Follow-up Questions – Options for Sealing Foundation Radon Barrier
idahobuild
| Posted in General Questions on
Hello All,
I am working to finalize the Air Barrier for the our new construction house.
Slab on grade foundation, 2065 sqft. in zone 5 U.S.A.
After much GBA and internet reading I came up with the following (also see atch’d image):
1. 6 or 10 mil poly vapor/radon barrier on top of the 2 inch XPS foam (R10) and under the 4″ concreate slab.
2. poly attach at bottom of OSB sheathing (OSB not in the image – forgot it sorry)
3. Prosoco MVP Air/WRB applied to OSB and over the poly at bottom of OSB
3.1. No interior vapor barrier (R10 XPS keeping condensation point away from interior walls — per table in code)
4. Sigo Majrex (or similar) over exterior top plate – connecting wall A/WRB to interior ceiling.
5. Ceiling sheet membrane (Intello or similar as air barrier under truss cord and above Gypsum Board sealed to top plate Majrex .
6. Vented attic with 1″ XPS baffles at top cord to direct air from soffit over the top of the insulation at the 16″ raised hip/heal roof trusses.
QUESTIONS:
1. Is it recommended to seal the slab’s poly barrier at exterior wall (to WRB). If not, where should that seal happen?
2. I am thinking thinking either Stego Crete Claw tape or similar to seal the foundation slab’s poly barrier at the exterior to the exterior WRB. A search of GBA details/QA didn’t turn up much. Any thoughts on alternative or more readily available options?
Thanks All!
WD
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Replies
It is not necessary to make the poly continuous from under slab to walls. Because your concrete foundation is air tight, you can seal the under slab poly to it, and also seal your wrb/framing to it. This will make the foundation a part of the continuous air barrier, just the sheet materials will not be continuous.
Plumb - that is a great point. Please see atch'd detail. The blue is the proposed seal location that makes sense to me given the clarification that you've provided. Does that seem like a viable location to seal the poly to the foundation's stem wall? Thanks.
Your original sketch shows the slab terminating below the top of your foundation walls, so if you try to bring your poly up on top of the foundation walls the corners will be all bunched up and quite hard to detail correctly. I often see the poly sealed with acoustic sealant to the side of the foundation walls with the sealant bead somewhere mid depth of the slab, and then any poly that sticks out above the slab can be trimmed off.
The materials and locations can be changed as long as the process and result are thought through. I had a great teacher that said anything can be part of the air barrier if detailed correctly- 2x4, concrete, glass, etc.
You should also look at constructability. The poly is often moved around and beat up by the concrete guys that are pouring the slab, something to think about.
If you have not already done it, go to Building Science Corp. website, they have tons of free articles and will cover this detail well. I know they often show a bead of sealant where the top of the slab intersects the foundation wall, I have not done this but in theory should work, especially if done in conjunction with the poly detail as above.
Thanks Plumb...I've looked on Bldg Science and DOE sites. I got some good info., on multiple topics, from the sites. I just hope I'm properly interpreting it.
On the 'constructability' front, most of the finicky details of the design I'll be doing myself. I hope to get these details set and then ensure that I work slowly and methodically to get them done right.
I posted Pt2 of this question in the QA for consideration and feedback.