeat the purpose of even using the closed cell in the walls?

5.) blown in cellulose in the whole house.

6.) Owens Corning Energy Complete System sprayable caulk to seal the walls and rafters up, then use blown in fiberglass insulation.

7.) or do you have any better suggestions?

I have allergies and am allergic to dust pretty bad. I realize dust is everywhere and it is hard to avoid, but if I could minimize it in the next house it would be best. I have read that the spray foam does help to lower the amount of dust in the house. I am planning to install a whole house hepa air filter regardless of what type of insulation i go with. Would using the blown in cellulose insulation increase the possibility of dust in the house. And if you use the blown in cellulose is it any better than helping to contain the dust over just using regular batt type insulation?

What it all boils down to is that i would like to use a product that will help to provide a better air quality inside of the house and at the same time will help to lower the utility bills. So what are your recommendations?

From what I read, the best bang-for-the-buck in stick construction is to put foam panels OUTside of your sheathing, coupled with cellulose or equivalent in the stud bays. I would say 2" foam would be best, along with lots of careful attention to sealing air leaks, which appears to be at least as important as good insulation. The idea of the foam on the outside is to eliminate or reduce thermal transfer through all of those studs. I would consider going with 2X6 walls for the extra strength and insulation. If framed correctly, you can space the studs farther apart, resulting in better insulation and material savings.

As for the attic- careful sealing and lots of blown-in insulation is effective. As for the dust- if everything is properly sealed up, dust shouldn't be a factor. Keep reading- there's lots of info about air-tight drywall and a lot of other techniques to build a tight, well-insulated house, using a wide variety of techniques.

It pays to view the house as a whole system. It's great to hyper insulate the walls and attic but at a certain point the other components begin to govern (like windows and doors. You need to run the numbers.... That said, it may depend on what type of exterior siding you plan on. If it's something lineal and horizontal (like clapboards or hardi plank then then the exterior foam makes sense as you can apply vertical furring to create an air space and a true pressure equalized rainscreen system. With something like vertical board and batten it gets trickier as the furring may need to run diagonally or be interrupted to allow air to flow and water to drain - then wood shingles get trickier still as they need nails just about everywhere.....

I would say that if the siding allows, go with the exterior rigid foam and then cellulose on the interior. This requires careful detailing as to where int eh stack up the air barrier is and what it is. If one of the sidings is chosen that makes that difficult then I would go "flash and blow" by applying an airsealing layer of cc foam on the inside surface of the sheathing and then the rest of the cavity filled with cellulose. In either case the balance between the thickness of foam and the thickness of cellulose is important in order to prevent condensation on the sheathing or the foam (in the flash and blow) The more cellulose you have the more foam you'll need - which seems counter intuitive, I know, but it's true. I would use a 2x4 construction for the structural frame (using advanced framing method) then fur it out on the interior side ala a "Mooney Wall" That's the best bang for the buck and eliminates the thermal bridging of the framing (mostly). With a flash and blow in the mooney the cc foam can be applied right over the 2x4's. The Mooney also simplifies wiring and and I suppose plumbing too (although I still avoid plumbing in an exterior wall no matter how much insulation is there...but if you must at least it's safer and PEX doesn't burst as easily).

One last thing, be very careful to allow for drying of the wall assembly either to the interior or the exterior. Be careful not to install two vapor barriers that will trap moisture between them. The prescriptive nature of the code and a zealous building inspector can sometimes be your downfall here.....

Good luck!

As to the dust - I don't think the cellulose will be a contributor once the finishes are complete. My wife has horrible dust allergies and I think it's more what's in the dust than just dust in general. Dust mites and pet dander are her undoing.... The best things we did for that was a) hard surface floors (stone, tile, wood, cork) with radiant heat (no drafts, fintube etc.) and b.) a Central vacuum that exhausts directly outside. The central vac in particular has been a life saver. (others can chime in about make-up air for a central vac - our house isn't tight enoughfor it to matter)

One of the best resources I have seen in the last 40 years on this topic is "The JLC Guide to Energy Efficiency". Should be required reading for anyone considering building or even retrofitting and it's a great reference. http://jlc.buysub.com/jlc-guide-to-energy-in-the-home.html

Using 3" of closed cell in a wall cavity is a waste of money- the 3" thermal short- of the stud is an R3 short-circuit in an R18 layer, rendering a whole-wall R of ~R10, about where you'd be with mid-density batts or cellulose.

Closed cell foam is a GREAT air-sealer, but applied to the interior of a wall cavity it misses the stud-plate/subfloor gaps.

Closed cell foam on the interior of the sheathing forms a non-wicking condensing surface, protecting the sheathing from wintertime moisture accumulation form interior moisture drives, but that's true even at 1". At 3" it impairs the ability of the sheathing to dry toward the interior.

Birdman has it right- put the cheap stuff in the wall cavity, and use BLOWN or SPRAYED goods to eliminate gaps/voids/compressions, and save the foam budget for the exterior. With 2" of exterior iso (or closed cell spray) and cellulose cavity fill you have more than twice the whole-wall R value for the same or less money, and sufficient exterior R that you can use standard latex paint as the only interior vapor retarder without risking moisture accumulation in the sheathing, and the whole assembly stays drier. Using higher density fiber reduces any infiltration losses- either cellulose (any density) or 1.8lb Spider or Optima fiberglass is air retardent enough.

The flash'n'fill method would be advisable if going with less than R7 on the exterior of the sheathing to prevent wicking during condensing events, and in that case going with dense-packed cellulose (3.5lbs/cubic foot) would be advisable, otherwise the humidity cycling in the cellulose from seasonal condensation would cause it to settle somewhat over time.

Rigid iso above the roof deck & rafter is cheaper & more effective than 10" of any type of foam, and keeps the wood warmer, above the dew point of the interior air. Fatting out the R with cellulose blown in netting under the rafters also works, but you need at least R20 above the roof deck in a N.IL climate to then keep the roof deck warm (==dry) enough to do that without interior vapor retarders. With exterior foam on the roof deck you can use 2x furring though-screwed to the structural roof deck to hold down the foam, and put a cheap half-inch OSB nailer deck above the roof deck (fully vented) for the shingles.

For folks who might be new to this topic, I think there should be mention that the roofing related suggestions are for a NON-vented attic. Unless I missed it, don't see any reference to the type of venting.

Keep in mind the following important building science rules:
1. Avoid thermal bridges (with wood frame construction the easiest and most cost effective way of doing this is to use rigid EPS (expanded polystyrene insulation) as your sheathing material. You can then use fibreglass or roxul in the stud cavity. You should specify the ESP to have all joints taped and sealed so that it can act as your air barrier at the same time.)
2. Put the vapour barrier on the warm side of the wall. (whether it's closed cell spray foam or just poly sheet plastic - don't forget the poly if you are using batts in the wall cavity. Also, it's not as important that the vapour barrier be perfect if a good air barrier system is used - if there is no air movement through the wall there is little potential for moist air to be drawn through the wall and condensation to occur )
3. Make sure your air barrier is continuous - pay attention to continuity at windows, doors, wall to roof/ceiling connections, past floors etc. And remember, when it comes to your building envelope: HOLES ARE BAD
4. More is more. When it comes to insulation the more, the merrier. Yes, whole building is important, yes windows are likely to be your weak point (even if they are quadruple pane, low E, argon filled) but your total house R value is really the average of all components so don't get stuck on the weak points unless it is a glass house.
5. Do not trap moisture in the wall or ceiling space. As mentioned above somewhere, be careful not to use a vapour impermeable membrane anywhere but on the warm side of your wall. Where it is unavoidable (at the roof level - where an impermeable roof membrane is desirable!) the space in between needs to be vented to allow moisture to escape. (Note: tyvek homewrap is vapour permeable and can be used as a barrier in a typical rainscreen wall assembly.)
6. Last but not least, don't forget the earth. It may not be as cold as outdoor winter air but it is a huge heat sink - did you know that approx. 30% of heat loss can happen through the basement slab on grade? INSULATE UNDER THE SLAB!!!!! (EPS works well here too - try 4") AND all the way down the foundation walls.

Hope this helps with whatever insulation you choose.

p.s. Blown in cellulose in attic is a cheap way to get lots of R, but sealing at the ceiling is trickier - 1" of spray foam prior to blowing in can do the trick.

Ok, so after doing some more research, and taking your guys very helpful info into consideration i am thinking of doing the following. Please feel free to critique any of it!

Install 2" xps foam under the basement slab and garage floor slab, taping all of the seams, 10 mil visqueen laid on top of it, overlapping seams and taped as well. One question, do i install the foam all the way to the edge of the concrete walls, or leave a small gap there?

Install 2" xps foam on the exterior foundation walls, after the water proofing tar is applied. Do i need to install it all the way up to above grade where the vinyl siding (on sides & back) or brick (in front) would start, or is it ok to stop it just below the grade level with approx. 6-12" of the concrete wall being exposed at the top. If i did run the foam all the way up, what would i cover it with to protect it above grade?

What is the best and most cost effective way to seal the sill plates? Accoustic caulk, foam, or edpm rubber gaskets? I'm figuring on having them seal this at the top of the foundation to PT sill plate, 1st floor bottom sill plate to floor decking, and 2nd floor bottom sill plate to floor decking.

My builder uses standard 1" R5 foam board on the exterior of the house. I am using 2x4 walls 16" on center and live in northern Illinois, so climate zone #5. Would using foil faced Polyisocyanurate (polyiso) be a better bet. It is more expensive, but i assume cheaper than going to a thicker xps foam and it has a higher r value of R6.5 per inch.

Would the house wrap go on the wall sheathing, or on the foam?

And for the wall insulation i am leaning towards using wet sprayed dense packed 3.5 lbs cellulose insulation. Would blown in fiber or spyder insulation be a better choice here? And also have them install an air seal package, which should hopefully seal everything up pretty good.

And for the vapor barrier only use latex paint on the drywall.

Use the edpm rubber drywall gaskets on the insides of the exterior walls to create an air tight drywall system.

For the attic, blown in cellulose insulation at approx. R-60 at 16" thick. Is there a better type of blown in insulation to use here, like for example the different types for the walls?

And install the foam air baffles on the undersides of the roof decking for venting. How often should these be installed? Every 10' or so?


Any other suggestions to help a newbie out? ;)

Posted By tbm878 on 28 Feb 2012 01:35 AM
Ok, so after doing some more research, and taking your guys very helpful info into consideration i am thinking of doing the following. Please feel free to critique any of it!

Install 2" xps foam under the basement slab and garage floor slab, taping all of the seams, 10 mil visqueen laid on top of it, overlapping seams and taped as well. One question, do i install the foam all the way to the edge of the concrete walls, or leave a small gap there?

Install 2" xps foam on the exterior foundation walls, after the water proofing tar is applied. Do i need to install it all the way up to above grade where the vinyl siding (on sides & back) or brick (in front) would start, or is it ok to stop it just below the grade level with approx. 6-12" of the concrete wall being exposed at the top. If i did run the foam all the way up, what would i cover it with to protect it above grade?

What is the best and most cost effective way to seal the sill plates? Accoustic caulk, foam, or edpm rubber gaskets? I'm figuring on having them seal this at the top of the foundation to PT sill plate, 1st floor bottom sill plate to floor decking, and 2nd floor bottom sill plate to floor decking.

My builder uses standard 1" R5 foam board on the exterior of the house. I am using 2x4 walls 16" on center and live in northern Illinois, so climate zone #5. Would using foil faced Polyisocyanurate (polyiso) be a better bet. It is more expensive, but i assume cheaper than going to a thicker xps foam and it has a higher r value of R6.5 per inch.

Would the house wrap go on the wall sheathing, or on the foam?

And for the wall insulation i am leaning towards using wet sprayed dense packed 3.5 lbs cellulose insulation. Would blown in fiber or spyder insulation be a better choice here? And also have them install an air seal package, which should hopefully seal everything up pretty good.

And for the vapor barrier only use latex paint on the drywall.

Use the edpm rubber drywall gaskets on the insides of the exterior walls to create an air tight drywall system.

For the attic, blown in cellulose insulation at approx. R-60 at 16" thick. Is there a better type of blown in insulation to use here, like for example the different types for the walls?

And install the foam air baffles on the undersides of the roof decking for venting. How often should these be installed? Every 10' or so?


Any other suggestions to help a newbie out? ;)

For starters, using EPS rather than XPS under the slab would probably be cheaper, and it soaks up less permanent water under moist conditions.

It may be both cheaper and is clearly better to go with an R16-R22 ICF for the foundation walls rather than post-insulating on the exterior with R10 XPS.  If you run your sub-slab insulation right up to the interior EPS of the ICF you then avoid the thermal bridging to the ~48-50F subsoil through the foundation wall & footing.

From a heating-dominated climate point of view, the difference between 1" iso and 1" XPS on the exterior is negligible, since at 25F and lower you need to de-rate iso to about R5.6/inch, whereas XPS gains R with falling temp, and will be about R5.3/inch at your average January temps in N.IL.

But from a total-R point of view,  a 2x4 16"o.c. dense-packed studwall comes in at ~ R10, and you're only adding R5 in foam for a whole-wall R of merely R15, which is about the same as a no-foam 2x6 dense-packed wall.  If you're going with 2x4 16" o.c. use at LEAST R10 of exterior foam is going to be long-term cost effective, and would be plenty for being able to skip interior vapor retarders. Better still would be to go with 2x6 framing  &  2" of low cost Type-I EPS (~R8), or 2" roofing iso (~R13, either foil or fiber-faced) which would leave you at about R22(if EPS)- R27(if iso), and sufficient foam-R to still be able to use latex only in a N.IL climate.  Lap the seams of the foam with those of the sheathing by 12", use duct mastic (or caulk/tape) the seams of the foam, and foam-seal the edges

It's more reliable long term to use the structural sheathing as the primary air-barrier rather than the interior drywall, since it'll never be poked with picture-hangers, and has an order of magnitude fewer electrical penetrations.  A bead of acoustic sealant/caulk under the stud plates, and using construction adhesive or caulk at the edges of the sheathing as it goes up makes a world of difference on air-tightness of the assembly.

Spider or Optima is as air-retardent as dense-packed cellulose only if installed at 1.8lbs or higher density (and not 1.0 lbs, which is another standard density used for those products.)  It has the advantage of drying more quickly when wetted, but the dis advantage of not buffering moisture that enters the wall cavity, so the normal humidity cycling peaks in the structural timber is a bit higher than if cellulose were used.  Installers seem to like it because it's somewhat easier to install then cellulose in wall cavities. Dense-packed Spider/Optima only adds about R1 to the whole-wall R as compared to cellulose, but a bit more center-cavity, which leaves the sheathing a bit colder mid-winter for more condensing hours, more water absorption, so going higher R on the exterior foam becomes a bit more important.

Whether the housewrap goes outside the foam vs. between foam & sheathing depends on how you're setting & flashing the windows.  The flashing should be made continuous with the housewrap.  Check out all of the how-to-links on this blog page for details on how to do exterior insulation "right".  BTW: You may want to consider high-density rigid rock wool on the exterior in lieu of foam, which has the advantage of much better drying characteristics.

Air-sealing the upper floor sealing is more important than the spacing of the roofing ventilation. There is some argument for making the soffit vents signficantly higher in total cross sectional area than the ridge venting, which lowers the stack effect forces trying to suck air up through the attic floor.

2" xps foam on the exterior foundation walls- Do i need to install it all the way up? YES!!! That is where the heat loss is. Cutting it off below grade is only good for the manufacturer (they get paid, you lose). What to cover it with? That is the real issue - best is a good stucco finish over mesh but it is hard to do, which is why it is rarely done well. If it isn't done well enough or doesn't go far enough below grade, you'll get carpenter ants or termites using it as a super highway to your building. I've given up on exterior insulation and now use 2" of closed cell spray foam inside the basement, plus a 2x4 or 6 wall with cellulose or fiberglass.

The issue with polyiso vs XPS or EPS is moisture. The danger is that with insufficient interior insulation and exterior foil faced polyiso, moisture will condense on the sheathing & cause rot & mold. I'd suggest 2" of XPS or EPS. You may be able to find a more detailed discussion of this at Buildingscience .com.

Your builder is recommending a R-15 wall; pretty low. (R-13 less 20% loss for framing, plus R-5) Aim for R-25-30. (2x6 +2" XPS)

Use EDPM gaskets at the sill plates.

Use (dry) installed dense packed cellulose in your walls.

the Tyvek air barrier goes on the outside of the sheathing, under the XPS. Use 1x3 strapping for your rainscreen. Easy & cheap.
Bob

BTW: Setting it up such that the insulating sheathing aligns perfectly with (or slightly proud of) the exterior foundation foam is a good idea too, to not interrupt the drain-planes perfect and maintain the thermal break at the foundation sill. If you have "innie" windows and thus have the housewrap under the insulating sheathing, install Z-flashing at the intersection of the sheathing-insulation and the foundation foam.

A metal or membrane capillary break between the foundation sill and the concrete is also recommended. In termite zones that needs to extend over the foundation foam as well (both inteior & exterior, if ICF.)

I meant to mention:

"My builder uses standard 1" R5 foam board on the exterior of the house."

Did you realize that is code-minimum for a 2x4 house in northern IL? (He can't legally build with less.) While this is a big uptick from the R11-no-foam of 1970s there is both a comfort & economic rationale for R10+ exterior insulation on 2x4 framed homes in a N. IL climate.

Hey dear, considering your goals of cost-effectivenes dust reduction and energy efficiency I recommend a combination of 3" of closed-cell foam in the walls and 2" of closed-cell foam in the trusses followed by 8" of open-cell foam on top. This approach balances insulation efficiency and cost. Consult with a professional for proper installation.

Hello, I understand the code-minimum requirement for 1" R5 foam board on the exterior of your 2x4 framed home in northern Illinois. Adding this exterior insulation is a wise choice and it complements the internal insulation strategy I previously recommended. It will significantly enhance comfort and energy efficiency in your new home..