Hello

I am building a new construction home on a slab in PA 1500sqft and we have pretty cold winter. What is the best way to insulate it? What would you do or what do you suggest?  I was thinking of putting joists above the slab so I can insulate better and also I dont want to walk on concrete since its cold and hard.  Should you still insulate the slab if its going to be raised with2x6 or 2x8 joists? It will be a very tight home with r30 walls and r60 attic. I am at the very beginning design.

A full basement is alot more and I dont want it or will ever use it.

Thanks in advance.

First, PA covers a wide range of climate, from US zone 4A to US zone 6A, and what's appropriate or economic varies. Got ZIP code? Or just find your county on this map and report the climate zone:

https://energycode.pnl.gov/EnergyCodeReqs/?state=Pennsylvania

Then take a peek at table 2, p10 of this document, and find the row appropriate to your climate zone:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

For sub-slab insulation (even for full-basement houses) they're recommending R7.5 for the full field. A slab-on grade house would want that much for the full field, and a bit more at the grade beam or stem wall. A minimalist R16 insulated concrete form stem wall, floating the slab on 2" of EPS foam (R8.2-ish) would work. Put the vapor barrier on top of the EPS- it can take the ground moisture just fine. Putting it under the foam sometimes traps moisture that takes forever to dry, potentially affecting the finish floor. With an insulated slab you don't need joists to provide a bit of flex to the floor- 1x sleepers would be fine, but stained concrete or ceramic tiles work fine for most people. (YMMV)

Note that the R-values indicated are "whole-assembly R", with all the thermal bridging of the framing factored in. Assuming you're in zone 5A (most of PA) an example of the recommended R30 wall would be a 2x6 /R20 framed wall with 3" of continuous exterior rigid foam sheathing. The thermal bridging of the 2x6 reduces the whole-wall average down to R15 from the center-cavity R20. Another R30 whole- wall would be a double-studwall with 9.5" of cellulose between the exterior sheathing and the wallboard. For variations on the theme see the details for Case 2b (p 33) and Case 4 (p37) in this document as well as Table 3 (p9):

https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf

Since you're at the beginning of the design phase note that complex footprints with lots of bump-outs, ells and dormers adds a lot of thermally bridging framing and makes it harder to air seal. A simple rectangle is most efficient, but limiting it to 6 or at the very most 8 corners gives it a bit more flexibility.

Simple gabled roofs and orienting a clear wide pitch toward the south can make it pretty easy to hit Net Zero Energy with an array that's affordable and fits on the house, if you heat/cool with high efficiency ductless (or mini-ducted) heat pumps.

A trussed roof using "plenum trusses" makes keeping the ventilation and heating duct systems (usually separate systems) fully inside the building envelope. See:

http://www.greenbuildingadvisor.com/blogs/dept/building-science/use-plenum-trusses-keep-ducts-out-your-attic

Reclaimed roofing insulation foam is far cheaper and greener than virgin stock, if using foam in any of the assemblies. For sub-slab foam you can use roofing EPS or XPS if it's only holding up the slab & partition walls. Foam under footings and load bearing walls would have to be specified by an engineer. DON'T use polyisocyanurate (usually a pale yellow, sometimes brown on the edges if well aged) under slabs, since it's hygroscopic and will lose performance when waterlogged. If using virgin-stock sub-slab foam, use EPS- it's greener & cheaper than XPS which uses HFC blowing agents (powerful greenhouse gases) in manufacturing. As the blowing agents leak out over a few decades it's performance drops to that of EPS anyway, inch for inch, so derate any USED XPS to R4.2/inch for design purposes. Roofing polyiso is just fine to use for exterior insulating wall sheathing, but derate to R5/inch for exterior sheathing applications in your climate. It may be labeled anywhere from R5.5 to R6.5, but the mean temp through the foam for testing for labeling is 75F, which is a higher performance operating temperature than you'll be seeing.

First, PA covers a wide range of climate, from US zone 4A to US zone 6A, and what's appropriate or economic varies. Got ZIP code? Or just find your county on this map and report the climate zone:

https://energycode.pnl.gov/EnergyCodeReqs/?state=Pennsylvania

Then take a peek at table 2, p10 of this document, and find the row appropriate to your climate zone:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

For sub-slab insulation (even for full-basement houses) they're recommending R7.5 for the full field. A slab-on grade house would want that much for the full field, and a bit more at the grade beam or stem wall. A minimalist R16 insulated concrete form stem wall, floating the slab on 2" of EPS foam (R8.2-ish) would work. Put the vapor barrier on top of the EPS- it can take the ground moisture just fine. Putting it under the foam sometimes traps moisture that takes forever to dry, potentially affecting the finish floor. With an insulated slab you don't need joists to provide a bit of flex to the floor- 1x sleepers would be fine, but stained concrete or ceramic tiles work fine for most people. (YMMV)

Note that the R-values indicated are "whole-assembly R", with all the thermal bridging of the framing factored in. Assuming you're in zone 5A (most of PA) an example of the recommended R30 wall would be a 2x6 /R20 framed wall with 3" of continuous exterior rigid foam sheathing. The thermal bridging of the 2x6 reduces the whole-wall average down to R15 from the center-cavity R20. Another R30 whole- wall would be a double-studwall with 9.5" of cellulose between the exterior sheathing and the wallboard. For variations on the theme see the details for Case 2b (p 33) and Case 4 (p37) in this document as well as Table 3 (p9):

https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf

Since you're at the beginning of the design phase note that complex footprints with lots of bump-outs, ells and dormers adds a lot of thermally bridging framing and makes it harder to air seal. A simple rectangle is most efficient, but limiting it to 6 or at the very most 8 corners gives it a bit more flexibility.

Simple gabled roofs and orienting a clear wide pitch toward the south can make it pretty easy to hit Net Zero Energy with an array that's affordable and fits on the house, if you heat/cool with high efficiency ductless (or mini-ducted) heat pumps.

A trussed roof using "plenum trusses" makes keeping the ventilation and heating duct systems (usually separate systems) fully inside the building envelope. See:

http://www.greenbuildingadvisor.com/blogs/dept/building-science/use-plenum-trusses-keep-ducts-out-your-attic

Reclaimed roofing insulation foam is far cheaper and greener than virgin stock, if using foam in any of the assemblies. For sub-slab foam you can use roofing EPS or XPS if it's only holding up the slab & partition walls. Foam under footings and load bearing walls would have to be specified by an engineer. DON'T use polyisocyanurate (usually a pale yellow, sometimes brown on the edges if well aged) under slabs, since it's hygroscopic and will lose performance when waterlogged. If using virgin-stock sub-slab foam, use EPS- it's greener & cheaper than XPS which uses HFC blowing agents (powerful greenhouse gases) in manufacturing. As the blowing agents leak out over a few decades it's performance drops to that of EPS anyway, inch for inch, so derate any USED XPS to R4.2/inch for design purposes. Roofing polyiso is just fine to use for exterior insulating wall sheathing, but derate to R5/inch for exterior sheathing applications in your climate. It may be labeled anywhere from R5.5 to R6.5, but the mean temp through the foam for testing for labeling is 75F, which is a higher performance operating temperature than you'll be seeing.

thank you so much for the info. Those links are great. Im in zip 18062.

Posted By Natethegreat848 on 24 Oct 2017 10:39 PM
Hello

I am building a new construction home on a slab in PA 1500sqft and we have pretty cold winter. What is the best way to insulate it? What would you do or what do you suggest?  I was thinking of putting joists above the slab so I can insulate better and also I dont want to walk on concrete since its cold and hard.  Should you still insulate the slab if its going to be raised with2x6 or 2x8 joists? It will be a very tight home with r30 walls and r60 attic. I am at the very beginning design.

A full basement is alot more and I dont want it or will ever use it.

Thanks in advance.

If you are considering building a floor system over a slab, have you looked at a sealed, insulated crawl space with pier and beams and a thin rat slab?  That way you still have access to all of your plumbing and whatever else you elect to put under the floor (ducts, wiring, etc.).  This also avoids most if not all of the cost of compacted back-fill under a raised slab foundation.

The best way to do this would be Insulated Concrete Forms (ICF) for the foundation stem walls and 2"-4" of EPS insulation under a 2" rat slab.  If you can find reclaimed EPS, I would go with 3"-4".

You're in zone 5A, so the wild guess was apt(?) .

Reclaimed foam often shows up on the local craigslist, if you search on the terms: rigid + insulation

eg:

https://harrisburg.craigslist.org/search/sss?query=rigid+insulation

https://philadelphia.craigslist.org/search/sss?query=rigid+insulation

Just assume it's out there- you're way early in the game, with a lot of designing to do first.

You're in zone 5A, so the wild guess was apt(?) .

Reclaimed foam often shows up on the local craigslist, if you search on the terms: rigid + insulation

eg:

https://harrisburg.craigslist.org/search/sss?query=rigid+insulation

https://philadelphia.craigslist.org/search/sss?query=rigid+insulation

Just assume it's out there- you're way early in the game, with a lot of designing to do first.

what about radiant in the concrete and stain the concrete or engineered wood on top of it?

i don't think basement is much more expensive than slab, (at least not here where they dug 4' down for the slab alone). 

Also i you live in an area where basement is normal, a slab  is a big negative for resale.