Hi. I recently purchased a ranch style home built in 1968. It is approximately 1900 sq feet with a full unfinished basement. My first step to improve the energy efficiency of the home will be to air seal the attic and add blown cellulose.

The attic floor joists are actually constructed of 2x4s and there is only about 3-4 inches of loose fill fiberglass. My original plan was to rake the fiberglass to one side of the house so that I could air seal all of the penetrations on the cleared side with spray foam from a can and caulk. I would then clear and seal the other side. I would finally spread the fiberglass evenly and blow cellulose over top.

I am now wondering if I could get a more thorough seal by simple laying tyvek house wrap over the existing fiberglass and sealing the perimeter to the top plate all around the house. I would also seal all seams with tape. There are some electrical wires running through the attic. I was thinking about slitting the wrap to allow the wires to sit on top of the wrap. I would seal the slit with tape and use foam so seal around the penetration point of the wire. I would then blow cellulose on top of the wrap. Thus, from the room below the layers would consist of latex paint, drywall, fiberglass and ceiling joists, house wrap, and finally cellulose.

I have a couple of questions.
1. Which method will be easiest?
2. Which method will result in the tightest seal between conditioned house and unconditioned attic?
3. Which method will result in the best value when comparing initial cost with expected savings?
4. Will method 2 result in any unforeseen problems? Thanks!

I don't know, but my guess is that house wrap can be put anywhere in the layers and it will reduce infiltration.

The Tyvek might work if you can get it completely sealed but I'll bet that is going to be more difficult than it sounds. For example,sealing to the top plate will be difficult as you probably only have a few inches of height there for access. So while pulling back the fiberglass will be a PITA, it should result in a more thorough job.

When you get the attic done, look at your basement, unless you have a slab on grade. The top of the basement walls at the sill & rim joist are typical places for infiltration and the basement walls lose a huge amount of heat. If you have a slab on grade, sealing the wall/slab joint is critical.

Independently of the air-sealing, floor loading on 2x4 rafters with a cellulose overblow could run into some limits. A foot of open-blow cellulose is nice, but adds on the order of 1.5lbs/square foot of static load. To add much R value without ceiling sag you may have to put at least some of the insulation at the roof deck and going with a semi-conditioned unvented attic ( which presents another set of issues, and will be more expensive, but may be worth considering.) How much cellulose were you planning on adding, and what's the spacing/span/species of the 2x4 joists?

Air sealing with housewrap is more easily done at the rafters than a the attic floor, but the de-rating of 4" of low-density fiberglass at any temperature difference would be less with a top-side air-barrier OVER the fiberglass. As little as 3" of cellulose rectifies the convection R-derating issue, but not the air-transported moisture potential. As Bob said, detailing the floor as an air-barrier would end up as a better solution to the purely air-infiltration aspects.

Also what Bob said about sealing & insulating the foundation sill and band joist. Insulating your unfinished basement walls with a combination of semi-permeable rigid foam + unfaced batts in a studwall is probably also cost effective. See: http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

For climate/weather references, where are you (nearest city or zip code)?

Thanks, Dana.

I now have a couple of more questions.

First, some additional information. 

• The home is located in Clemson, SC (29631)
• Heating is supplied by natural gas fired furnace
• AC is supplied via a 3 ton split system.
• Air handler is located in semi-conditioned basement
• All ducts are insulated but very leaky
• The attic is constructed of 2X4 site built trusses
• Roof is a five year old 30 yr architectural shingle roof.
  

I was concerned about the 2X4 joists, but wonder if trusses will support the additional weight of of 12" of cellulose over the current fiberglass.  I am not an engineer and am not sure how to calculate static loads present before and after the addition of cellulose.  If anyone is willing to provide me with the correct equation and parameters I would need to calculate the load I would appreciate it. 

If the weight load will be too great with blown cellulose I will resort to sealing off all of the vents and using spray foam to insulate the roof deck.  I have spoken with the building codes director for our city and he said that he is amenable to sealed attic, provided it is done properly and meets the specifications of the foam product manufacturer.

If I have to go this route, how can I maximize my insulating dollars? An inch or two of closed cell applied to roof deck and gable ends with several more inches of open cell over that? 

I am open to any suggestions one might have for reducing my energy consumption and decreasing my carbon footprint in an economically feasible way.  I wish that paradox didn't exist, but until our politicians actually read science and think critically we will continue rapidly toward our own destruction.

There are some freebie online truss calculating tools, but you basically have to be an engineer to interpret the results.  Assuming anything more than 2x loading if the 2x4 used simply as a JOIST might be pushing your luck for un-rated site-built trusses (unless you model it correctly.)

In Clemson, SC with a composition shingle roof you could use wet-spray cellulose blown-in-blanket (behind mesh) against the roof deck (rather than all foam) without risk of roof-deck rot.  If you want to be extra careful a 1" shot of closed cell foam on the roof deck and rafter elements prior to the cellulose would guarantee that it works with ANY type of roofing material. See:

http://www.buildingscience.com/docu...of-systems

The inch of spray foam could be your air-barrier/air-seal, and would glue the roof deck to the trusses, making it more hurricane-proof too. Giving the rafters the full inch, not just the roof deck is worth maximizes the glue effect with less material than a full 2" shot on the roof-deck only.

Open cell foam on the roof deck would also work, but is usually more expensive per unit R than spray cellulose BIB.

If you want a full foot of cellulose at the roof deck you'd probably have to build out the rafter elements Larsen Truss style, to which you would attach horizontal furring to staple the blowing mesh to.  That would also add some structural rigidity to the assembly, but if it's a lower pitched roof this could be very awkward.  Since the additional truss element is not strictly structural for supporting the roof (it only needs to support the insulation), you'd probably be able to do it every-other truss just to have something to staple the blowing mesh to at a consistent depth. 

At the gable ends, again, an inch of foam as an air seal is sufficient, the rest can be done in fiber at a fraction of the cost.  If you want higher-R than the gable studs would give you, a Mooney Wall approach and cellulose blown in mesh will work. See: http://www.builditsolar.com/Project...eyWall.htm  

This Mooney Wall approach simper to do for the roof deck as well, but would be thinner insulation.  A 1" shot of foam + 6" of cellulose would deliver ~ R27, but combined with a total of 3-4" of cellulose on the floor it adds up to ~R40.  If you used 2x6 24" o.c. on as the Mooney horizontals you'd get ~ R35 out of it at the roof deck, and another R12-14 at the floor, which would be simpler than a Larsen Truss approach.  (This is what to do if that part is contracted out rather than DIY.) The more of the framing elements you can set up as a DIY, the less out-of-pocket you'd spend.  If you use wet-spray cellulose it won't sag over time (it uses water based adhesives), and it can be applied at low density (much cheaper than dense-packing.)

If that's all too complicated, an 8-9" shot of 0.7lb density Demilec open cell foam would give you the R value and about the right vapor-permeance but not the additional structural rigidity.  It'll probably be significantly more expensive though.

With R6 of foam and a foot of cellulose you'd be looking at an ~R50 roof. While not cost effective on a net-present-value against projected utility savings in a 10 year time frame, both the peak & average heating cooling loads will come down substantially. And you'd be at the recommended R levels for a longer term payback for climate zone 3 (which is where you live)  See table 0.2, p 10:  http://www.buildingscience.com/docu...mate-zones 

While you're having the foam guys in, insulating the foundation sill and band joist with an inch of foam seals one of the largest air leaks the basement too. If you can get to it ahead of time, put 0.5-1" of extruded polystyrene (XPS- pink or blue foam-board) on the basement walls first, and use the foam to seal the top edge of the foam.  To meet fire code on the wall foam you need to put 1/2" gypsum between the foam and the rest of the basement, and the best way to do that would be to put up a 16" or  24" o.c. studwall (single-plated top/bottom- it's not structural, but putting XPS under the bottom plate acts as a capillary break to protect the framing from moisture).  In the studwall you can cheaply add R value using UNFACED R11 or R13 batts.  The seams of the XPS should be sealed with housewrap tape or fiber-reinforced duct tape as you go. The foundation needs to be able to dry toward the air-conditioned interior in order to keep the foundation-sill dry enough to avoid rot, which it can do through an inch of unfaced XPS or closed cell spray foam.  If there's a  history of partial flooding of the basement, only extend the batts down to the high-water mark.  The interior gypsum can be painted with latex (which is semi vapor-permeable), but avoid oil paints or vinyl/foil wallpapers, or it can be left unpainted.  The XPS on the wall reduces the rate of moisture reaching the studwall from capillary draw and vapor diffusion thereby protecting it from ground moisture, as long as you don't block the drying path at the gypsum layer.  Alternatively, you could use 2.5-3" of EPS (bead-board) instead of XPS as long as it has no facers on it, and use furring through-screwed to the foundation wall on which to attach the gypsum.  An all-foam solution has the advantage of being tolerant of bulk-water incursions.

See Case 4, p49:  http://www.buildingscience.com/docu...y-analysis 

Note, Clemson is much warmer than the Minneapolis climate where the hygric analysis was done in that document, which is why you can get away with much less XPS without wintertime accumulation of moisture.  Unfaced EPS is much more vapor permeable than XPS, but anything between 1.5" and 4" would work from a moisture point of view (but you're looking for R10+, which would mean 2.5" minimum for EPS.)   Rigid foam with poly or foil facers MUST be avoided, or you'll risk rot the foundation sill.

By insulating the basement you're earth-coupling the house to the thermal mass of Clemson's nearly ideal ~65F deep subsoil temps, which for low and moderately well insulated house has benefit in both the heating & cooling seasons.

By air sealing both the basement and the attic you will reduce to near-zero the natural convection "stack effect" infiltration, and you will then have those leaky ducts within condioned space where the leakage does less harm.  It's still worth trying to rectify the leakage wherever you can though.  Duct-mastic is best, but if you must use tape, 2" aluminum FSK tape designed for ductwork is the best option.  With leaking unbalanced ducts the air-handler drives air infiltration at higher rates than natural forces would otherwise.  Having a professional air-sealing contractor fix all of the low-hanging fruit for air leaks in the whole house after you've dealt with the attic & basement further reduces the efficiency losses of leaking ductwork too (and is by far the most cost-effective way to reduce heating & cooling loads on most homes.)

With a high-R roof you may end up lowering the sensible cooling load to the point that you may need to run dehumidifier to keep the interior humidity at healthy levels in summer.  In a moderately well sealed house setting up a room-dehumidifier in the basement set to keep the basement at 60% RH or less is usually enough to keep the (typically warmer) first floor at a very healthy 50% max.  If it can be set up to drip-drain into a sump or basement-sink it won't require manual dumping of the water, just seasonal cleaning of it's filters, etc.

Bottom line, foam is an expensive way to go high-R, but is useful for adjusting the moisture profiles of the assembly.  In your mixed climate a little foam goes a long way, so when looking at maximizing insulation dollars, use the foam primarily for air-sealing and vapor control, let cheap fiber do the heavy lifting on R value.  Sprayed/blown fiber nearly always performs to spec, batts only do if installed perfectly, and stay at moderate temperature ranges, so blown fiber is worth paying a moderate premium for, but often is pretty close to batts cost-wise, and foam NEVER is.  Cellulose is usually the $/R value-leader for blown/sprayed fiber.

in 1968,  a lot of trusses were site built ... the designs were fairly standard, and  they normally used  plywood gussets instead of metal  joining plates....
so.. your trusses may , or may not,   be entirely adequate to supporting the  cellulose.. they may even be superior to a modern factory truss  ... MAY

what is the span of the truss ?   do you have a 24' house  ... a 28' house  ?   are the trusses 24 oc   or   16 oc
are they a standard design  ?

we use  24  to 30 " of cells in our attics, but we also  furr our ceilings and we use  insulmesh...... so the cellulose weight is actually carried by the  furring and insulmesh instead of the gypsum board

i like your original idea of raking the fg away and air sealing
 air sealing in conjunction with  blower door testing  will plug all of the leaks , making the  tyvek barrier redundant
when you  have finished your airsealing ...

i would think  a foot of cells would not stress anything

Air leakage, or infiltration, is a major problem in both new and older homes. Although windows, doors, and outside walls contribute to air leakage, the biggest holes are usually hidden from view and connect the house to the attic, crawlspace, or basement.

The key is to identify these areas during the design process, assign responsibility for sealing holes, and check to ensure that the air sealing was done effectively.Usually, seal all the big holes first, then the large cracks and penetrations, and finally the smaller cracks and seams.