Hello everyone!

OK, I need some help in trying to decide what to do next to further insulate and improve the envelope of my house.

Our house is 3500sq/ft total. Originally built in 1949 was 1600sq/ft. 1900sq/ft addition built on in 2010. 1.5 story
Old part of the house has no insulation in the wall cavity. New part is 2x4 walls with fiberglass bats (R-12 i think??)
All of the house has 1/2in "blue board" insulation under the vinyl siding
Attic and crawl space has been air sealed
New part has R-18 in between floor joists. Old part has none. Crawlspace/basement in not insulated but stays approx 65F year round.
R48-60 in all attic spaces
All double pane windows
House is in South Carolina

Results from blower door test.
MVG (minimum ventilation guideline) 2683
3680cfm @ -50pa
ACH50 = 7.4
ACHn   = 0.37 (calculated as close as possible without actually doing a tracer gas test)

Right now I think the attic is more then sufficiently sealed/insulated for our area. Overall, air sealing is about as good as it can get without adding a ERV. The floor joists are only insulated in half the house, but the crawlspace/basement stays at a relatively consistent temp so I don't consider this the top priority

My thought is that I need to handle the exterior walls next. I'd rather not have to rip out Sheetrock as this gets messy/costly very quickly. I have considered blowing in loose cellulose in the old part of the house since the walls are empty. However I'm thinking I may be better off adding more insulating sheeting under the siding. A) I can easily do this myself. B) I have a couple areas where I need to pull the siding off to fix other problems with it anyway. C) This will add insulation and prevent thermal bridging.

I was thinking of adding 2-3 inches of blue board to the exterior. This will give me an additional R12 (using 1/2" sheets with R3). So far I have not found a good source of 2"-3" foam board that gives the same total R value per inch.

Opinions? Suggestions? Better ideas?

Thanks!
David

You might want to provide your zip code so suggestions can be more precise regarding insulation requirements.

29706

ACHn   = 0.37 (calculated as close as possible without actually doing a tracer gas test)

You don't need tracer gas to get a much more accurate value than the wild guess of ACHn = ACH50/20 or even a LBL or N value. But you do need to measure pressures on a room by room basis. If your hvac system is pressuring/depressurizing some rooms (common), then ACHn will increase by several 100%. With every room at approx the same pressure as the outside, it will decrease.

To answer your question - I would use software like BEopt to model the options.

Was there an infrared scan done during your test? Your Cfm50 is still quite high, dense packing the walls would most likely significantly help lower your leakage, an IR scan would have helped to verify where the leakage is coming from.

Posted By jonr on 20 Oct 2012 03:00 PM

ACHn   = 0.37 (calculated as close as possible without actually doing a tracer gas test)

the wild guess of ACHn = ACH50/20

I can't help but notice that people around here love to attack a  ACHn calculation. I never said anything about guessing a ACHn or using the ACH50/20 method!

Anyway, back to the actual question.
Before having the air sealing done, the blower door pulled 7825cfm at only 33pa. (Couldn't get it up to 50pa) So the house is much tighter then it was. Seeing as though I', getting so close to the MVG, I was planning on doing more insulating before tackling installing a ERG and sealing the house tighter. Plus, I figured adding the foam board on the outside of the house and sealing the seams would also accomplish some air sealing at the same time.

I wasn't aware of the BEopt program. I have tried to download it about 5 times but it keep dropping. May be my connection here. I will try again later.

It may be tighter than it was but its still a leaker. Double check your MVG, why would you need that high of a flow? Your "MVG" is a mid to high blower door number at your square footage on new construction prior to remediation work. Ashrae 62.2 with 4 bedrooms puts you at 72.5 cfm ventilation flow.
Who came up with the term MVG, is this a new BPI thing?

7.4ACH/50 isn't exactly a tight house- it's pretty loose, in fact.  It's unlikely that you'd be able to get it under 2000cfm/50 without a bit of work, but it's work worth doing.  If you somehow managed to get it down to 1500cfm/50 or ~3ACH/50 you should celebrate, since it would then be at the IRC2012 code max leakage for new construction!

Adding 2-3" blue-board to the exterior is more expensive than blowing insulation to the empty wall cavities, but it's also a lot higher R.  The "whole wall" R of cellulose or fiberglass filled 2x4 cavities is about R10, including the sheathing & siding and interior gypsum.  With empty cavities it's about R1.5-R2, but empty cavities become flue-stacks driving air infiltration.  With the existing half-inch of blue board under the siding, with the thermal bridging of 10,001 nails penetrating the foam it only adds another R1.5-2, so call it R3-3.5, empty cavity, R13-14 if filled. (Steel is about 1000x more thermally conductive than blue-board, so it doesn't take a huge cross sectional area to have an impact on performance.)

At your leakage rates tightening up the house more important than going to R10+ with more exterior foam.  It's worth filling the cavities with cellulose, just for the air retardency, and WILL see a reduction in cfm/50 numbers when you do.  If you only add more exterior foam the empty cavities are still going to be thermal bypasses for air infiltration unless you meticulously chase down and seal every leak, and many of those leaks will have no reasonable fixes without a more extensive dis-assembly of the house.  But filling them with mid-density (2-hole method) cellulose yields a ~90% reduction in the volume of air moving through the wall cavities.  Dense packing them would reduce the air leakage even further, but may not be cost effective.

Contrary to your intuition, the fact that the crawl space stays 65F in winter is a BAD thing-  means it's leaking a HUGE amount of heat to the outdoors in winter through the uninsulated foundation, (which is probably only about R1 if poured concrete, or R2 if CMU.)  Putting up 2" of blue board on the interior (or 1.5" of fire-rated polyiso, which needs no interior ignition barrier) would cut that heat loss by 90%, and it will be measurable in the heating bill.  It might be the second largest heat leak type in your house, behind the empty wall cavities. Include either spray-foaming or cut'n'cobbled rigid foam (edges sealed with gun-foam) on the foundation sill & band joist as part of that project.

Whenever you replace the siding you have an opportunity to blow cellulose from the outside, which will also tighten up batt-insulated walls by filling in all the voids and thermal bypasses.  You can definitely add more rigid  foam than the existing half inch, but only up to a point. At some thickness the detailing requirements around windows & doors gets cumbersome unless you're at the point where you're ready to replace and re-flash those too.  In a South Carolina climate an inch or two of foil faced polyiso will do better than identical thickness blue-board, but if the wall assembly has any foil or poly vapor barriers stick with blue board, and keep it to  2"or less or it becomes too vapor retardent for reasonable drying rates.

jonr's comments about air-handler driven infiltration are dead-on too. If the ducts aren't mastic-sealed at every seam & joint, and the air-handler panels taped with FSK tape, with every register-boot caulked to the gypsum, air handler driven leakage in a house that leaky could easily account for 15-25% of the total heating & cooling bills.  (And if the duct design doesn't have dedicated returns for doored-off rooms its worth making some with door/wall grilles or jump ducts, etc.)  The tighter the house, the less the duct leakage matters, but that's not to say it doesn't matter at all. (If the ducts are in the attic minimizing duct leakage is still of critical importance, even with a fairly tight house.)

You can one hole dense pack from the outside as your house is now, would be quite beneficial, you don't need to pull your siding off.

Thank you Dana for the detailed info!

I have contacted the company that did the initial air sealing work for me to find out how they determined the MVG. Haven't heard anything back yet. I'll let you know.

I didn't realize that the empty cavities could present such an issue. Does it make a difference if they are sealed at the top plate & rim joist? Wouldn't that stop the stack effect inside the wall?

Now I have not had a tracer test done or pressures tested room-by-room to determine an exact ACHn, but I did use the LBL factor equation so it is better then the ACH50/20 method. However, our house is a completely open floor plan. The only doors are those for the bedrooms & bathrooms all of which have 2 inch gaps at the bottom to allow plenty of return air when the doors are closed. Now I'm not saying that the eliminates any potential for pressure differential, but I believe it does help. If I'm wrong, please explain.

I understand that 7.4 air changes/hour is a big strain on the HVAC. Now, that is at -50pa. With that equating to APPROXIMATELY 0.37ACHn. Decreasing the air infiltration down to 3ACH50 would be APPROXIMATELY 0.15ANCn. Does a 0.13 air changes/hour difference make this the number one priority over insulating? Seems rather minute to me.

You mentioned the vapor barries on the exterior making a difference in what foam to use. Inside to out, I have:
1/2" Sheetrock
2x4 walls (New part (1900sq/ft of the house) has insulated walls, Old part (1600sq/ft) has hollow cavities)
New: 1/2 plywood sheeting. Old: 1x4 slats.
New: Lowes Vapor barrier. Old: Tar paper & old Asbestos siding
1/2" blue foam board
Vinyl siding

Do you still think 2" blue board would be the best way to go if I did decided to add more foam board?

The main AC unit has its duct work in the basement/crawlspace with is unconditioned. Upstairs unit has ducts in the attic. Both of which I have sealed with mastic. The attic ducts are also completely covered by the attic insulation.

Thank you all again!

There you go, you have empty 2x4 walls with slats. Think of the gap between each slat, multiple that narrow gap by length and multiply by the number of slats and equate to the size of opening that equates to. It will be significant, plugging that short circuit will go a long way towards reducing your overall cfm/50 which is above typical sq. footage and way above a treated house.

Does the tar paper, old siding, foam board, new siding not cover all of that? Now I know that none of those are air sealants like spray foam or even cellulose, but I would think that all those layers would slow the infiltration dramatically and then adding 2" foam board with taped seams would decrease the air leak even more AND provide more insulation.

I know it probably sounds like I'm just trying to argue my idea, but I really do want to understand this better so that you and please keep the knowledge coming!

Are you sure the Lowes product between the siding and sheathing is a vapor barrier, and not a vapor-permeable housewrap? (I suspect the latter.)

Does the stud cavity insulation (where it exists) have a facer on it?  If yes, which side, and which is it, kraft paper or foil?

It's nearly impossible to retro-fit air seal the stud cavites. On new construction while it's open the seams can be sealed with caulk, construction adhesive or foam, but there's nearly no way to reduce the air leakage short of filling it with dense fiber insulation (the denser the better, but 3lbs/cubic foot is "good 'nuff" if cellulose, 1.8lbs if fiberglass), or filling it with an expanding foam pour (which is more expensive, and has higher risk of blow-out, etc.)

Unless you have polysheeting or foil faced batts on the interior of the wall cavities, and no foil or vinyl wallpaper, you can put any type of foam on the exterior.  For both a higher R value and less damaging blowing agents I'd opt for polyisocyanurate (aka "iso", or sometimes "PIR").  With foil faced iso you can get a very good seal with 2" FSK tape, and the radiant-barrier effect of the exterior facer under the vinyl siding gives it a small performance boost beyond it's raw labled ASTM C518 R-value.  At 2" iso runs R12 to blue-board's R10, but the facer gives it another "effective-R" of ~R1-2 at the temperature extremes, enough to make a difference on the sun-baked siding during the cooling season.

Blue-board (==extruded polystyrene aka "XPS") is blown with HFC134a, which has about 1500x the greenhouse gas potential of CO2.  Iso is blown with pentane, with only ~ 7x the greenhouse potential of CO2.  From a lifecycle greenhouse gas point of view the carbon footprint reduction of the energy savings of a couple inches of iso pay off in the first year, but at 2" XPS could take several decades in your climate. More than 2" would likely have a higher lifecycle greenhouse effect than the energy it was offsetting( assuming you have at least some insulation in the wall cavities.)

With the ducts going into the attic air sealing becomes more critical, due to the air-handler induced losses. 

But ducts or no, insulating the walls of the sealed crawlspace is in order- it's just a big hole in the thermal envelope of the house.  Even if it's not creating a comfort issue, it's still an energy use issue.  (Under IRC 2012 the min-R for crawlspace walls in your climate would be R5 if rigid foam, R13 if 2x4 studwall w/batts.)  That would be an inch of XPS min, but you may find that 1.5" EPS (bead board, like coffee cups & cheap coolers) is cheaper, and would deliver R6. And like iso EPS is blown with pentane, with a very low greenhouse gas potential.  It makes long-term sense to even go higher-R, and double-layering 1" EPS, staggering the seams would not be out of the question.  It can be held in place with furring through-screwed with TapCons to the foundation 24" o.c. for minimal thermal bridging.

I think your right, it's house wrap, not vapor barier. (what function does house wrap provide anyway?
the insulated walls are bfiberglass with the brown paper facing on the inside
I didn't reaslise the XPS was so bad from a "green" stand point. good to know
so you think insulating/sealing the crawlspace walls would be the best thing to do next?

The housewrap provides the same function as the tar paper- any bulk water that gets blown by the siding in a storm is stopped and drains. It's secondary function is (or can be) as an air barrier, but it's not a very good primary air barrier.

Foam in general isn't all that green, but the differences in blowing agents makes a few orders-of-magnitude difference. The HFC134a used for all XPS products in the US is the worst at 1500x CO2, followed by the HFC245fa used for most closed cell polyurethane foams (ccSPF) at about 1000 x CO2. That said, but XPS and ccSPF are both very useful products when applied judiciously. Water-blown ccSPF is available in many locations, and is a preferred product if you can get it.

With kraft-facers on the batts you'd be better off going with fiber-faced iso (roofing polyiscyanurate with fiberglass or paper facers) and avoid any foil facers. You can use up to 2" of XPS (R10) or up to 4" of EPS (R16) before it's an issue. With iso the facers determine the overall vapor retardency, and with just one layer you can go a thick as you like, and it'll run ~R6/inch.

In order of importance, filling the empty wall cavities probably precedes the crawlspace, but the crawlspace would be a close second. In the crawlspace EPS would be better than iso if there is any question of bulk water or floods. Even in a dry crawlspace the exposed edge iso should not be in contact with soil at all, but it's fine if there's a 10-mil poly or EPDM vapor barrier between the iso-edge and the soil. Seal the edges & seams with 1-part foam as you go.

Thanks to everyone for the great info!
So I will plan on filling the empty walls as the first priority. The cellulose I have seen online for the big stores seem to be blown in for attic use. Is this the same stuff you would use in walls? Does the blower they rent for attic also work for blowing it in the walls? Can you also blow cellulose into walls that already have fiberglass bats to fill in all of the voids?

Thanks!

Posted By dcb1101 on 06 Nov 2012 03:33 PM
Thanks to everyone for the great info!
So I will plan on filling the empty walls as the first priority. The cellulose I have seen online for the big stores seem to be blown in for attic use. Is this the same stuff you would use in walls? Does the blower they rent for attic also work for blowing it in the walls? Can you also blow cellulose into walls that already have fiberglass bats to fill in all of the voids?

Thanks!

The stuff sold at box stores is the same stuff, but it usually contains sulfate fire retardents.   Blowing cellulose into walls with a box-store blower can be done, but you may have to fabricate some of the reducers & nozzles to get there, since they usually don't rent it with anything but the 2.5" hose (fine, for open blow.) This is one where it's better to hire a pro- and specify "borate only, sulfate free" material- they'll know what you're talking about.  Sulfates become corrosive to metals if they ever get wet, and since the cost difference between goods that only contain borate fire retardents is "in the noise" statistically-speaking, it's worth going with borate-only stuff.

It's possible to blow over R11 batts, even dense-packing over them is possible with enough dedication, but it's probably easiest/best to just drill every 4' and blow until the compressor stalls, move to the next and accept 2.5-3lbs density instead of 3-3.5lbs, since it can be pretty difficult to snake a dense-packing tube into a fully filled cavity.  (Dense-packing over half-depth R7 "econobatts" is a lot easier than with R11s or R13s.)

If you're a dedicated DIYer anything is possible though- there's a dense-pack cellulose guru named Rick Karg who has published a lot of tips & information online, if you want to google him.  A rental Force-1 blower is a bit slower than the multi-stage blowers, but you're also much less likely to blow the sheet rock off the walls with one of those no matter how you set them up, and you'd still be able to dense-pack to 3lbs+ on the empty cavities.  The box store might have to special order "stabilized formula" versions from their cellulose supplier, which is cellulose designed for damp-sprayed applications, and thus contains no sulfates.  But if they're nicking you $100/day for the blower it might be cheaper to let the pros do it. 

FWIW:  I ended up buying an old single-stage insulation blower several years back to handle a pain-in-butt project on my own 1920s vintage home. There was pre-existing half inch kraft/horsehair/kraft laminate "insulation" woven between the studs, crossing the studbays diagonally.  To get a decent fill required shredding the old stuff and pulling most of it out, some of which was in pretty good shape and tough to shred with hooks through a 2.5" dense-packing hold.  Knowing full well why the pros would only bid it on an hourly basis, I was able to do the project in pieces over a handful of months since there was no additional cost to keeping a blower around. With a rental blower it would have been pretty pricey, since most of the time involved was spent probing the cavities and shredding/pulling the horsehair out with long steel hooks (that I also had to fabricate).  Density-wise it came out in the very low 3s which is fine- dense enough to not have settling issues for decades (but not centuries, in my climate.)