# Rigid foamboard in attic insulation?



## strategery (Jul 18, 2011)

Have access to quite a bit of XPS rigid foam boards and I was thinking about incorporating it into my attic insulation. Would it be effective to cut some boards to fit in between the floor joists on the attic and just lay the boards down and cover them with blown insulation?

I know that xps has a plastic film on the surface and it's not very vapor permeable. Would it be advised to glue it down to the top of the sheetrock so moist attic air doesn't find it's way underneath it?

I'm currently remodeling some of my house and I'm re-rocking the ceiling. So I have a chance to insulate those areas from scratch. I'm working on air sealing all of the joints with canned sprayfoam.


Thanks!


----------



## de-nagorg (Feb 23, 2014)

The edges where the rigid foam is next to the attic floor joists will have air gaps where your precious dollars will escape. 

It would be better to use the sealing tape made for the rigid foam, that way the moisture is kept out as well, no glue down needed. 



ED


----------



## Windows on Wash (Aug 30, 2011)

You would be better served to spray foam/seal all the top plates prior to applying the foam. 

It is going to be much easier to get a proper seal on the top plates via the method as compared to spray foam. You can use the rigid foam as your baffles and vent chutes if you like but putting it down on the attic floor is not usually done.


----------



## Nailbags (Feb 1, 2012)

just seal around any penetrations in the ceiling. Then just add regular Insulation to your local code requirements. You have to ask your self this question how much air can blow through sheetrock? answer zero. A lot of this "air sealing" is Green aka Green backs for the contractors.The largest areas for air infiltration is around doors windows, bottom plates, penetrations for duct work, plumbing and electrical. seal those areas insulate and you save money in more ways then one.


----------



## strategery (Jul 18, 2011)

Sounds like everyone thinks I should skip the rigid boards. I was just thinking it would be a no cost way for me to get a lot of r-value. I already have the boards and they're not being used.

Also, I'm first sealing everything - joints, penetrations with canned foam so it should be airtight. The idea of rigid boards first was just for r value.


----------



## AndyGump (Sep 26, 2010)

Use the foam but I am not too sure about the plastic liner on them you mentioned.
What is that all about?
If it is there I would take it off as I have no idea what climate you are in and I am not a big fan of vapor barriers in walls and ceilings except in certain locations, climate zones.

Andy.


----------



## Nailbags (Feb 1, 2012)

strategery said:


> Sounds like everyone thinks I should skip the rigid boards. I was just thinking it would be a no cost way for me to get a lot of r-value. I already have the boards and they're not being used.
> 
> Also, I'm first sealing everything - joints, penetrations with canned foam so it should be airtight. The idea of rigid boards first was just for r value.


The Mud and tape on the Sheetrock make it air tight. your just wasting time and money doing what your doing.


----------



## Windows on Wash (Aug 30, 2011)

Nailbags said:


> just seal around any penetrations in the ceiling. Then just add regular Insulation to your local code requirements. You have to ask your self this question how much air can blow through sheetrock? answer zero. A lot of this "air sealing" is Green aka Green backs for the contractors.The largest areas for air infiltration is around doors windows, bottom plates, penetrations for duct work, plumbing and electrical. seal those areas insulate and you save money in more ways then one.


Correct.

Solid section of drywall aren't leaking air and are, by definition, air barriers. 

Sealing up top plates absolutely does make and impact and we can verify this via blower door testing. 

The other areas you mention (i.e. duct boots, plumbing vents, electrical penetrations) are also big leak points.

Some homes are completely devoid of those items you mention and will still benefit considerably from top plate sealing.


----------



## Nailbags (Feb 1, 2012)

Windows on Wash said:


> Correct.
> 
> Solid section of drywall aren't leaking air and are, by definition, air barriers.
> 
> ...


the top plates are sealed by the sheet rock mud and tape. Proven fact and is not required to pass a blower door test.


----------



## Windows on Wash (Aug 30, 2011)

If that is the case then why can we verify via blower door and smoke pencil.

If there's no leakage at the top plates wide of interior wall show dust tracking and dirt tracking along the baseboard?

Why is it then can we pull fiberglass insulation back from a top plate and clearly see the evidence of dirt tracking and dust accumulation in the fiberglass specifically aligning with the top plate and the drywall mud joint?

Are we to assume that the lumber is the same dimension on the day that it is installed after several years of drying out? 

Sorry to be disagreeable I have personally witnessed Blower door test in and test out data where homes have just had the top plates are sealed and there is tangible, verifiable, and observable, airtightness improvement.

Plenty of folks who previously had ice damming issues from poorly sealed attic can also verify the benefit of top plate air sealing.


----------



## HomeSealed (Jan 3, 2008)

1) Unless it is tied to a program that is offering rebates or incentives for hitting certain performance numbers, a blower door is not a "pass/fail" test. 

2) Top plates most certainly are significant sources of airleakage. Yes, the ceiling to wall joint in the sheetrock is usually sealed, however top plate air leakage typically originates at outlet and switch plates, along with other penetrations in interior walls. Top plates along with all other penetrations to the attic should be sealed.

3)In this case, the xps board is not a big deal one way or the other. It will give you more r-value per inch, however it wouldn't negate the need for air sealing and probably isn't worth the hassle in this case.

4) The incinuation that air sealing is some type of money grab is way off-base. This is generally a fairly inexpensive procedure and returns a major bang for the buck. This is all verifiable fact.
Further, this is probably one of the worst home improvement jobs available. Who wants to crawl around in an attic full of God only knows what all day, everyday?... There is plenty of "snake oil" for sale when it comes to home improvements and energy efficiency, but air sealing is not that.


----------



## strategery (Jul 18, 2011)

The baseboard trim can also leak air through the wall and the bottom plates.

Everything WoW and HomeSealed said about air sealing is true. I had a blower door done on my 100 year old house before these measures and it leaked almost 19 ach per hour at 50 cfm. Now I'm down to about 6. I've also done some other huge improvements including removing an unused masonry chimney and replacing some old windows.

A blower door combined with a thermal imaging camera reveals a surprising amount of air leakage in places you don't expect it.


----------

