Remodeling the office (new floor, ceiling & adding insulation) passive house retrofit - Page 6

MN Renovator · 03-28-12, 02:19 PM

"But at any rate, it will be fine as long as you don't put foam on the outside of the sheathing. then your sheathing would be sandwiched between vapor impermeable materials"

Uhh oh. I've got a house built in the mid 80's with 3/4" foam(pretty sure it's XPS or Polyiso, it's a white smooth board, the brand is R-Max) on the outside over the sheating and it has 2x4 construction with fiberglass in the inside with a vapor barrier against the drywall. This vapor sandwich sounds like exactly what you are warning against but it seems my entire neighborhood was built this way and I haven't heard of any issues. Are we all just lucky?

S-F · 03-28-12, 02:23 PM

Yes, you are just lucky. That won't pass code on a new building. The house won't be as durable as one with only 1 vapor barrier. And you never know. Many of those houses could have rotting or moldy sheathing under the foam boards. They won't know until either the house fails in some very noticeable way, or the foam gets removed or the wall opened up.

Daox · 03-31-12, 01:17 PM

Almost forgot about this thread. I was going to mention that I do have 1/2" of xps on the outside of the house already under the siding. Are you saying my only option (if I leave the xps in place) is to use all cellulose on the inside?

I've been thinking about this a lot recently and I do have some places (mainly the upstairs) where I don't want to loose much floor space. The rooms are already on the very small side upstairs and it would be great to add insulation to the outside at some point.

S-F · 03-31-12, 01:20 PM

Quote:

Originally Posted by Daox

Are you saying my only option (if I leave the xps in place) is to use all cellulose on the inside?

Yes. aside from the fact that it's not a safe idea the code won't allow it. If you already have foam on the outside you shouldn't need to worry about air sealing inside. Let the foam be your air barrier.

Daox · 03-31-12, 01:31 PM

Well, I highly doubt its done so things are airtight. Is foam normally taped once its hung on the outside to make it airtight?

S-F · 03-31-12, 01:34 PM

It is definitely taped. I would just dense pack your walls now and in the future put 3" of polyiso on the outside of the XPS. Don't worry about air sealing the existing layer. Just stagger the seams with the new foam and tape the outer layer. Tape it really well.

Daox · 03-31-12, 01:44 PM

I didn't think polyiso was good to use on the outside due to the fact that it absorbs moisture? I've heard of people using it though.

S-F · 03-31-12, 01:51 PM

You don't want to fill earth over it. It's usually what people use for this kind of outsulating. The foil faced sort can be taped with foil duct tape and provides a fantastic water barrier. When I do this to my house this summer I'll be putting 1 layer of the 2.5" fiber faced stuff I used in the basement on and then a layer of 2" foil faced over it to air seal. Some people do use XPS and EPS but there are issues with those materials in this application. 1, lower R / ". 2, it's pretty hard to get any tape to stick to it long term. There was recently a BSC article where they noticed their building was leaking heat even though they had put some crazy amount of EPS on the outside. When they opened up the building they found that the EPS had changed shape and the mastic they used (because you can't really use any tape on EPS/XPS) had broken and not moved with the expansion and contraction. You just shouldn't try to outsulate your foundation with polyiso or put it under a slab.

Daox · 03-31-12, 02:25 PM

Alright, I guess that clears all that up, thanks.

NEXT!

About the dew point being in the middle of the wall... How do you go about calculating where the dew point is going to be?

S-F · 03-31-12, 02:30 PM

Just make sure you have at least 60% of your R value in the foam on the outside. Calculating where the dew point will be can be tricky. I don't have the statistics on the top of my head but as you add more insulation it has diminishing returns. So it's hard to calculate where a dew point will be at a certain temperature. There might be a table somewhere which lists dew points for specific R value walls at different temperatures and humidity levels. I don't have one though.

03-31-12, 01:17 PM	#53
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Almost forgot about this thread. I was going to mention that I do have 1/2" of xps on the outside of the house already under the siding. Are you saying my only option (if I leave the xps in place) is to use all cellulose on the inside? I've been thinking about this a lot recently and I do have some places (mainly the upstairs) where I don't want to loose much floor space. The rooms are already on the very small side upstairs and it would be great to add insulation to the outside at some point. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

03-31-12, 01:31 PM	#55
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Well, I highly doubt its done so things are airtight. Is foam normally taped once its hung on the outside to make it airtight? __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

03-31-12, 01:44 PM	#57
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	I didn't think polyiso was good to use on the outside due to the fact that it absorbs moisture? I've heard of people using it though. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

03-31-12, 02:25 PM	#59
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Alright, I guess that clears all that up, thanks. NEXT! About the dew point being in the middle of the wall... How do you go about calculating where the dew point is going to be? __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

03-28-12, 02:19 PM	#51
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 940 Thanks: 41 Thanked 117 Times in 91 Posts	"But at any rate, it will be fine as long as you don't put foam on the outside of the sheathing. then your sheathing would be sandwiched between vapor impermeable materials" Uhh oh. I've got a house built in the mid 80's with 3/4" foam(pretty sure it's XPS or Polyiso, it's a white smooth board, the brand is R-Max) on the outside over the sheating and it has 2x4 construction with fiberglass in the inside with a vapor barrier against the drywall. This vapor sandwich sounds like exactly what you are warning against but it seems my entire neighborhood was built this way and I haven't heard of any issues. Are we all just lucky?