Cathedral Ceiling Vent-Barrier-Insulation Question

DrMo · 01-08-2012, 11:14 AM

Newbie Here...

I am retrofitting a ranch home in Michigan. Any comments on using a double layer of rigid foam board in a Cathedral Ceiling design as described in the link to the following article? -

http://www.homesandcottages.com/Arti...x?ArticleId=41

I appreciate any comments as I have a very similar situation - I am using 2" rigid foam board with a 1" vent channel from eaves to ridge between every rafter. Then I plan to add batting below the foam.

I am wondering about if/how to seal the joints and seams around the foam board and whether to add more rigid foam board below - and/or a plastic vapor barrier.

I am concerned about the possibility of creating a "vapor sandwich" between the bottom layer of foam and/or plastic and the top layer of foam, with batting in-between.

Additionally, would there be any problem with adding multiple layers of rigid foam directly on top of one another? Would this create a possibility of condensation in-between?

Finally, in using a plastic barrier, what products should I use to seal the overlaps and outer edges?

thanks for your comments! DrMo

**Gary in WA** · 01-09-2012, 11:25 PM

Welcome to the forums!

I couldn't get your link to open.....

Does this cover it?: http://www.buildingscience.com/docum...n-roof-venting

Gary

Windows on Wash · 01-10-2012, 09:43 AM

Quote:

Originally Posted by DrMo

Newbie Here...

I am retrofitting a ranch home in Michigan. Any comments on using a double layer of rigid foam board in a Cathedral Ceiling design as described in the link to the following article? -

http://www.homesandcottages.com/Arti...x?ArticleId=41

I appreciate any comments as I have a very similar situation - I am using 2" rigid foam board with a 1" vent channel from eaves to ridge between every rafter. Then I plan to add batting below the foam.

Don't bother with the vent channels. Make your own out of strips of 1-2" foam. What is the pitch of the roof?

Make sure that the eave and ridge openings are screened to keep the bugs out as well.

I am wondering about if/how to seal the joints and seams around the foam board and whether to add more rigid foam board below - and/or a plastic vapor barrier.

Seal the seams with sealant and/or foam. Foam board will have a higher (XPS and ISO) R-Value per inch so you will get more out of that assembly with foam than you will with fiberglass etc.

Adding a layer of rigid foam to the interior side of the rafters is a great idea because it helps eliminate the thermal bridging of the structure.

Even better is to put the rigid foam to the roof layer but obviously that requires removal of the roof.

I am concerned about the possibility of creating a "vapor sandwich" between the bottom layer of foam and/or plastic and the top layer of foam, with batting in-between.

Use non-vapor barrier (i.e. non foil faced or non poly faced) foams to the outside edge so that it can dry to the vent space and make sure that your air barrier detailing is spot on to the inside.

Additionally, would there be any problem with adding multiple layers of rigid foam directly on top of one another? Would this create a possibility of condensation in-between?

Not if they are sealed up properly.

Finally, in using a plastic barrier, what products should I use to seal the overlaps and outer edges?

If you are using rigid foam to the interior, use a product that has the poly facing and skip the plastic. Acoustical sealant is easy to work with if you go the plastic route. You can also just use a vapor retarder paint when you re-drywall.

thanks for your comments! DrMo

see above...

DrMo · 01-11-2012, 11:36 AM

Thanks for both comments - so far - still would like to hear other thoughts as well...

Gary (GBR in WA) - I am sorry you could not get the link to open - it works for me... Perhaps if you copy and paste into your browser??? The article you provided does not seem to address my situation - not an attic or unvented cathedral roof. Thanks for your efforts!

Windows on Wash - thanks for your detailed response!

I don't know the exact roof pitch, but it is not an extreme slope, typical of a single-story ranch home - very easy to walk on. Good news - I have already installed my own hand-made strips - not the manufactured vent channels.

Can you tell me more about the "product that has the poly facing and skip the plastic" ? Is there a name brand? Finally, where can I purchase the acoustic sealant? Is there a name brand for that as well that you can recommend?

Thanks so much for your input! DrMo

shazapple · 01-11-2012, 12:03 PM

Windows on Wash has it right. If you go with a thin enough rigid insulation it wont act as a vapour barrier, then you could fill the space with fiberglass and then cover everything with rigid insulation as WoW mentioned. Sometimes you can find (I haven't seen it around here) rigid with a plastic or foil face that acts as a vapour barrier, then you only need to tuck tape the seams. If you go with the 6mil polyethelene sheet, acoustical sealant is a terrible black sticky gooey stuff that will stick the perimeters of the sheet to most surfaces. It can be found at any big box store.