Insulation and venting

I'm to the point that I have to insulate on top and close the gables. I'm getting conflicting sides on where to insulate [ on top of ceiling or under roof deck] Wheather or not to vent the attic. There will be no duct work up there. It will be closed off.

This is a ICF house.

I was thinking of having spray foam applied to the top of the ceiling sheet rock thus sealing the living space completely like a igloo cooler. HAve been reading about not venting the attic area but am concerned with condensation. Metal roof installed.

I have not covered the gables yet so gable vents will not be a problem.

So the questions are

1] where do I insulate?

2] vent or no vent

I hope to get back up the middle of this month. Will take me about

5-10 days work then I can get living in this baby!
Reply to
Raider Bill
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Proper roof inuslation in some areas of the country is an important factor concerning condensation. In our area, the north pacific coastal area, I insulate above the ceiling, and provide venting at each end in the gable or with a ridge vent and bird holes in the blocking @ the plate. With the new fire safey regulations this blocking has been eliminated.....

Reply to
jloomis

Top of ceiling, never the roof deck unless you make provisions to ventilate the space between the insulation and the actual decking.

Spray foam? I'd conside blow in cellouse insulation myuself.

You want venting. Do not block or obstruct the vents.

IMHO, ceiling

IMHO, vent.

Where is this house? That is a major factor in the decision.

Reply to
PeterD

If attic is unheated it must be vented or you will mold then rot all wood. The attic floor is insulated. ICf so what is on floor now and R value, what is wall R value, what Zone are you? Foam attic floor to R

50-60 or more for Zone 5. Fiberglass looses efectivness as it gets real cold
Reply to
ransley

I think your intuitions are correct - spray foam to the top of the sheetrock of the ceiling below, sealing the envelope of the house as best as possible. this has several positive consequences. Use high density spray foam, as the low density foam is not an effective vapor retarder. As others noted, where you live is an important consideration.

- puts the insulation where you need it. since there's no attic mounted utilities, there's no reason to keep the attic in the conditioned space of the house.

- blocks moisture movement into the attic

- blocks air movement through the walls, at the top plate, wire penetrations, etc.

- can provide good insulation value

I've been evaluating a lot of homes, and given what I've seen, I would definitely foam that boundary. IMHO, the attic/living space boundary is one of the most important ones in the house to seal tightly. I have never seen construction tight enough to block all air and moisture movement through walls, at joints, etc.Assume that the house is built leaky and foam the heck out of it.

Where my opinion diverges from the foam guys is I still believe in super insulating the ceiling. Most foam guys will tell you that you need less foam R value than fiberglass because foam is a more effective insulation. That's true to a point. However, using enough foam to fully coat the framing is beneficial to reduce thermal bridging and increase the true overall R-value. If you want to save a little, spray 3"-4" of foam then add a foot of cellulose on top of that, giving a total R-value of about 60. In hot climates, this will keep the rooms below the attic cool on hot, sunny days (assuming you do your windows and walls properly). In cold climates, it will keep them warmer.

As for venting - if you seal the attic with foam, there's no place for moisture to migrate into the space, so venting becomes far less important. The reason people get moisture problems in the attic is that they've got leaky construction that allows warm moist air from inside the house to migrate up there during the winter. Usually this is exacerbated by central humidifiers that dump too much moisture into the living space. A tight house shouldn't require added humidity. with that said, many building codes are requiring vented attics, so let them vent. Most of the time, they don't vent it properly anyway because almost nobody measures the "net free area' of the soffit vents and matches that to the ridge vent, so it's all for show.

If you're in a cold climate, keep in mind that soffit vents on the south side of the house can cause ice dams. Some building science studies have shown that the warm air rising off the south wall, heated by the days sun, can travel up the wall, through the soffit and heat the underside of the roof deck, leading to major ice dams.

See:

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great information on attic venting.

Reply to
Astro

Foam guys lie, at least they have to me telling me foam is really R14, I think its to sell a job since foam is expensive.

Reply to
ransley

Dont know where the house is, but here in Texas, South East Texas, I dont go hog wild with the foam, house and the attic needs to breath, if you seal one up that tight, think of a fresh air return added to your HVAC system, to positive presure the house, and get some fresh outside air in there, cause we dont live in our refridgetors, we keep food in them. but to seal up so tight is not the best way to go, when I say not best way, I am talking healthy way as well.

Reply to
Craig M

FYI, I happen to know the house is in Tennessee.

Reply to
DouginUtah

In central Texas, where I live, tend to agree on that, especially the attic area. You don't want the attic sealed up as it won't vent the heat, very important especially in the summer. Found the ridge vent system with an adequate number of soffit vents works better than the typical gable vents. Even when using fan aids in the attic with gable vents.

I am confused about the metal roof question though. I thought the insulation for that type of roof was placed between the rafters and the material the metal roof is attached to. Something like Thinsulate for instance.

Reply to
Dioclese

ed text -

Doug is right Athens. tenn.Good memory!

Sorry I posted and ran I'm back up here and internet and cell is very spotty at best.

I'm freezing my butt off. Icf sure is a great insulater as it has retained all the cold.

I like the idea of foaming the ceiling then blowing in some cellouse on tp.

But what about condensation in the summer from the the metal roof?

To vent or not to vent....... that is the question?

Reply to
Raider Bill

text -

That question was answered by almost everyone-- you *must* vent. Period, not two ways about it.

Now, why do you feel you need foam? Why not just cellouse?

Reply to
PeterD

uoted text -

Loose fill cellulose does nothing to stop air/moisture movement from the living space up to the attic - the primary source of problems with winter moisture buildup in attics. 2-3" of high density spray foam seals everything up, greatly, greatly reducing the opportunity for moisture to get into the attic and drastically reduce the chance for any moisture damage up there during the winter.

During the summer, the moisture drive is from the outside air. The metal roof cools rapidly and condensation forms under it if there is any moisture between the metal roof and the underlying sheathing so there must be a moisture impervious layer between the metal roof and the sheathing as well as room for ventilation between the metal and the sheathing to avoid moisture entrapment.

Reply to
Astro

quoted text -

Even a 1/4 inch of foam or sheet plastic works, a better reason is fiberglass and cellulose loose R value as it gets very cold, near 0 it might be 20% less R value. Fiberglass and cellulose settle easily 10%. But many foams can go bad and deteriorate if not painted

Reply to
ransley

The issue is "interstitial condensation". Washing, bathing and simply breathing creates water vapour in the house. It's important that this does not find it's way through the structure and condense out on cold wooden roof members encouraging rot. In the UK the normal proceedure is to install a vapour barrier (a plastic sheet) on the warm side of the insulation AND ventilate the cold side of the insulation. If you put the insulation at ceiling level then the whole attic is cold and ventilated. If you put the insuation between the roof timbers then you must leave a 50mm/2" ventilated gap between the insulation and the underside of the roof membrane/tiles. The alternative is to use a vapour permeable membrane under the roof tiles to allow vapour to escape.

An alternative to a ventilated roof is to build whats called a "warm roof" (at least in the UK it's called that). This puts all the insulation totaly above the wooden parts of the roof structure rather than between them. This means there is no cold wood for the vapour to condense onto.

Reply to
Cwatters

de quoted text -

All foams are somewhat vapor permeable. 1/4" of foam, even high density polyurethane, is quite permeable. Perm rating is non-linear. At 1", it is 2.6. A thin coat would block bulk air movement however, which would be good for most houses and could be a compromise. But better to use at least 3" everywhere. See:

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Cellulose increases R-value with decreasing temperatures:

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Settling not such a problem with cellulose as thermal performance doesn't vary much with density as it does with fiberglass. It will still decrease, but not a big deal since you applying lots of it -

12-15" typically ..

Foams only go bad if exposed to UV. Not much UV gets through shingles and roofs :-)

Reply to
Astro

Hide quoted text -

Uffi foam once banned and now back in some use turns to powder in sealed walls, Have foams been around long enough to really know what will happen?

Cellulose that I have seen disentegrates over years, its just newspaper, and any roof leak makes it worthless, Cellolose settles is alot.

Reply to
ransley

Contrary to popular belief, modern, properly installed cellulose insulation does not settle.

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Cellulose increases R-value with decreasing temperatures:

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Settling not such a problem with cellulose as thermal performance doesn't vary much with density as it does with fiberglass. It will still decrease, but not a big deal since you applying lots of it -

12-15" typically ..

Foams only go bad if exposed to UV. Not much UV gets through shingles and roofs :-)

Reply to
Steve Barker DLT

Expain why, even fiberglass batts settle mine did, its like saying water doesnt evaporate.

Reply to
ransley

mawillif had written this in response to

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: I am wanting to put fiberglass insulation in my attic myself. I was originally going to use Kraft Faced Fiberglass Insulation...with the faced side against the heated ceiling and just placed inbetween the joists. However, I am really worried about the fire-hazard aspect of using faced insulation. What does the warning mean exactly when it says to not leave exposed? Does this mean I have to lay drywall over all the joists to cover the insulation? Is the benefit of having a vapor barrier worth the fire hazard risk? Thank you.

Reply to
mawillif

Installing it that way won't give you a vapor barrier. I'd assume you are saying you have *no* insulation there now, and are going to install some. Just get non-faced insulation and you should be OK. Or get foil backed to reflect radiant heat, but as for the concept of a vapor barrier, that's meaningless at this stage.

Reply to
PeterD

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