You might want to look at the foil faced foam board it has a higher R-value of R-7 per inch instead of R-5 per inch for the tuff-R or formular. Tuff-R is stronger because it is used for sheathing, but it is the same type of foam board.

Thank you. That is helpful. We will see if we can find the foil-faced foam board locally.

A 1 1/2 story house is one with a second story above the first but only about 2/3 of the second story is habitable because the roof eave is at or just above the second floor line. What you describes is a 2 story house with 2 habitable floors plus an attic above, so I assume you are insulating the floor of an uninhabited unheated attic.

Time for a reality check about rigid foam insulation boards.

Styrofoam is the brand name for Dow's extruded polystyrene foam insulation board. It is used against foundations, in roof assemblies, and walls either over structural panel sheathing or in a brick cavity because of its high compressive strength and resistance to water. It comes in many thicknesses with an R of 5 per inch.

Formular is the brand name for Owens-Corning's extruded polystyrene foam insulation board. It is licensed from Dow and is used in the same way as Dow's Styrofoam.

Tuff-R is the brand name for Dow's polyisocyanurate foam insulating exterior sheathing board. It is used for exterior wall sheathing because it has a tough 3 layer facing on one side and a foil facing on the other. The maximum thickness is 1 inch and it has an R of 6.5 per inch. The foil-facing does nothing if there is no air space immediately in front of it.

None of these insulation boards is normally as cost effective as blown-in cellulose assuming there is room for the proper thickness. If you need a greater R value you should consider closed-cell sprayed polyurethane. I would think cutting Tuff-R to fit would be prohibitively expensive even if you install it yourself.

Probably an R value of 7 for typical polyisocyanurate insulating sheathing is the initial value instead of the aged value.

Hi there, mightyanvil

Thank you for the response. We have a 1 and 1/2. Put fiberglass in the crawl space a few years ago. We are insulating the roof, basically. Have only two rooms up there - a landing with a closet, and one big room. The landing was done two years ago with the rigid pink styrofoam. A jigsaw cuts through it like a knife through butter.

We have an air vent at the roof peak and under the eaves used the flexible pink vent funnels from the Orange Box store. For this room, DH will nail in wood strips to keep the insulation spaced away from the roof.

So I think we are all set. The tedious part is measuring between the rafters every 15 inches because they are not uniformly spaced and some have warped just a little.

I don't really understand what you are insulating but there is no reason whatsoever to create a vent space above foam insulation in a roof.

Maybe it is a question of regional differences. Here in the middle east coast, attics now have a vent at the peak and under the overhang. The air is supposed to flow from the peak down the vent. The insulation is not supposed to touch the actual roof. Hence the need to replace the insulation and insure there is a vent for air flow, which keeps the ice dams from forming. An ice dam on the roof can cause leaks which mean plaster damage.

The primary purpose of eave and ridge vents when insulation is installed between rafters, is to allow dry air to travel from the eave to the ridge by convection to allow porous insulation to dry if it gets wet. (A much better method is to cover the entire roof with Ice & Water Shield) This venting is not required if non-porous foam insulation is used.

Some think the venting lowers the temperature of the roof enough to allow asphalt shingles to last longer but that has been proven untrue by field testing even in the worst climates. The idea of venting preserving shingles is perpetrated by asphalt shingle manufacturers to reduce successful claims since it is virtually impossible to properly vent all parts of a roof.

If the roofing is relatively new, check the wording of the warranty. Some manufacturers require venting, some will reduce the warranty protection, and some allow non-vented roofs. Many have a clause that allows the use of composite insulated roof sheathing which is simply a foam insulated unvented roof system since they don't want to lose that market. Remember that roofing warranties only reimburse for material costs, decrease each year, and are often frustratingly difficult to enforce so they should not always be your first priority.

The more important warranty is from the installer who should not have any objection to an unvented roof.

The best way to reduce ice dam damage is to insulate the roof well, seal air leaks from inside, and install Ice & Water Shield as far up the roof as you can afford.

I worked in a ski resort town in CO and we designed many roofs to be "cold". This technique involved two waterproof "roofs" with a substantial air space between them. In a cold climate an inch of vent space above insulation is no more effective in reducing ice on a roof than filling the space with insulation because air does not move through a small vent space the way you might imagine it would. In a humid climate, it can allow moisture vapor to enter the rafter cavity.

It is important to remember that the first priority in designing a house is to keep it dry. We used to put this statement on our drawings: "It is the intention of these drawings to provide a weather tight enclosure" in case a contractor didn't understand that priority. Today, there is so much emphasis on energy conservation that the primary purpose of many details is overlooked or misunderstood. DuPont's invention an air barrier that takes the place of a weather barrier is a good example. You would need to save a lot of energy in order to pay for major water damage repair like replacement of the sheathing and siding.

I should also mention that if you vent a roof you must provide an air barrier at some point to keep outside air from entering the roof space and cutting rigid foam insulation to fit between uneven rafters isn't a very tight barrier so there would need to be a membrane on the underside of the rafters or elsewhere. It is quite possible for air leaks to short-circuit the installation and substantially reduce its effectiveness.

The design of energy efficient and water-tight buildings is not as simple or easy as homeowners want to think it is.

"I don't really understand what you are insulating but there is no reason whatsoever to create a vent space above foam insulation in a roof. "

This is absolutely untrue....and a building code violation in most applications.

While 'Open cell' foam insulations can act as true 'air barriers', they are completely permeable to liquid and gaseous water vapor.

This means they not only require a vapor retarder on the warm-in-winter side to keep interior moisture from entering and condensing in wall and ceiling cavities...these 'open cell foams' legally require mandatory air ventilation in rafters by Code...typically a minimum of 1" space between the insulation and roof deck... to assure that water vapor can be ventilated up and out through attic roof vents to prevent damage to framing members from mositure.

Only where 'closed cell foams' are used can roof assembly ventilation spaces be avoided, or where SIPs (structural insulated panels) are used which have been approved for installation without ventilation by their listing.

Otherwise, what mightyanvil has claimed is entirely false.

As the OP has made clear, this is a DIY job using rigid "Formular" extruded polystyrene foam and he was asking about using Tuff-R polyisocyanurate foam sheathing all of which would be bought at Home Depot.

Both materials are closed-cell foam and there has been no mention of using open cell foam.

I have mighty disagreement with mightyanvil. manhattan is correct.

Further, tuff-r is not "cut to fit" within rafter space. it should be placed on bottom side of rafter, if used at all.

Use shallow enough batts of insulation between rafters to leave a 2" space (per Dow Corning web-site) or use deeper insulation but also use foam / plastic vents that staple to underside of roof deck and provide ventilation to roof ridge vent and (hopefully) from soffit vent.

Look at manufacturers' websites for detailed instructions how to do cathedral ceiling, which is basically what you have above knee wall in a 1 and 1/2 story house.

ALSO read the printed stuff on the material (batt insulation, tuff-r, whatever) you are using.

Get a handle on the whole "moisture" thing before doing insulation. It's important. Warm moist air against a cold surface will cause condensation on the cold surface, which will lead to damage. Don't want to have this condition in an unventilated area.

Also, aluminum faced insulation (ie tuff-r or rolls of plastic bubble wrap coated on both sides with aluminum) has the remarkable ability to reflect back certain wavelengths of radiant (i think it is) heat (3 types of heat - convection, radiant and ???). Only true IF there is a mimimum 3/4 inch air space against warm side of aluminum coating. This explains printing on the tuff-R board about "extra R value if airspace"

I recall that State of Florida did tests on aluminum coated insulation against bottom side of rafters in attic. Oustide shingle temperature is increased by 10 to degrees as I recall, leading to somewhat shortened shingle life offset by lower energy consumption.

Probably way to late for original poster, but maybe somebody will see it! :-)

This thread could serve as a warning to the well-intentioned homeowner wishing to improve their home comfort and save on energy outlay.
On the one hand we have the traditional, by the book code approved way-of-doing-things that on average yields trouble free results, yet makes no advance.
The other, represented by Mightyanvil, provides newer out-of-the-box thinking that is likely to become normative down the road a bit. Builders and code enforcers are VERY resistant to change.
The problem, assessing moisture transmission, way out of the typical homeowner's purview. There is the secondary issue of satisfying roofing material warranty requirements.
The former while likely having geographic standards (as in primarily heating vs. cooling) is better seen as site specific, and ignorance prevails.

"Get a handle on the whole "moisture" thing before doing insulation."

That's gospel.

Found this thread by google'n :)
I have a split level house. One upstairs bed room wall backs up to the middle level rooms attic, about 4' up on that wall. Its a small room so i decided to open up into that attic and make a loft for my sons bed. 2 of the walls in the loft will need insulation, one will only be about 2' hight and the other 4.5' down to 2'.
Can i use dow tuff-r board on the back side of the 2x4's of these 2 walls then stick some r15 on the inside of the walls? I would seal the dow board from the inside to eliminate any air leaks.
As far as the ceiling goes which would now be the roof, i guess i should just use the staple on vents and stick r19 inbetween the 2x6's?
Im just tying to keep it as warm and as cold as i can in both the winter and summer months!
Thanks Chris

weigh the cost of spray foam insulation (open cell in La.)
against lower material cost of foam sheathing boards and lots and lots of time to install, tape and caulk.
in my area it breaks out fairly evenly.

also you may get a price break for spray foam insulation
if you foam the floors. makes a big difference in comfort
& utility costs.

best of luck.