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Help with new construction insulation

Posted by vanmicnatlog (My Page) on
Sat, Oct 4, 08 at 22:26

I am new to posting on any forum. I have been browsing (addicted to) for several weeks. We are building a new home and just wanted to verify our plans for insulating our new home. We have 2 storys (1400 sq. ft, 800 sq ft and full basement 1400 sq. feet. 2x6 wall construction. In the basement i plan to have 2 inch foam, 2x4 wall and batt insulation. On the upper levels, i was just reading the "Gimme shelter" and other sites. (I would love to have spray foam, but we are paying as we go and don't think we will be able to afford that option)
Have 3/4 inch foil faced foam on entire exterior

Would it be ok to:

put sheet foam between the studs, then blown in celullose in the wall cavity.. will caulk and spray foam all crevices and around electrical outlets.

Attic/ ceiling: we have some attic space, some cathedral ceilings. Will do blown-in in attic spaces, and was just thinking about what to do for the cathedral sections. What if i use stack foam sheeting in the rafters (ten inches). Will have rafter mates for ventilation. Or would we be better off using batt insulation in the cathedral section.

what about that foil faced vapor barrier, can that be used for holding in the cellulose insulation?
Any help is appreciated.

Follow-Up Postings:

RE: Help with new construction insulation

Insulators get really pissed off at me when I bring up the subject of what happens down the road when your roof leaks. They insist their foam is 'open cell' but I have never seen a sample that allows water to pass through it. If a seamless layer of foam is sprayed on the underside of the roof deck, a leak will never be noticed until something serious like collapse of the decking due to rot occurs. Foam in the walls is a good idea but I think the roof insulation is something that will come back to haunt us later just like the urethane insulation proved too hazardous in fires back in the 70's and was banned. For my roof, I used foil-faced (goes foil down) plywood radiant barrier decking and R-30 blown in. Below is my 'energy rant' with some other ideas for you to consider.

Currently the U.S. is sending an exorbitant portion of it's energy money overseas. Anyone who does not take some initiative to cut their power bill (and therefore energy usage) needs to realize that they are providing 'aide and comfort' to those who would love to see us dead and are taking great delight in our current difficulty. Drilling for more domestic oil is not the panacea for this mess as prices are not being controlled by the mechanism of supply and demand, rather by a combination of speculators and OPEC who control such a large portion of the world's oil supply that they can easily negate any of our drilling efforts by simply closing the tap at their end just a little. Any oil we produce won't save us any money. We currently produce a large amount of oil in Alaska but do you see us getting a break on it because it's domestic? No, we pay the going world rate, set by the aforementioned OPEC and speculators...and provide another windfall for the oil companies. They sit before Congress smiling saying their huge profits are necessary to allow them to explore....excuse me....we are talking about NET PROFITS here...what's left AFTER exploration expenses. We the people have no control over this sort of manipulation of our government, whether we have Republicans or Democrats in charge. . .there's too much money involved. Our only hope is to cut energy usage, not drill and give them more avenues for profit. We all need to realize the best thing we can all do to help the energy situation and economy is to take individual action. Rather than waiting around for the government to do something we can each cut our electric usage dramatically....up to one half of your bill if you are willing to spend a little time and money. Every time you see your power bill shrinking, that's money for you rather than some oil producer. Money you can spend here to help the economy or save as you prefer. Here are several sites worth visiting that aren't selling anything but tell it like it is. Plenty of simple and painless techniques to cut your power bill.

Mr. 'Bluejay' does a great job explaing away a lot of scams...see his section on surge suppressors for example.

Be sure to look at the 'Half' program.

The current situation has people grasping at all sorts of non-cost effective solutions that sound great on paper but don't hold up to scrutiny in the real world. Solar electricity is one of those as would be wind power in cent. Florida. Solar PV (photo voltaic) just costs too much at the present. Hopefully ongoing research will change that in the future but it's not ready for prime time yet. The rental program below will probably be practical if it gets underway.

Check out this site for info on a possible contender for solar PV at home.

The 'Build it Solar' site also has some good info about costs.

Plenty of outright scams getting too much press also. The 'run your car on water' crowd is one I can think of (apparently many people didn't pay attention in high school physics). A good site explaining the physics that negates that scheme is here along with other interesting info about some of those behind the current revival.

Another scam that's been revived is the 'phase controller' that supposedly saves electricity by making motors run more efficiently. Home show demonstrations with an electric motor show it's electricity usage dropping dramatically when the motor is plugged into the device rather than straight into the incoming power. These devices actually do work in the applications they are designed for...this has the unfortunate effect of helping the scammers (or to give them the benefit of the doubt, uneducated-in-electricity folk) sell these things. An industrial application that produces savings would be a big table saw in a woodworking shop that runs constantly. When it's not cutting wood, the motor is under no load and at that moment, a phase controller will save energy. The goal of electrical engineers is to make sure anything with a motor runs that motor at rated capacity at all times...that's the point a motor is most efficient. Their striving for this is what has made ACs and refrigerators much more efficient lately. The point here is motors in your home all run at rated load all the time. There is no 'off load' time like with the big table saw. A 'phase controller' saves nothing in this situation.

In hot climates a proven strategy that works well is a heat recovery unit attached to your AC unit. It provides free hot water and also increases the efficiency of the AC a couple of SEER points by reducing head pressure (and electricity draw) on the compresser. You're paying for your A/C to move a great deal of heat out of your house, you might as well use that heat to provide you with hot water. Adding one of these to an older AC unit that still runs well is a good way to cut $30-$50 off your monthly power bill during AC season (which here in cent Fl can be about 9 months). The neatest thing about this solution is that it's cost-negative. In addition to being free hot water as opposed to other 'high efficiency' heaters that still consome energy, it actually lowers your cost of air conditioning. Here is a manufacturer's site for two of these and a power company document about it as well. The second mfg site has a neat calculator for your savings...but my experience shows it to be somewhat under-optomistic. I don't think it's taking into account how a hru will improve your AC's efficiency. There are claims that they don't work with higher efficiency unit, but I have two HRU connected to 13 SEER heat pumps that provide plenty of hot water.

Another real saver in hot areas is a radiant barrier in the attic or metal roof. Both create similar savings. If you are re-roofing, consider changing to a metal roof. When installed over a normal wood-decked roof, noise is not a problem. Most people thinking of a noisy metal roof have heard one that's been installed over wood 'stringers' as was the case many years ago...very noisy in a rainstorm. If you have a hip roof house or a lot of dormers metal may be too costly due to the waste from all the cutting to fit...metal is most cost effective on a two-slope 'shed type' roof. Radiant barriers can either be installed during easy way is using foil-backed plywood roof decking ...or added later. See this site for info and material.

Tankless water heating is also a problem area for accurate claims of savings. A gas tankless heater can be much more efficient since it eliminates the flue that passes through the center of a conventional tank heater and becomes a chimney to carry away your water's heat once the flame shuts off. Electrics are another story... Big savings are claimed that are not supported by data. No studies are shown that give specifics telling you what make/model/year tank unit is being compared with the advertised tankless. Maybe they use an ancient cast iron tank heater insulated with sawdust? Verifiable details please. Somehow electric tankless heaters dodge the requirement for the yellow government 'EnerGuide' label that every other appliance must have. If they had to carry it, their claims would show to be false. Claims of better durability because of 'no tank' are a red herring also...the 'tank' still exists, only it's now smaller. Buildup from hard water deposits may be more of an issue with tankless, possible surcharges from the power company later due to high instantaneous current draw, heavy wiring needed, new construction with planned tankless can make retrofit of tank type later very difficult due to lack of dedicated hot water piping, incompatability with future solar or other alternative power sources due to aforementioned high current draw. These are all serious disavantages of the tankless electric. All electric hot water heaters are 100% efficient at heating water. That's a function of an electric element immersed in water. The only added efficiency of a tankless is the reduction of standby losses which are very minimal in new technology insulated tank heaters. Put your hand on one and you'll see they pretty much run at room temperature...very little standby loss.

Another myth is that shading a A/C condenser saves '10-20% or more. The reality is not much. Similar situation to a garden hose left out in the turn on the water and what comes out is scalding hot....but in a few moments the water is cool...sun impinging on something will make it very hot but that is a function of 'accumulated heat'. What heat is striking something and being carried away by a fan is very small. See the following link about this subject...

A free, online calculator that can help you plan upgrades is here:

Two technologies have the potential to really turn things around..see these two links for something both companies say will arrive sometime next year...Still better to get moving with your own plan that to wait with your fingers crossed..

Please use due diligence when deciding on ways to save energy. There are LOTS of scams out there. The internet is a great source of info and you can easily see both points of view on any energy saving strategy. Just simple conservation by time-tested means is still the best way to approach our dilemma. If we all cut our power bill in half, we'd all have much more money in our pocket and our nation would be much more secure. This action, along with Mr. Picken's ideas on wind power, WILL free us from imported oil and rather quickly.

RE: Help with new construction insulation

garymunson- This is just my view, nothing documented at all.
The American people of today are damn spoiled- there are very few people who are willing to 'have less', in order to gain more at a later date.
How many of your friends- this is asked of each of you reading- have made a significant try to lessen the impact? How many people have really cut down on driving- not making a trip to the convenience or local store for a soda, beer run, something they forgot when they went shopping yesterday?
How many people are using cold water only when doing laundry?
How many people are drying clothes outside rather than using the dryer?
How many people decide not to go somewhere on the weekend-(I can afford it- why should I make myself suffer attitude).
How many people are still buying brand name products instead of store name? (most plants are manufacturing more than one name- they are making store brand at the same place that makes name brand- take Tropicana Juices for instance- in Florida where I lived, on Tuesday they made 'off brand' juice, with the same orange lot that they used yesterday to make Tropicana).


Exactly how many cd's does your kid have? (you)?

Well- I'm not going to stop driving my Suburban- I need it to take the kids to soccer practice. I'm not going to squeeze them into that little car that is so efficient on gas!

I'M not putting a compact flourescent in MY $4,000 CHANDELIER!!

MOM-DAD- I JUST HAVE TO HAVE THAT NEW PAIR OF JEANS BY XXX BRAND!! (that an 8 year old child in South Korea has spent 6 hours making in a dirty factory).

OH- ISN'T THIS SOOOO CUTE- LITTLE JOHNNY JUST HAS TO HAVE IT! (little Johnny has so many damn toys that he could care less- I'm sure little Johnny is aware of how much petroleum and energy is used in the making of that worthless plastic toy)

WHAT DO YOU MEAN, I CAN'T HAVE THE 52 INCH TV IN MY BEDROOM! (Is that the teenager screaming or the adult- I can't tell).

Honey- I saw a new gas-riding lawnmower in the Sunday paper....I think I'll go buy it. (so he now gets even LESS exercise than ever, because the time that he has saved mowing the grass, he now will spend going to the football game and drinking 9 beers, then going home and watching all the rehash/reruns on tv, where he'll drink another 9 beers, then get more heart disease- eventually having a fatal heart attack,( using more items in the hospital that were made using coal/petroleum energy, mass amounts of pressurized oxygen), using the skills of doctors and nurses, costing himself and the insurance company hundreds of thousands of dollars, plus using a plethera of items that were made of plastics- made by using coal/petroleum energy- all because he had a heart attack caused by the lack of exercize caused by buying the riding lawnmower-...
you get it- (I hope).
Exactly how much petroleum products are we using just supporting auto racing? (it might be small in comparison to the millions of daily drivers, but do we NEED it?
I really don't think the big three auto makers need to be building ANY auto/trucks right now- I think there are plenty enough out there, even if you have to drive a 3-5-10 year old model- how many autos that really could be used are going into the dump everyday? And yes- there are many that go to the dump, not recycled- just go to your local dump and see what is thrown out!
(Oh- excuse me- people in New York, San Francisco, Ventura, wherever, don't even KNOW what a dump IS- they think all cars, stereos, cd's, refrigerators, tv's kitchen sinks, beds, clothing, sofas, bedroom furniture, etc, etc go to Heaven.) Oh yeah- I think I saw about 9 good bicycles in the 'metal' part of the dump yesterday when I took trash there.
Speaking of trash- why do I have to tear off useless plastic off of the recyclable batteries that I bought- why is cereal wrapped in both a cardboard box AND plastic?? Must we really have so much 'advertising' on something? Again- all of that 'advertising' is energy that didn't need to be used up-( the energy used to make the outer layer of cardboard for the cereal).

Again, I am not suggesting that a certain person in particular is not doing a great job of making a smaller footprint- but how many people can actually say they cut down in EVERY area- not just where they HAVE to?? (commuting to work with another person just isn't enough!)

There are so many that the list could go on and on. And on.
Why CAN'T we look into solar more- even if it is expensive now- how much will it cut dependence later?

I am sorry garymunson- but until it gets much, much worse, I do not think the average American will change their lifestyle enough to make any impact.

There are people who will argue that slowing down on production of all the 'stuff' that I have mentioned above will decrease jobs- somehow I think we could use these worthless jobs to a better use-

Oh- and just don't get me started on the guys in upper corporate!!..

Nope- until people are not so greedy, nothing will change- at least nothing worth mentioning.
I know there are many ,more complex ways to help decrease our footprint, but these are things that I am aware of now- things that I try to practice. I will learn more as more ways are made public to me.

My POV- we as a whole are just too greedy/spoiled to care at this point. Maybe when there is NOTHING, as in the Great Depression....
And no- I am not suggesting that everyone go buy things at Goodwill- but I think each and every person CAN make a significant impact if only they will try.

RE: Help with new construction insulation

not sure what any of this has to do with my insulation question.............. I REALLY WANT TO KNOW WHAT TO DO WITH MY NEW HOME... but since the conversation is coming up///// i walk to work every day..... i wash my clothes in cold water and hang them on the line outside or on hangers inside to dry//// i go shopping once a month, and go to the local store 3 blocks away for intermittent shopping trips/// my biggest down fall is my children, i try to make sure they don't want for anything.. they have name brand clothes ,,, that i BOUGHT AT GOODWILL... I am doing all of this to try put 2 and soon to be 3 kids thru college and build a dream home with out a payment..... so if any one can help me with my orignal question and not a political battle, i would appreceiate it... Should i put foam sheet insulation, layered in the rafters of my cathedral ceilings or standard batt insulation?

RE: Help with new construction insulation

Sheesh....what brought that on?? I was just pointing out ways to save energy.....I would think the foam would be most effective. My concern is about the spray on the underside of the deck. I'll attach some other observations I made building our new home. Most strategies may only save a few percent but when you combine many of them, you can make a real dent in your energy consumption.

How to keep your electric bill low

Pick plan with garage shielding house from sun...i.e. not one with a garage on the side or end of house. Remember in Florida, it's all about solar heat gain..through the roof, windows, and walls.

Choose a lot that allows you to orient house with garage on west side facing afternoon sun. This allows garage to block afternoon solar heating of the part of the house 'hiding' behind it. Minimize size and amount of west facing windows and doors. Use double pane, low-e glass in vinyl frame windows and fiberglass or vinyl doors. Plant trees and large shrubs to shade the west side of the house.

10' ceilings are desirable. Although you have more cubic feet to cool, you'll feel more comfortable at a higher temperature. A higher ceiling allows the air to stratify. The warmer air will accumulate at the top, well above your head where you can't feel it, the cooler air will sink, making the 'lower level' you live in more comfortable. With a high ceiling, activities that generate heat such as refrigerators, dish and clothes washing, showers, etc., that introduce heat into the living area have their heating effect minimized by the heat-at-ceiling stratification. Ceiling vents in a high ceiling house should be directed down rather than trying to spread the air sideways. Sideways deflection mixes the air, defeating startification. Ceiling fans are not as effective with the higher ceiling as they tend to dislodge the hot air and send it down, causing the air conditioning to run more. Spring or fall, when the windows are open, is the best time for ceiling fan use. Small pedestal fans and the popular 'tower' fans work better during AC use, circulating the cooler, lower level air. Air movement from them may let you set the thermostat higher and save energy. Running a ceiling fan in the winter to 'get the hot air back down' is counter-productive as any reclaimed heat won't compensate for the wind chill effect you'll feel. With 10 foot ceilings, a floor level air return is preferable to minimize disturbance of stratification but with normal 8 foot ceilings, a ceiling return will make the structure more comfortable by evening out the temperature.

Have 24" roof overhang to shield windows and much of the wall area from midday sun. The goal is to allow maximum view of clear sky from all windows to minimize need for interior lighting during the day without direct sun entering house during the summer. A 24" overhang will usually do this in the summer , while in the winter, the lower position of the sun in the sky will allow it to shine in for some solar heating although the desired low-e window glazing will minimize that. Be sure the engineering is correct for the wind load where you are building.

Two story structures in Florida require extra planning to minimize heat gain as they usually present more wall area to the sun. You lose the 24" overhang advantage shielding the windows and walls on the lower story. This can greatly increase solar heating through the windows and walls of the lower floor which will then convect the heat to the upper level. This is an advantage in northern climates but a liability in the south where cooling is the main concern. There is some advantage here in less roof/attic space, however..

Use hip roof construction...much stronger than gable design...will qualify you for a substantial discount on your windstorm part of your insurance. Check with your agent.

Use either galvanized ('bright metal', no colors) roofing...expensive, especially on recommended hip roof designs (a lot of waste cutting the metal with all the angles of a hip roof) or use a radiant barrier. This barrier can either be an aluminum/Mylar film draped over the trusses before the decking is applied or applied on bottom of trusses in existing structures or plywood decking with pre-applied barrier. The radiant barrier will keep the attic much cooler. Plywood makes a much stronger deck material than OSB at a minimal premium in price.

Use a roof design with 6/12 pitch. This gives sufficient attic height to maximize convection cooling. Also 6/12 is the 'sweet spot' for wind resistance. As roof pitch gets lower, the tendency of the roof to act as an 'airplane wing' in a windstorm increases raising the chance of roof lifting off. A greater pitch begins to act as a direct barrier to high winds increasing chance of 'push over' failure. A 6/12 allows wind to slide up and over but also creates turbulence on the lee side minimizing lift as well as down force on the upwind side.

Try to keep square footage as small as comfortable for you. A 4000 sq. ft. home requires far more energy use than a 1500 sq. ft one.

In Florida, build concrete block on a slab. We have hurricanes AND termites here...Code requires ground pre-treatment. Continuing the termite contract is cheap insurance. An extra benefit of on-slab construction is being able to take advantage of the cooling effect of a slab floor. The earth under a house stays fairly cool and especially with some hard surface flooring the small difference in temperature between the earth and the floor surface means very little heat gain from below. Its helpful to think of heat as a fluidjust as water flows more forcefully as height of fall increases, heat moves quicker across a greater temperature gap.

Use several solar powered attic roof ventilators along with numerous passive vents. In Florida you can't have too much attic venting. We don't get enough cold winter weather to be an issue if you're well insulated.

Use R-30 or better in the attic. Insulated cells in concrete block walls are probably not worth the expense. Florida requires so many cells to be poured concrete now for windstorm resistance theres little empty cells left to insulate. Check the Florida Solar Energy Center website and youll see that less than 17% of heat gain occurs through the walls (not entirely accurate for two story construction). Also keep in mind that there are many windows and doors that deduct from the wall area and upgrading them will have far greater effect than some fluff in the block cells. A better solution is a foam underlayment beneath the stucco on the west side of the house. Be wary of spray-on underside roof insulation. This may prove to be a time bomb as the roof ages and begins to leak. The decking and trusses may be seriously compromised before you become aware of any leak problem. The 'sealed' nature of the insulation envelope may well lead to extreme damage in the long run.

Use fluorescent lighting wherever possible in place of incandescent. LED lighting will be preferable when it becomes more cost efficient

Refrigerators are a substantial user of power. Since about 2000, efficiency has been greatly increased in the new units. If yours is old, consider replacing it. Check the energy labels and youll see that top or bottom freezer models have consistently lower power usage than side-by-sides. Small access doors to get drinks out are handy and keep all the cold from getting frequently dumped out if you have children opening the fridge frequently. As in house sizes, get the smallest refrigerator you can be comfortable with. Note that external ice and water dispensers take up a lot of interior room and tend to make people buy a larger unit to compensate.

Unless you are really tied to using a separate freezer, todays high energy prices far outweigh any savings youll get by buying food in bulk and stockpiling it at home. The possibility of increased power failures from more frequent storms and the accompaning food loss also makes freezer use less attractive than it used to be.

Point of use hot water heaters can't prove measurable savings. Tank type heaters have such good insulation now that standing heat loss is minimal. Point of use systems will quite likely lead to surcharges from the power companies in the future as the high instant current draw becomes more of a problem during peak power use time. Only tankless gas units can lay claim to significant energy savings due to the elimination of the central flue in a conventional gas tank type heater.

There seems to be a lot of misinformation being pushed about AC efficiency. Code requires a minimum of 13 SEER rated Heat Pump equipment. 14, 15, 16, and higher are available at much greater costs. Research will show the only difference between the very expensive high efficiency units and a higher quality 13 SEER is the amount of condenser coil and the controller PC board (also a fancier cabinet). They all use the same brand and model compressor (Copeland Scroll), the same fans, reversing valves, driers, etc. Most companies trumpet the boards as being the key to their high efficiency, this is mostly smoke. When the 'rubber hits the road', what counts for amount of electricity used is how efficiently the compressor can transfer the heat from one place to another. The compressor is what's putting that 12 to 20 amp load on the line. The more you can reduce that value, the lower the electric bill will be. The compressor is moving the refrigerant gas from an area to low pressure to high. That's the work it's doing that runs up your electric bill. As the gas enters the high pressure area, the condensing coil's job is to cool it and cause it to shrink. This shrinking lowers the high pressure and puts less back pressure on the compressor. This has the effect of reducing the work the compressor must do and therefore lowers it's amperage draw. An amp meter is a handy tool for the energy conscious homeowner as it will let him see in real-time what effect his efforts to conserve energy have. You can put it on your heat pump as it runs and perform a simple experiment. First hold a couple of sheets of cardboard against your condensing unit's grill as it runs. You'll see the amperage begin to increase as the compressor's workload goes up since the coils can't shed as much heat and shrink the gas. Second, remove the cardboard and spray a fine mist from your garden hose on the coils and you'll see the amps go down as the coils become more efficient from the evaporative cooling of the water and shrink the gas quicker. Keeping the coils wet all the time would lower your cooling bill but would also corrode the coils so is not a good idea. The way to accomplish this safely is to add a unit called a 'heat recovery unit'. An AC professional can install this by cutting the discharge line from the compressor and sending the hot gas through the recovery unit then back to the condensing unit. The heat recovery unit is a 'heat exchanger' that has two pieces of copper tubing wound tightly together and enclosed in an insulated box. One piece of tubing is the loop carrying the hot, high pressure gas from your condensing unit. The other makes a loop to your hot water heater. Whenever the AC is running, the small water circulator pump in the heat recovery unit turns on and in effect does the same thing squirting water on the condenser coils did as it circulates water through your water heater.The heat from the hot gas transfers through the walls of the tubing into the water causing the gas to shrink and cut the compressor workload. In addition, the unit now provides free hot water for your house, allowing you to shut off your water heater in the summer. These two savings together can easily total $50 a month (!!!!) on your electric bill. Remember, in addition to lowering the workload on the compressor and saving electricity there, your water heater is no longer online. An interesting side benefit is that the more you use your hot water, the lower your electricity bill goes. This doesn't mean using more water (longer showers, etc) but you can set your washing machine to use hot water rather than cold or warm and get cleaner clothes AND save energy. You are using that water anyway and by using it hot, you are 'shedding' heat from the system and increasing it's efficiency. Heat recovery systems are typically less that $1000 and FAR more practical than solar water heaters that have much more complicated control systems, are far higher maintenance items and cost much more. Pushing solar hot water in Florida is doing the consumer a great injustice. Heat recovery units are said to not be practical on high efficiency systems (15 +SEER ratings) but the addition of a heat recovery unit to a 13 SEER system probably raises the entire system's efficiency to the 15 or 16 SEER range if you bring the savings of the water heater into the equation. Water heaters are typically resistance element heaters...the most inefficient way to heat. The industry is cashing in on energy fears by adding more coils to their units and charging a premium for the 15 and 16 SEER units that's much higher than the cost of adding a recovery unit to a SEER 13 system. The higher efficiency unit may save cooling cost but then 'wastes' the perfectly good heat that could be put to use for your hot water by just blowing it away outside..

RE: Help with new construction insulation

To the OP:

Your plans are similar to ours. We've got 1,800 sq ft basement, 1,800 main floor and a 2,200 second floor, all with 2x6 walls. This very week, the 2 inches of Closed Cell Foam will be sprayed on the walls, under roof decking and under any exposed floors (like the garage ceiling). Then I'm going to install unfaced R-13 batts in the walls, and R-19 in the garage (that doesn't share a living space wall).

Munson is very negative about the roof leak issue with foam. As long as you stick with a vapor permeable underlayment for your roof (15# felt over 30# and new space age polymers), then any leaks will dry out, limiting the chance of rot. With an open cell product, water does pass through, but not if it's a small leak. The open cell is like a sponge, and will soak up quite a bit of water before you see a stain on the ceiling. That's why we're doing closed cell that won't even let it in.

Back to your house, with 3/4 inch foam on the exterior (and I assume you were fastidius about taping the joints), I'd go with blown cellulose everywhere over the spray foam. You're foam sheathing is already a very good air barrier, if you caulk and foam openings, the blown cellulose should give you a house nearly as tight as doing the spray foam. Remember that the foam isn't a better insulator, it's a better air blocker than fiberglass and cellulose.

Then, I'd cut foam boards into the rafter spaces, foam the edges to seal them up, and then blow cellulose on the attic floor. That will seal up air leaks and will keep the attic a little cooler than standard.

Just my opinion though.

RE: Help with new construction insulation

I still have doubts about the long term safety of foam sprayed on the underside of roof decking. Although you may use a vapor permeable membrane under the shingles, the shingles are not vapor permeable. Eventually they will leak and the water will spread between them and the underlayment where it will be trapped and be unable to dry out. Any water that gets past the underlayment ( the REAL roof leak scenario) will soak into the plywood/OSB decking and/or find a joint in the wood decking and start wetting the truss lumber. None of which would be a good thing since the foam will keep you from realizing this mischief is going on.

RE: Help with new construction insulation

I think I would stay away from foam directly under the roof deck for the water reasons just in case. You have many more really good options to block a large amount of heat with radiant barriers and either cellulose or bats. I would just go thicker on the ceiling insulation and maybe put up a radiant barrier and also make sure you have good ventilation up there.

OK here's to you ilmbg:

I drive an escort over 90% of the time instead of my liberty, I was my clothes in cold water and we try to line dry the bulky items. We grocery shop for the best deals per Oz and not by who makes juice on Tuesday. BTW some name brand items are cheaper per oz than generics, you have to keep your eyes open. Now that I think of it.. this is a wast of KWhrs proving that your not the only one that has a brain in this country.

RE: Help with new construction insulation

Sorry there's so much stuff here that's irrelevant to your OP.

If I was starting from scratch, I'd go with SIPs over stick-built. The downside is how much planning needs to go into any wiring, etc, runs in the exterior walls. And the difficulty of adding anything in the future.

If not SIPs, then I'd do something similar to what you're proposing- putting sheet foam on the outside of exterior walls to reduce conduction. Inside, I'd foam/caulk all potential air paths (IMHO, foam sticks better and has broader application but is more expensive).

The foam boards you're proposing putting in stud bays are going to be expensive. I'd vote for the wet-sprayed cellulose to the full depth.

You'll lose a little insulation but it'll cost less and require less detailing (I'd probably be picky and foam the edges of all the sheet foam you put in the bays). You also would be creating essentially 2 moisture barriers with your sheathing in between- potentially a future moisture problem.

bdpeck's idea for the rafter bays might help, but it could be an adder that you decide further into the project whether you can afford. FWIW, we remodeled and put in the foil-faced OSB for sheathing on the newer part of the house. It seems to keep things cooler up there.

FWIW, in the future you might get less "ideological" responses on the building a house forum.

Here is a link that might be useful: Southface on air sealing

RE: Help with new construction insulation

thanks for your help, i will cross post ing the building a house in the future. I have another question though about taping the joints of the exterior foam insulation boards. The tape that they sell is extremely expensive. Can I use the ever popular DUCT TAPE?

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