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Insulating an addition
Last Post 06 Dec 2011 03:59 PM by Dana1. 16 Replies. |
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jkieffer
New Member
Posts:18
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| 28 Nov 2011 10:40 AM |
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We are adding onto our home by enclosing an existing screened-in porch and making it part of the conditioned space. I have a few questions about insulation.
Background: 2 story home with basement, climate zone 4 (inland, not coastal). The existing screened in porch was added by the previous owner. It is less than 10 years old. It is built on a slab with the proper footers, and it has its own roof. It has 4x4 posts for supports, and they are spaced about every 4 feet. It has a vaulted, uninsulated ceiling under the gable roof.
With the walls, I plan to add studs between the 2x4 posts, then sheet the exterior with OSB, if necessary. I want to add 1.5" EPS to the exterior, then batts between the studs. Can I put siding directly over EPS of that thickness, or do I need furring? I really want to avoid furring for cost and time reasons. Furring is not typically used in my area (in fact, I've never seen anyone use it on the exterior of a home around here), so I am not concerned about quality or durability issues. Also, should I use foil faced EPS? I can get faced or unfaced for a relatively small cost difference.
I have two thoughts about the ceiling:
1. I can frame out a flat ceiling and blow in insulation; or
2. I can blow insulation into the existing space between the rafters and then add some foam board to the underside of the vaulted ceiling to get the total R up to code. I presume I would have to use a couple of inches of iso, which is fine. Then, I would need to drywall over that.
Choice #1 is relatively straight forward. I don't have any concerns about that. But, if my wife wants to maintain the vaulted ceiling, is #2 an option? Are there drying/moisture issues with that approach? Adding the foam board above the rafters is not an option. I am not going to tear off the roof to add foam to the top side.
Insulating the slab: I am going to install floor joists over the slab to match the floor level of the home. I will have 6-8" between the slab and the bottoms of the joists. Because there will be plumbing and mechanicals between the joists, I want to make sure that the area is insulated. The exterior foam and batts on the walls will go all the wat to the slab, so that will insulate the exterior walls. I am thinking about adding a couple of inches of EPS foam board directly on top of the slab. Is that acceptable? Should I use foil faced foam board?
Thanks in advance for the help. |
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Dana1
Senior Member
Posts:6991
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| 28 Nov 2011 04:06 PM |
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If you try to nail/screw the siding through the 1.5" of EPS into the OSB you'd be looking at some pretty long nails/moment-arm for the siding to hang on. Over time it would sag, leak air, and 1001 nails adds up to a considerable thermal bridge to boot. If you used furring and through-screwed it to the studs 24" on center the it would be more mechanically secure, wouldn't sag, and would have lower thermal bridging from fasteners. It's not rocket-science.
wallboard | studs(batts) | OSB sheathing | EPS | furring | siding
If just nail through the EPS (yes, people have done this) you'd get about zero benefit out of an aluminum facer. With the furring approach, the facer can deliver pretty-good radiant barrier effect during the cooling season, and can add up to another ~ R1-2 or so during the heating season. If you use foam with facers, you're better of using unfaced batts between the studs. But it would be better to use blown goods rather than batts, to eliminate all voids, especially since you'll be looking at non-standard stud spacings & cavity widths, making perfect batt installation very laborious indeed. With 1.5"/R6 of EPS on the exterior of a 2x4 studwall it's perfectly safe to go without interior vapor retarders/facer/poly etc, and it is in fact preferable- nothing more vapor retardent than standard latex paint would give the wall maximum drying capacity toward the interior, which is the way to go. With foil facers on the exterior is has zero drying capacity to the exterior.
On the slab 2" of EPS (R8) would be fine, so long as the walls of that micro-crawlspace are perfectly air-tight. Foil facers are fine, and would add a tiny amount of performance, but aren't as useful here as they might in walls/roofs (when properly applied, with real air-gaps.)
To blow fiber between the rafters you need to put foam board ABOVE the roof deck, not below, and it needs to be R15 or more in zone 4A & B (4C== coastal). But since you're not going to do that, you'd need to put 1-2" of closed cell polyurethane (it would take more like 2.5" to be code-legal, but 1" is actually enough to be protective in zone 4), at which point you could fill in the rest of the R with fiber, using only standard latex as the interior vapor retarder. If you want to then use EPS as an interior thermal break on the rafters that can work, but it needs to be semi-permeable, and foil-faced iso is IMpermeable. You might consider adding 2x4s laterally on the interior side of the roof "Mooney Wall" style to achieve some thermal break and get you to code on center-cavity R. But without some closed cell foam on the interior of the roof deck it will get punky and rot over time. If you use rigid foam rather than the Mooney wall approach you can go as thick as 4-5" with unfaced EPS (R16-20), but only 2"(R10) with XPS. (The cc foam at the roof deck can be a DIY project using a kit- a 600 board-foot kit runs $700, and would cover ~300 square feet of roof deck ~ R12. The 200 board-foot kits are about half the price, but less than half the coverage.)
http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_8_sec006_par003.htm
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
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Farmboy
Basic Member
Posts:356
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| 29 Nov 2011 01:12 AM |
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Floor. I highly recommend insulating and using a foil radiant barrier.
We built out an efficiency apartment inside part of a metal building over a 5" concrete slab that was not insulated below. We framed the floor with ripped 2x6s on edge 24" oc and plastic moisture barrier underneath. Then 2" pinkboard between "floor joists" and heavy aluminum foil (radiant barrier) adhered over the joists and foam. That leaves a 3/4" airgap under most of the 3/4" floor deck. Finished off with stick together vinyl type flooring. Last winter in when it was below 0F outside, 36F inside the building and 68 inside the room, the floor was comfortable and didn't suck the heat outta your feet like the concrete floor outside the room. Dave |
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Dana1
Senior Member
Posts:6991
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| 29 Nov 2011 11:11 AM |
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Posted By Farmboy on 29 Nov 2011 01:12 AM
Floor. I highly recommend insulating and using a foil radiant barrier.
We built out an efficiency apartment inside part of a metal building over a 5" concrete slab that was not insulated below. We framed the floor with ripped 2x6s on edge 24" oc and plastic moisture barrier underneath. Then 2" pinkboard between "floor joists" and heavy aluminum foil (radiant barrier) adhered over the joists and foam. That leaves a 3/4" airgap under most of the 3/4" floor deck. Finished off with stick together vinyl type flooring. Last winter in when it was below 0F outside, 36F inside the building and 68 inside the room, the floor was comfortable and didn't suck the heat outta your feet like the concrete floor outside the room. Dave
Putting the pink board below the floor joists rather than between them would be roughly 50% more effective, since you would then have eliminated the thermal briding of the wood. With the pink board between the joists you left 2" of wood as thermal bridging, reducing the "whole assembly" R to ~R7.5 from a potential of R10 (the 2" pink board.) Laying down the subfloor floated directly on the foam (no joists or sleepers, and no air gap) would be warmer, and use less vertical space. The compressive strength of XPS is high, and even 1/2" t & g OSB sheathing distributes the weight sufficiently well to allow residential type floor loading without deforming the foam, but using standard subflooring rated for 16" o.c. joists would be stiffer still. The bulk of the insulative value was still the pink-board. At the relatively low temperature differences between the slab and the RB, the effectiveness of the RB in that stackup was ~R1, on the coldest day of the year. RB is more useful on sun-exposed walls & roofs where peak delta-Ts can be much higher. If the RB isn't perforated and the air-gap between the RB & foam isn't vented, by putting down vinyl flooring you've installed a moisture trap between the RB and the vinyl floor- any moisture that finds it's way in may take decades to get out. Vinyl flooring & wallpapers (as well as foil wallpapers) are unintended vapor barriers, that sometimes come often with unintended consequences. |
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jkieffer
New Member
Posts:18
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| 29 Nov 2011 12:10 PM |
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Is there any cheaper way to insulate the ceiling without spray foam? It's not a particularly large addition (220 sq ft +/-), and I don't want to spend $1000 insulating the ceiling alone. I'm quite certain that sprayfoam is not used in the vast, vast majority of cathedral roofs in this area. What are my other options? |
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Dana1
Senior Member
Posts:6991
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| 29 Nov 2011 12:31 PM |
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Posted By jkieffer on 29 Nov 2011 12:10 PM
Is there any cheaper way to insulate the ceiling without spray foam? It's not a particularly large addition (220 sq ft +/-), and I don't want to spend $1000 insulating the ceiling alone. I'm quite certain that sprayfoam is not used in the vast, vast majority of cathedral roofs in this area. What are my other options?
You can insulate the ceiling if you leave 1" of vent space between the roof deck and the top of the insulation. If you cut some spacers out of 1" XPS or unfaced EPS to use as standoff strips for cut'n'cobbled the rigid foam between the rafters (air-sealing the edges to the rafters with 1-part foam) you could then use and amount of cellulose or fg below the foam with abandon. You'd need to drill at least some soffit venting to keep the air in the 1" cavity at the same humidity as the outdoor air. You can then hit any arbitrary R-value with low thermal bridging using a Mooney-Wall type structure on the interior of the rafters. There are some who will advocate dense-packing cellulose unvented, (the folks at National Fiber advocate it even for zone 5 ) so long as the density is 3.5lbs per cubic foot or greater at thickness of 9" or more utilizing the moisuture buffering capacity of the cellulose to protect the roof deck, but that's a code violation, and still puts the roof deck at some risk in zones 4 & up (maybe even zone-3). Venting the roof deck is a safer from a long term point of view. |
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rbisys1
Basic Member
Posts:142
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| 29 Nov 2011 01:22 PM |
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Greetings,
QUOTE> With the walls, I plan to add studs between the 2x4 posts, then sheet the exterior with OSB, if necessary. I want to add 1.5" EPS to the exterior, then batts between the studs. Can I put siding directly over EPS of that thickness, or do I need furring? I really want to avoid furring for cost and time reasons. Furring is not typically used in my area (in fact, I've never seen anyone use it on the exterior of a home around here), so I am not concerned about quality or durability issues. Also, should I use foil faced EPS? I can get faced or unfaced for a relatively small cost difference.
Your best insul material is reflective insulation (RI). A two layer material will do. SEE fifoil.com It's 97% eff and does not have any of the inherent problems the materials you suggest have.
IF you do use your suggested method, putting foam on the out side of the house could cause more condensation in the FG as you are using a low perm covering. Foil covered foam would be worse.
You can just about eliminate the energy transfer thru the studs by covering the interior surface with an additional single sheet RI material, perforated, and 1/2'" drywall "Z" strips attached to the studs, over the RI, and then drywall. The dry wall panels would be installed vertically instead of horizont'. This will give you about 2 btu/hr/sf under worst conditions. The elec boxes have to be brought out an add'l 1/2" plus you may have to furr out around doors, etc. You can't get better as this sys will give you a 2btu/hr/sf performance under the worst conditions.
I have two thoughts about the ceiling:
1. I can frame out a flat ceiling and blow in insulation; or
2. I can blow insulation into the existing space between the rafters and then add some foam board to the underside of the vaulted ceiling to get the total R up to code. I presume I would have to use a couple of inches of iso, which is fine. Then, I would need to drywall over that.
Vaulted ceilings are the worst for transmitting energy, especially thru the rafters. Using the same method as the walls will be easier and more efficient. The only difference is that you use 7/8" steel, fur'g strips, primed. , Or if you want a wood ceiling use 2x2 furr strips. You also want to install a good ridge /soffit vent sys, see: cor-a-vent.com
QUOTE>Insulating the slab: I am going to install floor joists over the slab to match the floor level of the home. I will have 6-8" between the slab and the bottoms of the joists. Because there will be plumbing and mechanicals between the joists, I want to make sure that the area is insulated. The exterior foam and batts on the walls will go all the wat to the slab, so that will insulate the exterior walls. I am thinking about adding a couple of inches of EPS foam board directly on top of the slab. Is that acceptable? Should I use foil faced foam board?
Lay a double 4 mil plastic VB. lay one layer one way and the across. The plastic should go up the sides and be attached to the side of the top plate. Lay a single RI sheet over the VB and it should be attached to the side of the top plate. This will easily out perform the foam.
You will probably not have to increase the heating unit as this sys will not use any more energy than the amount of energy you now lose thru the existing exterior wall. To heat the room you can supply heated air brought to a hole cut in the foundation and the space is used as a distribution to out lets in the perimeter.
Codes. If you do not hook up heat to the room right away you can probably circumvent the ridiculous
code "R" requirements. Just claim it's an enclosed porch for summer use. AND, keep the glass area to a min.
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WindowsonWashington
New Member
Posts:96
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| 29 Nov 2011 07:29 PM |
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Posted By Dana1 on 29 Nov 2011 12:31 PM
Posted By jkieffer on 29 Nov 2011 12:10 PM
Is there any cheaper way to insulate the ceiling without spray foam? It's not a particularly large addition (220 sq ft +/-), and I don't want to spend $1000 insulating the ceiling alone. I'm quite certain that sprayfoam is not used in the vast, vast majority of cathedral roofs in this area. What are my other options?
You can insulate the ceiling if you leave 1" of vent space between the roof deck and the top of the insulation. If you cut some spacers out of 1" XPS or unfaced EPS to use as standoff strips for cut'n'cobbled the rigid foam between the rafters (air-sealing the edges to the rafters with 1-part foam) you could then use and amount of cellulose or fg below the foam with abandon. You'd need to drill at least some soffit venting to keep the air in the 1" cavity at the same humidity as the outdoor air. You can then hit any arbitrary R-value with low thermal bridging using a Mooney-Wall type structure on the interior of the rafters.
There are some who will advocate dense-packing cellulose unvented, (the folks at National Fiber advocate it even for zone 5 ) so long as the density is 3.5lbs per cubic foot or greater at thickness of 9" or more utilizing the moisuture buffering capacity of the cellulose to protect the roof deck, but that's a code violation, and still puts the roof deck at some risk in zones 4 & up (maybe even zone-3). Venting the roof deck is a safer from a long term point of view.
+1 Have you considered an overdeck venting approach? Best of both worlds in my opinion. Completes the thermal break and allows you to use less expensive rigid boards to the outside surface. They make a poly iso nail base as well that will save you a step. |
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Farmboy
Basic Member
Posts:356
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| 29 Nov 2011 07:39 PM |
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Posted By Farmboy on 29 Nov 2011 01:12 AM
... We framed the floor with ripped 2x6s on edge 24" oc and plastic moisture barrier underneath. Then 2" pinkboard between "floor joists" and heavy aluminum foil (radiant barrier) adhered over the joists and foam. That leaves a 3/4" airgap under most of the 3/4" floor deck. Finished off with stick together vinyl type flooring. ... Dave
Dana1. "...pink board below the floor joists rather than between them would be roughly 50% more effective, since you would then have eliminated the thermal briding of the wood. With the pink board between the joists you left 2" of wood as thermal bridging, reducing the "whole assembly" R to ~R7.5 from a potential of R10 (the 2" pink board.)
Laying down the subfloor floated directly on the foam (no joists or sleepers, and no air gap) would be warmer, and use less vertical space. The compressive strength of XPS is high, and even 1/2" t & g OSB sheathing distributes the weight sufficiently well to allow residential type floor loading without deforming the foam, but using standard subflooring rated for 16" o.c. joists would be stiffer still.
The bulk of the insulative value was still the pink-board. At the relatively low temperature differences between the slab and the RB, the effectiveness of the RB in that stackup was ~R1, on the coldest day of the year. RB is more useful on sun-exposed walls & roofs where peak delta-Ts can be much higher.
If the RB isn't perforated and the air-gap between the RB & foam isn't vented, by putting down vinyl flooring you've installed a moisture trap between the RB and the vinyl floor- any moisture that finds it's way in may take decades to get out. Vinyl flooring & wallpapers (as well as foil wallpapers) are unintended vapor barriers, that sometimes come often with unintended consequences."
To Dana. Your point is well taken. I had planned to drill ventilation holes around the exterior floor band which could be open during summer and closed during the winter, but hadn't yet. Your point is that the moisture needs an exit plan. After reading many of your posts, I should have been more aware. thanks. Dave
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jonr
Senior Member
Posts:5341
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| 29 Nov 2011 07:49 PM |
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Can I put siding directly over EPS of that thickness, or do I need furring? I would consider the weight of the siding and the density of the foam. |
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WindowsonWashington
New Member
Posts:96
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| 29 Nov 2011 08:10 PM |
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Posted By jonr on 29 Nov 2011 07:49 PM
Can I put siding directly over EPS of that thickness, or do I need furring?
I would consider the weight of the siding and the density of the foam.
+1 Anything but vinyl will be happier with a rainscreen wall. |
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jkieffer
New Member
Posts:18
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| 30 Nov 2011 09:42 AM |
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Posted By WindowsonWashington on 29 Nov 2011 07:29 PM
Posted By Dana1 on 29 Nov 2011 12:31 PM
Posted By jkieffer on 29 Nov 2011 12:10 PM
Is there any cheaper way to insulate the ceiling without spray foam? It's not a particularly large addition (220 sq ft +/-), and I don't want to spend $1000 insulating the ceiling alone. I'm quite certain that sprayfoam is not used in the vast, vast majority of cathedral roofs in this area. What are my other options?
You can insulate the ceiling if you leave 1" of vent space between the roof deck and the top of the insulation. If you cut some spacers out of 1" XPS or unfaced EPS to use as standoff strips for cut'n'cobbled the rigid foam between the rafters (air-sealing the edges to the rafters with 1-part foam) you could then use and amount of cellulose or fg below the foam with abandon. You'd need to drill at least some soffit venting to keep the air in the 1" cavity at the same humidity as the outdoor air. You can then hit any arbitrary R-value with low thermal bridging using a Mooney-Wall type structure on the interior of the rafters.
There are some who will advocate dense-packing cellulose unvented, (the folks at National Fiber advocate it even for zone 5 ) so long as the density is 3.5lbs per cubic foot or greater at thickness of 9" or more utilizing the moisuture buffering capacity of the cellulose to protect the roof deck, but that's a code violation, and still puts the roof deck at some risk in zones 4 & up (maybe even zone-3). Venting the roof deck is a safer from a long term point of view.
+1
Have you considered an overdeck venting approach? Best of both worlds in my opinion. Completes the thermal break and allows you to use less expensive rigid boards to the outside surface.
They make a poly iso nail base as well that will save you a step.
Thanks to all for the helpful responses. At the risk of showing my ignorance, what is an overdeck venting approach? I presume you mean leaving a ventilation gap between the deck and the roofing, but The roof is already on this particular project. I have no desire to tear off a perfectly good roof to add insulation or venting between the roof and rafters. Maybe your comment was just a general one. If so, that's fine, but if your suggestion was something that might help me, I need a little more explanation. |
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WindowsonWashington
New Member
Posts:96
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| 30 Nov 2011 10:33 AM |
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An overdeck vent approach would require removal of all the shingles but the roof deck plywood would be left intact.
You would apply a given thickness of Poly Iso foam to the structural roof deck, a vertical set of 2x4 sleepers across the foam, and a new layer of plywood to the top of the sleepers.
It give you the venting, the thermal break, and any moisture control you might need.
The rafters can then be insulated with non-exotic materials like fiberglass, cellulose, roxul, flash and batt, etc. |
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jkieffer
New Member
Posts:18
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| 06 Dec 2011 10:13 AM |
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I think overdeck venting sounds great, but I'm just not willing to tear off the shingles. My HOA requires a pretty expensive shingle, and if I were going to go with that approach/expense, I'd just save the time and spray foam the rafters.
That being said, my wife really wants the cathedral ceiling. It's steeply pitched enough that I could frame out a lower pitched ceiling, creating enough space for an air gap. If I do that, vent the roof, and install batts (I know, the dreaded batts), can I add a little extra R by putting an inch or two of rigid polyiso on the underside of the rafters? I would use blown cellulose, but I'm not sure how I would keep it in place with this configuration. |
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Dana1
Senior Member
Posts:6991
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| 06 Dec 2011 12:26 PM |
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Cutting some 1" XPS spacer blocks/strips centered between the rafters and doing a 1" XPS cut'n'cobble of XPS to face the vent channel would allow you to use cellulose blown in mesh/netting between the rafters. (With a span of ~10" the 1" XPS is rigid enough to dense-pack against if you like, but wet-sprayed at 2.5-3lbs density is fine up against XPS with an exterior vent space.) Using lateral 2x4s mounted to the rafters do deepen the cellulose and thermally break the rafters would be as effective as 1.5" of interior iso from a thermal point of view, and allow better drying capacity toward the interior.
If going with interior foam rather than the lateral 2x4s, 1.5-2" of XPS would still allow some inward drying and have a comparable thermal break. If you have the space 3" of EPS would allow even better drying, but at that thickness it's something of a PITA to through-screw the gypsum to the rafters with 4.5" screws. |
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WindowsonWashington
New Member
Posts:96
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| 06 Dec 2011 01:51 PM |
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Posted By Dana1 on 06 Dec 2011 12:26 PM
Cutting some 1" XPS spacer blocks/strips centered between the rafters and doing a 1" XPS cut'n'cobble of XPS to face the vent channel would allow you to use cellulose blown in mesh/netting between the rafters. (With a span of ~10" the 1" XPS is rigid enough to dense-pack against if you like, but wet-sprayed at 2.5-3lbs density is fine up against XPS with an exterior vent space.) Using lateral 2x4s mounted to the rafters do deepen the cellulose and thermally break the rafters would be as effective as 1.5" of interior iso from a thermal point of view, and allow better drying capacity toward the interior.
If going with interior foam rather than the lateral 2x4s, 1.5-2" of XPS would still allow some inward drying and have a comparable thermal break. If you have the space 3" of EPS would allow even better drying, but at that thickness it's something of a PITA to through-screw the gypsum to the rafters with 4.5" screws.
Dana, I think 1" vent space is plenty but I was reading something from Joe Lstiburek the other day stating that he prefers a 2" minimum. I tend to agree with you that 1" is plenty but this is what he wrote...
The IRC calls for 1 in. of airspace, but I call
for a 2-in.-minimum airspace between the
back of the roof sheathingAt 2" of XPS, would that be similar per rating (roughly less than 1) to a fiberfaced ISO. Are there issues with the fiber faced ISO being compressed when drywalled over? |
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Dana1
Senior Member
Posts:6991
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| 06 Dec 2011 03:59 PM |
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2" of fiber faced iso will be in the same range as 2" of XPS, but it's probably not as well-controlled as a manufacturing process, since it's all a function of what's in the facer. The specs usually say <1 perm, but that might be anywhere from 0.3 perms to 0.9 perms (I've never tested it, nor have I seen test data) but more likely it's somewhere in the middle. Typical XPS sheathing runs ~0.6 perms @ 2", not a whole lot more, not a whole lot less.
Iso (any facer) has somewhat lower compressive strength than XPS sheathing, but is comparable to Type-II EPS. If you smack a hole in the drywall with a hammer it'll make a bigger dent in iso than with XPS, but I wouldn't sweat the compression it gets just from installing gypsum.
As for the vent spacing, 2" is huge, but guaranteed to work in any climate at any slope. Pulling some numbers out of thin air, I'd expect 1" with venting both top & bottom and pitch greater than 3:12 to be adequate almost anywhere in North America south of the 48th parallel. If vented only at the bottom or only at the top or it's a low slope roof (jkieffer's is described as high-slope) it may be a good idea to give it more. |
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