I'm doing a cost effective (Pretty Good House approach) energy upgrade on a 2 story 1920s house (with a full basement and attic) in the northeast.  The house currently has a minimally insulated UNVENTED attic.  I did the first stage of the energy audit and we've agreed to insulate and air seal the house in a number of stages starting with the walls and then the rim joists.  The house has no A/C and so we're going to add 2 geothermal or A/C forced air systems - an air handler in the basement for the 1st floor and basement, and another air handler in the attic.  My insulation team wants me to spray foam the underside of the roof, of course, which theoretically makes sense to bring the ductwork into the envelope, BUT this seems to make everyone else nervous, including me.  Add to that that the roof is a 60 year old spanish tile roof on top of the old shake shingle roof on top of a cross hatch for support (meaning there is no underlayment and you can see the shingles from inside the attic).  The roof is in decent shape, but it is old.  I priced out replacing the roof with an asphalt roof but besides it changing the look of the house, the cost seems prohibitive because of the large and complex roof, the need for deconstruction of the existing roof, and then cost of rebuilding.

So my choices are:
1-air seal and insulate the ductwork and floor of the attic and just deal with the added upfront cost and energy cost of the ductwork being outside the envelope.  I would also need to add venting to this complex tile roof.
2-spray foam the underside of the complex roof with open cell foam and hope I don't rot or otherwise cause any damage from water or moisture.

Any thoughts, comments, or suggestions?

Posted By geek on 25 May 2012 07:35 PM
So my choices are:
1-air seal and insulate the ductwork and floor of the attic and just deal with the added upfront cost and energy cost of the ductwork being outside the envelope.  I would also need to add venting to this complex tile roof.
2-spray foam the underside of the complex roof with open cell foam and hope I don't rot or otherwise cause any damage from water or moisture.

Any thoughts, comments, or suggestions?

I would go with option #1 -

Most new homes don't even have the ductwork within the thermal envelope. Spray foaming the underside of that 60 year old roof, in its current condition, will cause you problems with trapped moisture. When you have a 60 year old piece of wood that gets wet and starts to rot, you will have to replace that roof which will cost you $$$$$.

You need to vent that roof for longevity. I would focus on venting that roof. Wood needs to breathe, especially a leaky one like the one you have.

Where are you in the NE? How about a zip code?

What is the reason you are planning two complete air handling systems? Load?

Major rule of energy efficiency is DO NOT put HVAC systems outside the thermal envelope!!!!!

Posted By Bob I on 26 May 2012 10:06 AM
Major rule of energy efficiency is DO NOT put HVAC systems outside the thermal envelope!!!!!

The problem is that this home was built in the 1920's. If he spray foams that leaky roof in the attic, he will have major moisture/rot issues to deal with and the roof will have to be replaced. His main concern right now should be to vent that attic roof.

I would add "test" to option #1. Ie, verify that there are no air leaks in the ducts.

To answer some of the questions and comments: -the house is in Westchester, NY (zone 5, I believe), just north of NYC. -2 systems because the house is on the big size (2000 sq ft per floor + 1600 sq ft basement + full attic) and to run ductwork from the basement to the 2nd floor would not be practical for a number of reasons. The load on the house is about 70k-85k BTU depending on where we end the envelope. The 2nd floor accounts for 60% of the total load +/- any compensation for the loss from the ductwork. 2 rooms on the 1st floor account for 25% of the total house load due to a lot of windows. Those 2 rooms (formal living room, dining room), will be their own zone, can be closed off when not in use and have fireplaces the will be converted to energy efficient stoves for auxiliary heat.. BTW, the energy cost of of the ductwork outside the envelope is about 7000 BTU (and sizing up the system 1 ton, figure about $10k) -The insulation installer is also my energy auditor and will test and airseal the ductwork with foam. I also have a NYSERDA (NY State's energy conservation program) consultant assigned to the project and will be documenting the project for a case study, but no-one we talk to will go out on a limb. I understand energy efficiency theory, which is why I bother to ask the question and obviously I would prefer to bring the ductwork into envelope. I guess the crux of the question is how much is open cell foam a problem to an old and, inevitably, leaky roof (not just theoretically - I get enough theory from my insulation installer)? Technically, the water should be able to migrate thru the open cell foam (note I am not considering closed cell) and roofs are repaired from the top.. The foam will prevent airflow and restrict vapor somewhat. Is that still not enough to prevent rotting?. Any unvented roof experts out there?

Posted By geek on 26 May 2012 02:41 PM
  I guess the crux of the question is how much is open cell foam a problem to an old and, inevitably, leaky roof (not just theoretically - I get enough theory from my insulation installer)? Technically, the water should be able to migrate thru the open cell foam (note I am not considering closed cell) and roofs are repaired from the top.. The foam will prevent airflow and restrict vapor somewhat. Is that still not enough to prevent rotting?. Any unvented roof experts out there?

Open foam polyurethane is not waterproof and when it gets wet, it will eventually lose its R-Value and can breakdown.

In essence, you are trying to use a product (open spray foam) that requires the application it is going on (roof) to be water resistant, which it is not.

The reality is that it would be best to redo the roof and waterproof it. Then you can spray open or closed spray foam on the underside of it and have no worries.

My suggestion is to VENT that attic first and then fix the roof membrane.

Any chance of stealing part of a closet to get the air handler into the second floor envelope? If not consider building a well insulated room for the air handler in the attic to allow access for maintenance & serious insulation. Could duct chases be tucked under the second floor ceiling lowering part of it by 6" or 8"? Sounds like fun!

Posted By Lbear on 26 May 2012 01:17 PM

Posted By Bob I on 26 May 2012 10:06 AM
Major rule of energy efficiency is DO NOT put HVAC systems outside the thermal envelope!!!!!

The problem is that this home was built in the 1920's. If he spray foams that leaky roof in the attic, he will have major moisture/rot issues to deal with and the roof will have to be replaced. His main concern right now should be to vent that attic roof.

That is a gross exaggeration of the actual risk, and the risk is highly climate dependent. 

Also bear in mind that 1920s-style wood plank decking is less susceptible to wintertime moisture accumulation & rot than plywood, and plywood is less susceptible than OSB, not that it's a great idea to moisture-cycle it when you don't need to.

On the cool edge of US climate zone 4/warm edge of US climate zone 5 an all-open cell solution has fairly low risk under a tile roof, since with tile the deck is inherently vented toward the exterior (although drying toward the exterior is slowed down by whatever shingles were left in place when they installed the tile.) It would accumulate some moisture during the winters (when it's too cold for mold or rot-biology to be very active) but can dry relatively quickly toward the interior through open cell foam during warmer weather.  Only if the interior were maintained at a high humidity during the winter would rapid rot be a serious issue.

A safest approach would be to do a flash'n'fill, with ~1-2" of CLOSED CELL foam to air seal the roof deck then dense-packing cellulose or new-school fiberglass (Spider or Optima, or L77 @ ~1.8lbs min.) in netting below that to make the fiber layer air-retardent, but vapor permeable.  The foam provides a non-wicking condensing surface for wintertime moisture drives that won't draw moisture toward the roof deck, yet still allows reasonable drying rates toward the interior through ~1-perm of foam. The winter vapor drives in that climate through ~1-perm foam won't allow rot-levels of moisture into the wood via vapor diffusion alone over the modestly cool winters in Westchester,, where outside design temps are in low double-digits, and the mean January temperature is around freezing.   Most days it gets warm enough to provide at least some drying hours for the roof deck to dry toward a 40F dew-point interior even in January!  (70F, 35%RH= ~40F dew point air- whenever the roof deck is 40F or higher in winter it won't see a moisture drive from the interior, and will release some of the moisture it may have accumulated while colder.)

For more on the the flash'n'fill approach to unvented roofs, see:  http://www.buildingscience.com/docu...of-systems

In particular, see tables 3 & 4 on p.11 (p.12 in the pdf pagination), and consider Boston as the nearest climate comparison. With 2" of closed cell you'd be in pretty good shape even with a high-humidity interior and low density fiber for the remainder of the R.

So after getting a number of bids on rebuilding the roof, it seems that is cost prohibitive to put on a new rood.  Flashing the roof sounds like a good idea, but it seems that I might need to rebuild the roof in 10 years or so (time to start saving) but removing flashed closed cell foam would make it even more expensive to deconstruct the roof at that time, so I wanted to get some thoughts on this approach:
1-Airseal the attic with open cell
2-Airseal the ductwork with closed cell (need the rigidity to handle the vibration from what I've read).  I thought if done right, this could bring the ductwork into the envelope even though it's physically in the attic.
3-Build a foam room around the airhandler (maybe air seal edges)
4-Lay R-30 fiberglass insulation on top on the floor (and maybe over the foamed ductwork).  I saw prices of $0.50/sq ft for unfaced insulation at one of the big box home improvement centers.  At those prices, I can even put a 2nd layer if I wanted to go crazy with the insulation
5-Vent the attic.  Roofer suspects that given the age of the home that there should be enough openings in the soffits that it 'naturally' vented on the underside.  A large ridge vent would be too risky on this roof so he proposed 2 fan assisted top vents to get enough airflow.

It's not the most energy efficient, but it's more cost efficient, at least given the 10 yr time frame before I might need to rebuild the roof. Additionally, the cost would be about $2k-$3k vs $12k for closed cell foam and would have the benefit of being reversible (or can be added on to easily), when/if I need to rebuild (and foam) the roof in 10 years.

How does this approach sound?  Not perfect, but "pretty good"?

Posted By Bob I on 26 May 2012 10:06 AM
Major rule of energy efficiency is DO NOT put HVAC systems outside the thermal envelope!!!!!

On a major HVAC forum I am no, the "experts" still advocate using the attic for ductwork

I am assuming the original house has a boiler and rads. Assuming this, why would you change a good system for all the work and intrusion of ductwork? Cooling can be done by ductless splits of that is the main goal. If you were on electric baseboard then my comments go out the window.

If you can test that the duct has no leakage and you can put R40+ of insulation around it, why not put it in the attic?

Power venting the roof with fans depressurizing the attic to increase flow at the soffits is guaranteed to increase air infiltration, pulling conditioned space air up into the attic. Unless you have an extremely tight ceiling/attic floor and a blower door test to prove it, DON'T! It will cool the attic space in summer (mostly by pulling air-conditioned air in) increasing power use overall- it's a bad idea, even if it's a popular bad idea. Even with air tight attic floors the decrease in cooling load achieved by the fan has a lousier COP than handling that load-fraction with geo or mini-splits (or even code-min SEER13 bang-bang compressors.) In winter it would be just sucking heat out of your house for no purpose or benefit.

Foaming in the ducts and air handler works, but why can't you put all (or most of) that stuff in the basement to keep it inside of conditioned space, even that approachrequires a multiple chases to the second floor rather than one? The fewer penetrations between the attic and conditioned space the better!

OK so say you do route the ducts in the attic: With 2" of closed cell you can safely bury the ducts in cellulose without much condensation risk. Taking the attic-R up to R50 in cellulose is long-term cost effective (and will soon be code-min for most of New England once IRC2012 gets enshrined in local codes). Low density fiberglass is translucent to infra-red and isn't very air-retardent and won't be as effective as cellulose at the summer or winter temperature extremes.

If this is a full open attic, no attic rooms or kneewalls, detailing all ceiling penertrations, even vent-stack, flue, and electrical penetration can sometimes make it reasonably air tight. (This is less so in homes with partition walls that don't have top plates with the stud-bays open to the attic.) Air-sealing the attic floor, routing attic ducts between joists (and foam-sealing the ceilng penetrations) would allow you to blow a foot of cellulose over it and get pretty good performance. Blower door pressurization to find and fix all the leaks is recommended prior to blowing more R though.

If it has attic-rooms with kneewalls can still air-seal at the rafters rather than the attic floor using vapor-permeable housewrap (requires a lot of detailing & mastic at the soffits where it intersects joists. Attic spaces behind kneewall are notoriously hard to air-seal, and further opening up soffit vents invariably leads to thermal bypasses under the floorboards in the attic-rooms.

As long as the ducts are sealed and insulated, and the ceiling is pretty-tight and high-R you can do just fine. Air-sealing the ceiling/attic-floor lowers the amount of conditioned space leakage into the attic, lowering wintertime moisture accumulation at the roof deck, which reduces the need to increase the attic venting. Attic venting in a heating dominated climate is a "solution-problem", but it works- it increases stack effect driven air leakage, but dilutes that moist winter air to non-condensing moisture levels. But it comes at an energy use cost. When you can it's always better to air-seal to keep the humid air from moving up & through than to increase the ventilation rate.

Dana -

I was always under the assumption that open spray foam on a roof that is leaking is a bad proposition as it will prevent proper air flow and drying, resulting in rot and mold issues. But I stand corrected if I am wrong in my position.

There's no real evidence in the field ending up with more leak damage in a foamed roof deck (either open or closed cell) compared to vented roof designs when the roof itself is leaking. Finding leaks can be more difficult though. In climates colder & snowier than Westchester County NY there can be roof deck rot from vapor diffusion through open cell foam.

Roof decks vented from above or with inherently vented roofing and open cell foam below fare the best, since it will dry fairly quickly in either direction, even if it DOES load up with wintertime moisture. Tile roofs will sometimes fall into this category- it depends on the vapor permeability of what's between the tile an the decking. In this case it reads like there is no roofing felt (good!), and cedar shakes (even better!) between the roof deck and tiles, which makes it pretty well ventilated to the exterior. I'd be more concerned if it had 30# felt or Ice & Water Shield + 2 layers of asphalt composite shingles as the tile underlayment. Water that gets by the tiles, will mostly be diverted by the shakes, but even if it gets by the shakes will dry pretty quickly since the shakes are semi-ventilated. A chronic leak point will drip through the o.c. foam. If closed cell is used it'll dry primarily to the exterior, but will have a lower wintertime moisture drive from the interior- either way it's not a problem.

Rot an mold are more of an issue when it can't dry quickly in either direction, and has seasonal moisture drives during limited drying periods. A composition shingle roof loaded with snow for weeks/months on end won't be able to dry to the exterior in winter, and would dwell a substantial number of hours below the dew point of the interior air, demanding an interior side vapor retarder or under-roof venting to limit moisture accumulation. But that's neither the climate nor the roof layup we have here.

When re-roofing it would be advantageous to more fully ventilate the structural roof deck from above by building a vented nailer deck supported on 2x furring on which to mount the tiles/shingles/whatever. Tile roofs are fairly long-lived in general, but often suffers from freeze/thaw spalling and can harbor moss & algae growth in that climate.

Dana-
Is my understanding correct that if I do a good job of air sealing (with open cell foam) the attic floor (and testing, of course) and bringing the insulation up to R-40+, that I would not have a need to vent the attic?  Since my attic is currently "unvented" (although I think there enough penetrations into it between the old construction, lack of roof underlayment, spanish tile roof) and you mention that depressurizing the attic with power venting  is a bad idea anyway, could I get away without venting the attic or do I need to use closed cell foam on the attic floor to block the moisture?

Posted By geek on 31 May 2012 04:01 PM
Dana-
Is my understanding correct that if I do a good job of air sealing (with open cell foam) the attic floor (and testing, of course) and bringing the insulation up to R-40+, that I would not have a need to vent the attic?  Since my attic is currently "unvented" (although I think there enough penetrations into it between the old construction, lack of roof underlayment, spanish tile roof) and you mention that depressurizing the attic with power venting  is a bad idea anyway, could I get away without venting the attic or do I need to use closed cell foam on the attic floor to block the moisture?

Yes, if you can air-seal and insulate the attic floor, that will be a cheaper solution.  If the attic doesn't show signs of condensation problems now, it'll be even less likely to after you've air-sealed. 

Even a single square inch of air leakage into the attic from conditioned space moves 10x more moisture than can get through the entire area of a latex-painted ceiling via vapor diffusion, so there's no need to increase either vapor retardency or ventilation if there hasn't been a problem to date. 

It's easier to get an air-seal with closed cell foam than with open cell though, but it doesn't take a over-spray unless the ceiling is in really rough shape.  Most of the time a couple cans of  1-part gun-foam (the type that screws on to a pistol-grip gun, not the cheapie box store throwaway cans) can get you there, with some cut & stapled cardboard over the bigger holes, seams & edges sealed with foam or duct mastic.  Weatherstripping the attic access decently counts too.  A $20 FrothPak 2-part foam would be another option for air sealing foam if you have a LOT of holes to plug.

After air sealing you can blow in as much cellulose as you like (within reason), but be sure that you have 2" of clearance between the top of the insulation and the roof deck at the soffits.  If it's really tight in there, cut and stack rigid foam board (any type) on top of the studwall plate to maximize R at that thermal bridge, foam-sealing the edges to the joists, but leaving the 2" clearance to the roof deck to not impinge on whatever ventilation that it's currently getting.