Another Hole(s) in the… Wall
My Northern zone 5a home can’t be a Passive House (PH) because it was built in the 50’s. But I’d guess that imitating PH practices where possible as projects are done will help the house be the best it can without a gut rehab.
We’re putting in a 95+ efficiency furnace in the basement and the HVAC contractor will put a PVC combustion air intake and PVC combusted air exhaust through the 8″ brick wall a few inches above the concrete foundation (I imagine each will be about 4′ long). Not knowing enough to ask for specifics, I voiced a complaint about cold radiating radiating from the intake pipe and he said he’d wrap both pipes in bubble wrap insulation from the furnace to the brick. The current plan is to make each hole through the brick as small as it can be and still fit the PVC and then seal the outside and inside with expanding foam, which will expand into some of the 8″ between.
My question is, if you were making two plastic pipe penetrations in a PH retrofit to a historic building (this one isn’t, but the brick on the outside can’t be covered over with a new PH insulated shell), how would you do this?
With respect to the wall, is the plan above (which probably involves foam voids and PVC-to-brick thermal bridges) as good as it gets or would it be better to make a larger hole such that insulation can be put around the pipe so that 1) those 8″ are insulated and 2) they are thermally broken from the brick (again in this case spray foam would seal the outside and the inside)? The contractor is against larger holes because he thinks it likely the insulation will break down eventually and allow more air infiltration.
Is bubble wrap insulation along the 8′ length (two 4′ PVC pipes) between the brick and the furnace as good as it gets?
Is there a better sealant for this job than spray foam (I’ve read it develops cracks over time, but we can’t very well use high quality primer and tape on the decorative brick outside)?
Seems like the following Building Science Corp/Dep’t of Energy comment would apply. Do people agree? “Penetrations through building enclosure elements that also perform rain water management functions must be properly flashed. It is critical that airs sealing not interfere with drainage”
Other ideas? Maybe it would help to think about how fresh air intakes or plumbing penetrations in real PH retrofits where the exterior brick stays visible are put through the brick walls?
Thanks you for your time.
Gordon
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Replies
Gordon,
First of all, make sure that the installation meets the recommendations of the furnace manufacturer. All venting systems comes with instructions, including instructions about clearances and whether or not the pipes can be insulated.
Assuming you've done that, don't worry. You want to minimize air leakage, and you clearly understand that. Other than that, the heat loss associated with the furnace vent is small and unavoidable.
I would use the smallest furnace, shortest PVC, and the smallest diameter allowed by the manufacturer. This will probably give you 1.5" PVC on both sides. If possible, I would consider using a concentric kit to vent through the roof, rather than the wall, which most equipment manufacturers prefer. If you can't vent through the roof, use either a concentric kit at the wall, or a termination kit. For hole diameter just follow the directions of the manufacturer of the termination kit.
Flashing is typically installed in the plane of the weather-resistive barrier. So good luck with that. I also wouldn't worry about the bubble wrap thing. Save your money and ask the installer to put turning vanes at the top of the supply plenum and bottom of the return drop instead.
Thanks Martin and Jesse.
I'm off to read the installation directions and do a few searches on "high efficiency furnace concentric (or termination) kit" as I'm not sure what these are.
Of the three estimates we got, this is the smallest BTU furnace and A/C, so hopefully we're doing as well there as possible. And we already planned to move the new furnace, which will be much smaller, as close to the wall as possible (to create space in the basement, it hadn't clicked that this will help somewhat with thermal issues).
A bit baffled by the "turning vanes" - basically valves to block the airflow in the supply, return, or both? I'm sure it's completely obvious to you, but assuming my "valve" interpretation is about right, what would I turn off when to save energy?
Sorry to be dense. Thanks.
Equipment size is really malleable. It doesn't matter that you got 3 estimates for about the same size equipment. I deal with dozens of similar estimates and Manual J's every week, and they are nearly always over-sized garbage. A better way to approach the problem is to specify a very small unit - eg a 97% AFUE, stage furnace with 40kbtu/hr input. Then change the characteristics of the house so that the furnace will satisfy the heat loss of the house. Better yet, make it so that the unit satisfies the house on 1st stage (~25kbtu/hr). If you need to make shell changes gradually and want to be conservative, use a fully modulating 60kbtu/hr furnace that starts around 30%.
Turning vanes fix bad airflow, they don't restrict it. Once you've got a very small, high efficiency furnace the next equipment trick is reducing blower motor energy while effectively distributing air. Turning vanes, curved inside radius trunk ducts, over-sized filter openings, and low equivalent length fittings all help this.
http://www.acca.org/speed-sheets/manuald.xls
Martin and Jesse,
Thanks for the advice. The project is almost finished, Martin I'm sure the guys are sticking to the book in installing it, Jesse we did go with the concentric kit...
Emergency Sealing Question... So we'll soon have two holes. One with Armorflex and naked A/C lines through it and two low voltage lines (about 3.5" diameter very rough oval) and the other being the PVC intake/exhaust (about a 5" diameter very rough circle. The two holes are through 8.5" of brick and mortar.
The guys want to use cement or Great Stuff expanding foam to seal the holes. I think cement can crack and I know foam will crack, on the other hand, it will fill much of the 8.5" depth. A sealant will not crack but we'd only apply it at the thinnest joint we could access inside and outside on each penetration - most of the 8.5" gap (most interior to the seal and some exterior to it) would not be sealed or insulated.
Maybe those gaps along the penetration don't matter as long as there are two well sealed planes (inside and outside) or is it better to more or less fill all gaps through the 8.5" with cement or Great Stuff even though it may crack over time?
Please offer advice soon, thanks. And sorry to apply pressure.
Gordon
Gordon,
I think that you will be fine if you fill most of the gap with expanding canned spray foam. Once the foam has cured, you can use a bread knife to clean up the excess, and rake out the foam enough to seal the exposed foam on each side with silicone caulk.
Martin, thank you for the quick answer. I'll let them fill what they want with the foam and after it has dried will carve it flat and seal with silicone caulk.
I do need some clarification, though not on a rush basis. At the irregular areas near the surface where the face of the brick broke off, the spray foam may span from the surface of the tube(s) penetrating the brick out two to three inches until it meets the plane of the inside or outside wall. I'll trim the foam to these planes. But then, are you saying to put a thin layer of silicone caulk covering all the exposed foam (and bonding to a 1/4 inch or so of brick) from the pipe(s) to the normal brick surface - sort of like a cap over a few inches of material than a bead between two surfaces? Again no rush on this answer, and thanks again.
Gordon
Gordon,
If you want a slick-looking way to disguise the brick damage, finish the exterior with a stainless-steel escutcheon. The escutcheon can be bedded in silicone caulk.
Here is a link to a supplier of stainless-steel escutcheons:
http://www.awimfg.com/products/components/wall-flanges/
If you want to find more suppliers, just Google "stainless-steel eschutcheon."
One final comment: I'm not really sure that the escutcheon is necessary, since most sidewall venting kits include some type of exterior trim at the termination.