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Community and Q&A

Narrow shed roof for artist studio

mj62mj62 | Posted in GBA Pro Help on
Hello! Thank you for such a great site!  I’m now a paying member.
I have read (re-read 5x) the cathedral ceilings article and other GBA articles, but I have to compromise and I can’t determine the best way.
I’m hoping you can help:
I have a factory-made 18′ x 8′ brand new garden shed.  I’m making it into a fully-fitted office/studio with 2 rooms:
– studio for artists work
– kiln room (a pottery oven that will make the room very hot)
We are in the UK and can’t afford the closed-cell foam route.
Our goal is to be warm and protect our investment long term.

Roof: (see picture)
– Sloped
– Shed roofing felt on timber decking 
– *unvented*  (some air seeping in front and back but I’ll seal it)
– 2.5” rafters (not very deep!)
– Cold roof only – Adding insulation on top is not an option (We are already at height limit before planning permission is needed)
Ceilings – (the plan for)
– Drywall connected to the rafters in the kiln room (special fire-retardant drywall on all ceilings/walls in the kiln room)
– Wood cladding connected to the rafters in the studio room
– Just 6’ from the floor – we would not want it lower
GBA suggests:
– Warm Roof solutions – not an option 
– Cold Roof + Closed Cell Foam – too expensive 

GBA (Martin) replied to many posts on compromising, but I can’t see which is best for us. 

Q1) Ceiling Insulation – Martin said not to use fibreglass in unvented roofs – but he also advised against cut-and-cobble with rigid.   Rigid insulation is not sold in the depth we need (65mm) – only in 30mm, 70mm or 75mm (and beyond). I could extend the roof a down a bit to meet 70 or 75…

I wrote to the PIR companies here (celotex/etc) who said don’t their boards an unvented space. 
The mineral wool companies (rockwool) said their product in an unvented roof carries risk and a good VCL should be used.

How do we stay warm?

Q2) Ceiling covering in studio room – We want wood cladding but GBA warns that drywall should be used and wood cladding put on top of that, which would eat into our low height. 
Is this serious enough that we should change the ceiling to drywall?  (‘character’ matters in an artists studio and my wife may not be happy!)
Q3) Tiny front soffit (just 2.5″ high) – there are wires running through, making rockwool batts much easier to shape around them.   Is the objective to fill all the space within the soffit? (after to seal any air cracks)  Or if using rigid insulation do we block off the soffit with a vertical piece of PIR and leave it empty? 
Q4) Wall insulation – our current plan is:
Rockwool batts filling the cavity 
Tyvek Airguard Smart (like membrain) VCL 
interior wood cladding.  
We thought to spend extra $ on the tyvek because if tongue-and-groove cladding is used BOTH inside and out, some moisture will need to escape.
Adding a layer of drywall before the interior cladding would take up too much space. (shed is only 18′ by 8′!). 
Would PIR be better here?
Many Thanks!

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  1. Jon_R | | #1

    What is your average temperature for Dec/Jan/Feb?

  2. mj62mj62 | | #2

    6 degrees C, 42 F

    It dips below zero occasionally around that time - but it rarely stays below zero for long.

    It rains but rarely snows, mostly a 'wet damp' in the winter, rather than the dry cold you'd get on mainland

    Average Temperature Chart:

  3. Jon_R | | #3

    I assume that local building code has no input on this?

    Cut-and-cobble isn't ideal, but given:

    1) your constraints (eg, no lowering of ceiling)
    2) how close you are to avoiding excessive condensation with your moderate temps
    3) limited interior moisture sources (I assume)
    4) a smart retarder (well taped to be a second air barrier)

    I would fill the cavity with unfaced EPS/GPS (because it is fairly vapor permeable and an air barrier) insulation and use canned spray foam and tape to air seal it well.

    I would never use highly air permeable mineral wool against cold (ie, below dew point) wood sheathing that can't dry to the exterior. See here.

  4. mj62mj62 | | #4

    Hi Jon,

    Thank you so much for your suggestion. Can I clarify:

    Is it that the greater permeability is better because if moisture does get it, it will disperse within the EPS and/or 'dry to the interior' given the VPL?

    How do you deal with the soffit with EPS? Here is a photo of one bit of soffit - just 2.5" high, with numerous wires in it.

    And finally are you recommending the same solution for the walls?

    Thank you for your suggestion to check the building regulations, I found a few things:

    NBS (National Building Specification) says:
    " Cold roof voids are typically required to be ventilated; however, where vapour permeable underlays are specified, this requirement may be relaxed. "
    UK Building Regulations are lengthy and are divided between dwellings, non-dwellings, new builds, renovations, etc. They key points are:
    -- for flat roofs, warm roofs are better than cold roofs
    -- for flat cold roofs, aim to insulate both between and below the rafters to hit the U-value targets. (during renovations this is "except where ceiling height would be adversely affected.")
    -- should reasonably deal with thermal bridging, air tightness, etc, etc.
    -- you must Assess Condensation Risk as per BS 6229: 2003 (which you can't download without buying it)

    1. Jon_R | | #6

      Yes, EPS avoids a moisture trap (multiple impermeable layers). The same plan would work for the walls. I don't know about the soffit - somehow it needs to be blocked and insulated.

      1. mj62mj62 | | #16

        Hi Jon,

        That article you sent is pretty amazing - thank you!!! :-)

        Can you please clarify whether my roof and walls can 'Dry to the Exterior'? This is a key factor in understanding the article:

        I suspect for the roof it's a 'no' (solid pine with shed roof felt on top) and for the walls it is also a 'no' but because the walls are wood cladding and can't be 100% air tight that we need to think strongly about drying to the interior?

        I think Figures 11 through 13 are closest to my situation...

        1. Jon_R | | #19

          It's very likely that your roof is not very vapor permeable to the exterior (because of the roofing felt) and your walls are somewhat permeable (depends on paint). Air leakage to the exterior will add some unknown amount of drying.

          EPS/GPS (vs mineral wool) fill is the more conservative choice that works either way. Adding air sealed smart-membrane is also a conservative choice. But it's a reasonable argument that this conservatism is unnecessary and that mineral wool + interior side air sealing and below joist foam as a vapor retarder might be OK at a slightly lower cost.

          1. mj62mj62 | | #20

            The word 'reasonable' has made me feel a lot better, thank you :-)

            Just to confirm, your two suggestions are:

            EPS/GPS between the cavities, properly installed to get it really well air sealed (use foam + tape)
            + Smart VCL over that

            Mineral Wool
            + Smart VCL
            + Retarder (a layer of something across the rafters/studs - also very well air sealed. Either EPS, plasterboard, osb, etc. although EPS is good for drying to the interior)

            Have I got that right?

          2. Jon_R | | #21

            My recommendation is be conservative, follow codes (I only know US ones) and use EPS or GPS in the ceiling. Fiber in the walls is fine.

          3. mj62mj62 | | #22

            Uh oh!! EPS warning:

            I decided to go EPS in the roof. No sooner had I installed 25mm of it (with strong adhesive no less) did my wife find:

            Manufacture spec:
            - Don't allow any EPS to come in contact wtih pvc sheathed cables

            Shed article:
            - "Don't insulate your shed with EPS or Foil if you're having electrics installed - you'll be too close to wires and your shed could go up in flames!"

            My wife is in a state and wants it taken out :-(

            Our wiring runs through the soffit and along the studs for the lights. So it's both very-near and touching the EPS.

            I write this here for future readers - EPS may be great when you have larger empty cavities with wiring run separately, or maybe I'm over-reacting and this is just FUD.

            Whether in would be fine or not, if you have someone really worried about fire safety (such as in our kiln room) then it is not a viable choice.

            I presume I'll need a chisel or something to get all the bits of EPS out - she won't be happy until every bead is gone...

            (I'd still like to thank everyone for their advice - obviously I should have read the spec before installing it! Lesson learned!)

          4. Expert Member
            Akos | | #23

            Maybe different code, but ours sizes residential wiring for insulation contact, so wires can be buried in insulation. Running NM wire inside grooves of ICF fundations (EPS) is the norm.

  5. mj62mj62 | | #5

    ... and just to add one more question onto the list (Thanks again in advance for all your help!)

    In practice, will 65mm of EPS give me a 'worthwhile' amount of insulation? 65mm isn't on the market but i assume I can layer it (25+40).

    I appreciate 'worthwhile' is subjective, but it's hard to get a sense of 'real world' insulation value and given the thinness I just wanted to be sure...

    GPS is also available which gives better 30% better performance (for 50% more money) with the same benefits of EPS.

    Costs per m2:
    EPS = 3.88
    GPS = 7.5
    (PIR = 8.54 at full depth or 5.17 at half depth)

    Thanks again! :-)

  6. Expert Member
    Akos | | #7

    Assuming avg 7' tall wall and ignoring the ground losses, your shed has around 500sqft of walls and roof together.

    If you get that up to R13, at 42F outdoor 70F indoor, you'll need 1100btu (320W) to heat it.

    You can get there with 2.5" mineral wool batts in the rafters/studs with 1/2" foam over it. The foam should be permeable as Jon pointed out and you need to tape the seams on it to make it an air barrier. You can then install either wood or drywall directly over it as your finished ceiling. I would not skip the continuous layer of foam even if you do lose a bit of height.

    Cut and cobble is a waste of of time. The above assembly with polyiso cut and cobble in the rafters would only bump up your assembly R value to 16, reducing your heat load to 250W. I doubt that is worth the effort/cost.

  7. mj62mj62 | | #8

    Thanks Akos - I really appreciate it.

    Can I please confirm my understanding? It's:

    Mineral wool batts between the rafters
    Smart VCL
    1/2" EPS/GPS/etc - any type of permeable foam
    wood or drywall as the ceiling

    And do the same for the walls?

    I'll dig around to find out what to do with the soffit space - I guess I can fill it with mineral wool as it's easier to work around the wires.

    1. Expert Member
      Akos | | #10

      Pretty much. You can skip the smart VCL, not really buying you anything here.

      Because the studio is not occupied full time, there is no active interior moisture source thus you don't have to worry about condensation control. You just need an assembly that can provide drying in one direction at least.

      Air sealing is the important one as it can make a big difference in heating costs. Watch around your electrical boxes, transitions from wall/ceiling wall/wall and the details around your doors and windows.

      I would use the same assembly for the walls as well especially if it will be clad with wood. It is not a highest R value assembly, but in your warm climate, it doesn't make much sense to go for more insulation.

      P.S. Overall to the cost of the insulation plus your "free" labour costs, I would go for the higher R value foam.

      1. mj62mj62 | | #24

        Thanks again to Akos and everyone else for their input.

        In the end we were advised by a UK based insulation person to go with mineral wool plus intello (smart VCL) and for the soffit areas, closing them off with some osb wood and sealant rather than trying to fill them. We saved a few pennies by order the stuff from Germany via baunativ who had the whole range of related tapes/instaabox/etc.

        Everything is a compromise when the walls are so thin, but this provides a decent solution.

        About the EPS:
        "Maybe different code, but ours sizes residential wiring for insulation contact, so wires can be buried in insulation. Running NM wire inside grooves of ICF fundations (EPS) is the norm."
        Yes here in the UK grooves can be cut into EPS for wires, provided the wires don't touch the EPS. (but obviously take professional advice before making any such decisions - just sharing what I was told)

        Thank you so much everyone :-)

  8. Expert Member
    Peter Engle | | #9

    I like Akos suggestion for the walls and ceiling.

    For the soffits, I would stuff them with mineral wool, and then block with either wood or rigid foam and spray foam sealant to make it all airtight. Or, if they are already sealed on the outside, just fill the soffits with foam-in-a-can. Overhanging soffits could be stuffed with mineral wool first, then canned foam above the top plate for the air barrier. That would save some money over filling entirely with canned foam.

    For context, this is a relatively small building and it won't cost much to heat and cool either way. How often is it going to be used? That can also affect costs. You building codes might be relaxed or non-existent for seldom used auxiliary buildings. if so, any level of insulation will improve your comfort level. Air sealing is always most important.

  9. mj62mj62 | | #11

    Thanks Peter - it will be used most days, so I want to get it right :-)

    Which makes me so thankful again for all your input - Thank you again to Jon, Akos and Peter :-) :-) :-)

    Installing the EPS across the studs does not seem popular (no videos on it) but it seems like a good way of ensuring an air seal and adding a bit more insulation. The 1/2" EPS is sold in small sheets 39x20" so there will be lots to tape up.

    1/2" EPS may be too thin for staples from a staple gun - I'll try but I may have to double-sided tape it down.

    Is Jon's concern about mineral wool resolved because Akos's solution is drying to the interior and it's giving up headspace to airseal it even better?

  10. mj62mj62 | | #12

    I'm having doubts about the 1/2" EPS across the timbers - it looks like something no one else is doing (which doesn't make it wrong) but I'm also unsure about how to best fix it on there.

    - Staples - are you they likely to hold it down or blast right through the material? I suppose the wall cladding would keep it in place...

    - Double sided tape between EPS and studwork? I suppose if I get the really good tape this could work?

    - Some kind special nails with a large head? such things could get in the way when putting on the interior cladding or if in the future, you go to put up a shelf and hit it.

    I had hoped that if I researched this is really diligently I'd find the 'right' solution - silly me ;-)

    There are many options (the difference between what Jon and Akos are suggesting remains unresolved...)

    1. brendanalbano | | #14

      Usually you need a big budget and minimal design constraints to do the truly "right" solution you're wishing you could find.

      The rest of the time, it's all about finding the right compromise ;)

      1. mj62mj62 | | #15

        Yes it's quite an adventure :-) Especially on the internet where most sites aren't as good as GBA...

        If I had a lot more money I wouldn't worry so much about it but this 'shed' has become a major project! Lots to learn :-)

  11. Expert Member
    Peter Engle | | #13

    Just a few roofing nails to tack the foam in place will suffice. The drywall screws should be long enough to go through the foam and fasten both to the studs and rafters. Taping the drywall and sealing around penetrations will also help to establish your air barrier.

    1. mj62mj62 | | #17

      Thank Peter - Roofing nails is a great idea.

      I've been trying hard to understand the article Jon sent at the top:

      Whilst EPS-across-studs doesn't seem popular at all, I'm starting to see how important it is for my solution to dry to the interior. With wood cladding on the exterior (which looks air tight but probably isn't) I'm less keen on using mineral wool and perhaps more on EPS between the cavities as well as on top (with no VCL layer).

      Any thoughts on this? From what I've been hearing, I suspect that regardless of EPS or mineral wool that the Number One factor is installing it properly and having a really good air seal.

      1. Expert Member
        Peter Engle | | #18

        Yes, that's exactly right.

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