Hi all, It's time to make the final decision on our framing / insulation. We will use 2" of rigid foam on the exterior, then 2x6 advanced framing technique walls with dense pack cellulose. The 2600 dollar question is: We planned on horizontally strapping the 2x6's to get an extra 2" (nominally) of cellulose in the wall. The builder will do whatever we want, but it is budget crunch time and I'm curios on the forum's opinion whether 2600 is worth the extra cost in BTU savings. I realize that it is a small percentage of the total house cost, but when you need that last couple thousand to make it work... We plan to live here a minimum of 12 years, and the house is already planned to be sealed tightly. We've cut about everything else that can be cut. Thoughts?

Without knowing any of the important variables, I'll guess that it won't pay for itself. With any house design, you should have a model of the projected energy usage and be able to answer questions like this. Various software will work.

even if the extra insulation will not pay for itself in energy savings alone, 12 years from now when the gallon will be 10$ (wishfull thinking from my part) I'm sure you get your money back and then some by marketing your house as more efficient than your neighbours and you'll also have the bills to prove it. I think it is worth it for what would be around 10$ a month.

jon is right - you should try to project your energy usage - maybe the added insulation can alow you to use a smaller AC unit or a smaller furnace....and you might get some additional savings from there.

good luck
adi

The extra 1.5" will add roughly R5 to the roughly R25 whole-wall stackup you have going. Since you already have ~R10 of thermal break on the studs with the exterior foam and a low framing-fraction, the marginal effect of the additional thermal breaking is small. For less labor and about the same board-feet of lumber, going with 2x8 framing rather than 2x6 would be roughly comparable from a thermal point of view.

Whether an R30 wall even remotely makes sense from a financial point of view depends on your climate and anticipated heating/cooling fuel costs. Depending on the window & door package or the foundation insulation package the $2600 might be better spent on upgrades other than on walls. (At R20+ the windows & doors begin to dominate the heat loss in most homes.)

Where are you located? (nearest city, or zip code, if USA.)

Thanks for the replies. The home is near Saranac Lake, NY 12983. It is a heating only climate and one of my subs is sending a Man J calc. The heating subs up here don't seem to care that the boiler will be smaller, and only knock off an insignificant amount to account for this. So much for offsetting the cost of extra insulation. Heating does get pricey up here since the winters are very cold.

There's a fairly strong long-term rationale for R30 walls in Saranac Lake (US climate zone 6), but it won't necessarily be fully paid-off in 12 years if you're heating with gas. If heating with resistance electric, oil, or propane it's probably worth it (even R 40 might be for the very long-term.)

When you get the heat loads down so far the cost of getting them even lower with the insulation package goes up, and hardly affects the size & cost of the boiler, but it still affects the operating cost. Manual-J might not be the best tool for boiler sizing on an R25+ walled house, but if that's the tool of choice, use the 95.5th percentile ASHRAE design temp, which is probably something like -15F in Saranac Lake (I don't have a reference handy.) Sure, it gets down to the -30s there sometimes, but it doesn't dwell there long enough to matter, and even on those days it usually makes it up to the balmy positive-single-digits by noon.

While it has some temperature extremes on the cool end, it's still only a ~8500 HDD climate, not 12,000 HDD. If you've designed the rest of the building envelope commensurate with R25+ walls it takes a number of hours of BTU shortfall below design temp before you're actually feeling cold, and with typical inherent Manual-J oversizing you'd probably still be good at -40F designing to -15F with 0% oversizing based on a Manual-J.

But still, if you don't have insulated doors or U0.30 (or lower) windows designed in and at least R12 foundation walls, the $2600 might still buy back more annual BTUs in upgrades there than bumping the walls from R25 to R30. And every square foot of glass that you can replace with R25+ wall is huge.

We've got r-20 on the sub-grade exterior of the basement walls and r-30 under the slab per building science's report on high r value basements. The R-30 might be a bit much, but there was some gain shown on the curve and the cost wasn't that much more. We've minimized the windows and are planning on inline's double pane casements or picture for most of them.

R30 under the slab sounds like radiant heat? Or reclaimed EPS rather than virgin stock XPS? (In either case R30 probably a bit overkill, but certainly not a problem, and not worth adding $2600 more!)

Some builders/advisors (notably some of the BSC guys) use a 10/20/40/60 rule of thumb for slab/foundation/wall/roof R values in similar climates, but there's a lot of squish to that. (See: http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/how-much-insulation-needed ) Since the windows contribute a large fraction of heat transfer on the above-grade walls, it takes a lot of R to make the average heat loss out of above grade walls similar per square foot to R20 foundation walls. But if the windows are high performance and reduced in size R30-ish could get you similar overall performance to an R40 wall with a bigger glazing fraction. (On a 3-family retrofit I'm advising on locally they went with R20 iso on the exterior of dense-packed 2x4 to get to ~R30, but the glazing fraction was about half what most people put in single-family housing these days. There's 6" of iso going on the roof too, with dense-pack cellulose going into the full-dimension 2x6 cavities for a total of ~ R55ish. They're cheaping out a bit on foundation insulation though, with only ~ R13 in spray foam on the walls & R8 EPS under the slab, since it's shared but unconditioned space- they just need to be sure it never freezes. As a result, the first floor unit shows the highest design-condition heat load in the energy modeling, despite having fully conditioned space above it.)

Inline's double-panes all run a pretty-good ~U0.28-0.31, and I suspect would take more than $2600 to take them all up a notch in performance.

How about doors?

Not sure on doors yet. Any recs? I got r-30 under slab on my own off of the chart in high r value basements. Between r-20 and 30, the curve flattens a good deal, but it seemed like it was enough of an improvement to maybe justify it. What about this? Take the underslab back down to r-20 (walls will stay at r-20), delete the 2x2 furring on the 2x6's and pump up either the windows or the doors.

I've been in the insulation business in ustate NY for a long time and would like to caution you on one thing. Make sure you get a all borate insulation. There has been a lot of consolidation in the insulation business. One firm has purchased a lot of good small cellulose companies and the results are less than stellar. I have switched suppliers because of the amount of garbage I've been finding in the bags. I also suspect the the all borate product being produced may not be all borate. Also I know that the local plant now uses paste board boxes in it's mix. If you're going to depend on dense packed cellulose make sure the co. you choose can actually pack cellulose at the proper density. I actually bring my customers into my warehouse and blind blow some stud cavities to show them how the process works. As far as windows are concerned I've, I'm not sure I understand the whole fiberglass deal. They have been thr top biiling as far as windows are concerned. I've been sell vinyl windows for over thirty years and have yet to replace one because the vinyl failed. Last month as part of my icf, cellulose energy package , I sold a customer 36 vinyl casement windows that had a .01cfm rating with a .17 u-vlaue for about $266 each. The quote he had for fiberglass with a lower u-value and a higher infiltration rating was really astronomical.

I sold a customer 36 vinyl casement windows that had a .01cfm rating with a .17 u-vlaue for about $266 each.

Casements? What make? Those are awfully attractive specs.

u value of 0.17 is an almost "passive house" quality window. I can't see how that can be achieved without triple glazing and an insulated frame (I assume you're talking about whole window U value). If that is the case can you please tell us who is making that window at that price?

thank you
adi

Yes it is triple glazed and the sash and frame are filled with foam which increases the whole window u value. It's made by a small window manufacturer in Pa.  They only cover NY,NJ PA, CONN, MASS and some other states in this region.  I've been using their windows for 15 years and am quite happy with them.

My biggest issue against vinyl is just in the visuals.  I have yet to see one (admittedly I have not seen many) that did not have big massive joint welds and that squeaked when you touched them.  They just seem really cheap to me.  Fiberglass just seems like its a stronger (it is) window.  The joints are smooth and depending on the brand, practically invisible, and the color range is far greater then vinyl.  Not knocking high quality vinyl, as I have not really seen many, especially in our area (dominated by alum clad).

The vinyl profiles have changed a lot in the thrirty years I've been carrying them. I looked at fiberglass about twenty years ago and wasn't that impressed, then Owens Corning had problems with their windows. Everyone tends to get wound up in the u-value of the window. I'm concerned about how the window performs when it is shut as far as air infiltration. I sell very few new construction windows for new houses, because the nailing flange makes the window stick out past the insulated surface of the house. If you recess the window into the shell of the house it performs better.

Are we keeping the manufacturer of the great-sounding vinyl casements a secret?

I looked at fiberglass about twenty years ago and wasn't that impressed, then Owens Corning had problems

It's not that fiberglass windows are bad; it's just that Owens-Corning had bad windows when they first tried.

I'm not saying that they are bad but there are less costly options than paying $450 for a window when there other options than may actually perform better. One problem that is inherant with firberglass window is that same weight can become a problem. Take a 36" by 54" casement and crank it out. There is a tremendous amount of weight on the crank mechanism. More weight more the chance of failure. There was a women on the window section complaining about her crank failures with fiberglass windows last year.

Hey Smartwall,
What is the name of the manufacturer? I would like to look into them and we are starting construction in a couple of weeks so we need to order windows soon. PM if you'd prefer.

More weight more the chance of failure

OK, but isn't fiberglass more rigid than vinyl?

Crank outs are miserable, independent of vinyl/fiberglass/wood, issues. The window width and the friction/tightness of the weatherstripping is a far greater stess factor on crank-out wear than the mass of the the window. The higher coefficient of expansion with temp of vinyl makes the cranking stress (and air leakage) differ with temperature, but that's not a deal-breaker for vinyl windows in-general. I suspect more wear is put on cranking mechanisms by people trying to crank them too tight rather than relying on the lock to pull it in the last couple of mm, or by cranking to open before releasing the locks than any other issue though.

Push-outs are the only way to go, IMHO.