1.  I received a quote for some spray on foam insulation.  The contractor stated that I could leave the existing insulation in the attic and it would not cause a problem.  Keep in mind that he is talking about a complete seal of the attic spaces, to inlcude the ridge vent and the vents in the soffits.  I questioned him about mositure/condensation buildup, because it is my understanding the house needs to breath.  Am I totally wrong about this, or is the contractor just trying to sell me a product and move on.  Any information concerning this is greatly appreciated....  Thank you for taking the time to read and respond.....

Woolfski;

The insulation contractor is creating an unvented attic / conditioned space.

If you are in a hot -humid climate,  it is the best way to go, if you are in a different climate you may want to consider a different solution

I live in Pensacola, Fl.  The area is humid in the summer months May through November, but the humidity drops off ater that and we actually have some fairly cold weather in the Winter months, November through February and the Spring weather is mild February through May...   Thanks for the response

Pensacola is considered a  hot & humid climate, I would advise the unvented sytem which will put your HVAC ducts in conditioned space, greatly reducing your utility bill.

there is nothing to condense it is all now conditioned air the same as your living area
I build SIPs homes with the same concept...........never a condensation problem

Woolfski,

Did the contractor plan to use open cell or closed cell polyurethane spray?

All quotes are for open cell......

Condensation occurs only when humid warmer air comes into contact with cooler surfaces. How is the warm humid air going to get into your sealed attic space?

In winter with an vented attic warm air from the conditioned space below infiltrates through all the the attic floor leaks (large & small) upward into the attic due to negative pressures created by the outflow from the vents- if the vent flow isn't sufficient it CREATES a condensation problem in the attic, but with high enough flow it exhausts the infiltrating humidity. (Vents are something of a solution-problem- they can solve or create the same problem) But with suff In a sealed attic this just doesn't happen. Then it only becomes an issue of how far into the inuslation boundary the dewpoint of the interior air occurs- if it's in the foam, great- not a problem since the air can't circulate into the foam, depositing ever increasing amounts of moisture the dewpoint boundary.

If it occurs in the attic between the attic floor insulation and the rafter insulation, yeah, it may become an issue, but it's rare. But still, without a constant flow of moist air into the space the problem is small, and can be economically handled with a small dehumidifier. If you're concerned, put a humidity monitor there after the installation and check on every week or so. If the relative humidity ever exceeds 65% (possible, but not likely in an air-conditioned home in Florida), a dehumidifier with the humidistat set to 60% will fix the problem with a very low duty cycle- it won't cost anything like the increased heating/cooling costs that you would incur by removing the old insulation, lowering the overall R-value by a large fraction. (Mold only gets going big-time at 70% RH and above, so 60% gives you a healthy margin.)

In most of Florida the outdoor temps never get cold enough long enough to create a wintertime condensation issue- the source of humidity is the summertime OUTDOOR air, not the indoor air, and by sealing the attic putting inside of conditioned space (that is mechanically dehumidified by the air conditioning most of the year) you've fixed the problem. Vented attics in FL are more about cooling the attic (which it doesn't really do very well) than it ever was about moisture removal (which makes at least some old-skool sense in heating dominated climates.)

There's a ton of science to back this up- houses don't "need to breathe", if you design the insulation & vapor retarders correctly. Most houses "breathe" way too much for energy efficiency, creating as many material issues and indoor-air quality issues as some of the designed-in breathability was intended to mitigate.

Download the document from the link found on this page and read it carefully:

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/

as well as this one:

http://www.buildingscience.com/documents/reports/rr-0302-roof-and-attic-ventilation-issues-in-hot-humid-climates

Really? Prove it!

(Or at least tell us a compelling story of how that happens in Florida. :-) )

The attic room temperature will be warmer during the cooling season than it would be with rafter-insulation only, since it's between R-value layers.  That's not a disaster unless you're storing something extremely temperature sensitive in the attic (never a good idea.)

Moisture trap? Not likely- it puts the attic within the pressure boundary of the house, which is dehumidified by the AC.  The minimal air that infiltrates up from the house to the warmer attic loses relative humidity as the temperature rises.

Foam sealing and insulating between the rafters of an attic in FL will cause the average humidity in the attic to fall, not rise (whether the old insulation is removed or not.)  Leaving the old insulation in place increases the total R-value, lowering the cooling & heating load.

Good point about checking for spillage issues on atmospheric-drafted combustion appliances & fireplaces.  This should be done any time a major retrofit air-sealing project is undertaken- problems are rare, but potentially lethal.

Dana1;

Yankee fan has yankee thinking and is why for years HVAC systems in Florida attics were ineffective.
The key word is "attic" it is no longer an attic if the area is conditioned  space and regardless of the old insulation being left in place the "old attic" will be very close in temp& humidity to the remaining interior conditioned spce once it is sealed off from exterior

Why would you want to condition your attic space to room temps and humidity levels? If it's not usable space it seems like a lot of space to condition that doesn't really have to be. I'm in MA not FL but I have done it both ways and the energy saver ($savings)has always been the one with the unconditioned attic. Depending on how much space is up there this could be 10-20% of your enclosed space. I don't see the need to condition attics, but that is just MHO. Why not just completely seal the living space, vent the attic and call it a day. It takes a lot more insulation to cover the rafters than it does the ceiling. Am I missing something?


Tom Pittsley
[email protected]
www.eebt.org

Tom;

yes .....what your missing is most HVAC supply ducts (in Florida)are thru a hot humid attic, so not only are you losing efficiency thru duct leakage, you are also drawing in hot humid are via negative pressure. Both of these greatly reduce they efficiency of the unit and makes it work harder,

On the other hand when you seal off the "attic" and make it conditioned space you no longer have those HVAC issues and any air leakage falls back into the home


what work in MA does not in FL

from my last workshop with an ICC member, the de-rating of batt insulation comes from infiltration at gaps formed at the edge of the insulation at the studs or rafters. But, I personally dont have the data to back this up yet, and batt quality assuredly accounts for a part of that derating percentage also.
While im recalling the workshop,  he also touched on the perm rating of blown closed cell, if you have an overall perm-rating less than 1.0 or whatever your state mandates it fulfills the vapor barrier requirements.

2009 IRC
R806.4 Unvented Attic Assemblies

What works in MA does indeed work in FL- thermodynamics is the same everywhere.  But you still have to correctly assess the conditions, then apply the science accordingly.  While lot of ventilation & vapor retarder application practices are poorly understood by the designers/installers, it's not rocket science.

But foam sealed insulated attics work in heating dominated climates like MA too, especially as retrofits.  The SEALED part is critically lacking in attic floors in many/most existing buildings, and sealed attic floors can be very difficult to achieve as retrofit compared to the ease of foaming roof deck from soffit to ridge.  Foaming the roof deck also mitigates ice-damming issues far more reliably than any natural draft ventilation scheme for cooling the roof deck from the interior.  The difference in insulated surface area of an insulated roof vs. an insulated attic floor is quite small in most FL homes, since the typical roof pitches tend to be quite low.  In MA roof pitches tend to be higher, and it takes a somewhat more foam to pull it off, but the reduction in wintertime air infiltration from stack effect forces is usually worth it.  Slab on grade construction with ducts in unconditioned attics are far rarer in MA than in FL, but they do exist- in those cases foaming the deck is even more cost effective than in FL.

In new construction, foam-sealed attic floors and "energy heel" framing for high R-value blown insulation are usually a better value.  In FL it can cometimes be more cost effective to foam insulate & seal the ducts then heap cellulose over them, as long as the attic floor is well sealed (as verified by pressure door testing.)  Foams are great for making a perfect air barrier, but it becomes expensive at high R values.

Dana1;

HVAC ducts in MA are run from under, not over, so while the closed conditioned space may function the same thermodynamically. the HVAC systems will not

On My last project, I used SIP's for the ceiling/attic floor, creating a well sealed conditioned space inside of the home. In the one area of the home in which we had a cathedral ceiling I did use closed cell foam in the rafters. The energy modeling showed a reduction of 14% in the heating loads by not conditioning the entire attic as well as saving nearly $3,000 in construction costs. No ice daming problems with this approach but a very cold attic in the winter. 2,000 sq/ft Radiant slab on grade with HRV located in the unconditioned attic, the HRV and duct work were sealed with 2" of closed cell as well. The home is completely electric, not typical for MA with a geothermal water-water heat pump to provide heating and cooling. Total home energy costs during the winter were between $140-$150 per month for all of the energy needs not just the heating system. The cost of energy dropped to around $90 during the swing season when no heating or cooling was needed. The winter heating costs were under $300, or about what it cost per month in most homes here in MA. In this case insulating under the roof deck would have cost the homeowner $3,000 more and increased heating and cooling costs (according to modeling) by 14%. When attempting to build a home that is as energy efficient as possible, every little bit counts, and 14% to me is a big number especially when it costs more to do it.

Tom Pittsley
[email protected]
www.eebt.org

And that is just so NOT the typical retrofit situation, eh?

Many things can be different/cheaper/better in new construction. 

But in FL, even in new construction, putting the air handler & ducts in a (fully-conditioned) attic can still make a lot of sense (dollars too).  The performance hit in the typical lo-rise low-pitch FL attic won't be anything like 14%, as compared to the expanded wall area necessary to accommodate said air-handler & ducts to still end up with the same LIVING space.  Typical FL attics are more like "overhead crawl spaces" compared to typical New England attics, but I suppose there are exceptions to prove that rule too.

The house I am referring to has a 7 on 12 pitch hip roof.  The A/C unit is a water furnance GEO thermo heatpump.  Not sure if I will see enough energy savings to justify the cost.   Haven't heard enough about leaving the exisiting insulation in the attic.   Thanks to those how have responded with your comments/suggestions....