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Green Building Bible, Fourth Edition
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    • CommentAuthorJT101
    • CommentTimeJun 17th 2019 edited

    I am looking to insulate a newly built summerhouse but I have some questions on the process. Hopefully this will also be of use to others trying to do the same.

    Having been on this forum for a number of years, I've picked up the basics of timber frame insulating and construction do's and don'ts, so I want some feedback on whether my assumptions are correct.

    From the attached pics, you can see that I have an empty shell. A stud frame, which has been clad on the outside with T&G boards. And the other pic is the makeup I am proposing.

    Moving from the outside to inwards:

    1) Exterior cladding - Eventually knots will fall out the wood and gaps appear letting water and dirt in

    2) Install extra noggins to catch the edge of the interior cladding

    3) Breathable membrane - A breathable membrane is installed on the inside face of the cladding. By rights, I assume this should have been done before the exterior cladding was tacked on?

    So now the membrane must be cut to size, installed the correct way round between the studs. It seems common practice to just tack it with staples, but this seems to defeat the purpose, so I am thinking to lap it slightly up the studs, and secure with double sided breathable membrane tape.

    3) Next fit insulation between studs. Obvious choice for cost vs performance is phenolic or PIR boards.
    *** But the key question is air gap. Firstly, is one even needed? Maybe to prevent any moisture settling on the boards?
    And then I remember the magic 16mm number, the max gap to prevent convection currents reducing the efficacy of the insulation. So for ease of install, given that the studs are 70mm deep, I am thinking 60mm insulation, with 10mm airgap or 50mm with 20mm airgap.

    4) Foil tape the face of the boards to the studs.

    5) Fit the internal cladding pref moisture resistant.

    Any comments?

      Stud wall.jpg
      Wall makeup.jpg
    • CommentTimeJun 17th 2019 edited
    The idea of insulating a summerhouse seems a bit odd, but I'm sure you have your reasons.*

    For myself I'd be tempted to insulate externally. Stick a load of EPS on the outside of the current planking, cover that with a breathable membrane, then some battens and finally some more planks to act as the rainscreen. The whole lot can be held in place with screws through the battens, membrane and insulation into the original structure, and then the new rainscreen is nailed to the battens.

    edit: * FWIW our conservatory (actually a sun room) is insulated and triple-glazed :- the spec we gave the architect was 'should overheat as often as possible'.
    • CommentAuthorGreenPaddy
    • CommentTimeJun 18th 2019 edited
    Hi JT,

    you're not really starting with a "timber frame" construction, which sounds daft, but timber frame relies on a ventilated cavity outside of the frame. That's prob why djh gave the option above...effectively turning it into a timber frame.

    So assuming you stick with the existing cladding staying on the outside, you don't have a ventilated cavity to allow any moisture to evaporate. What are you using planning to use the shed for. If there's little or no moisture being generated inside the shed, then do you really need to have cavities and VCL's.

    If you want a vented cavity, then your detail as above would work (50+20mm) but drill and fit circular soffit vents, about 70mm diam. Fit pairs top and bottom between each set of studs. That sorts the walls, but what about the roof? I think that's an external insulation over the roof boards, with a membrane bonded to the insulation. Then leave the existing ceiling exposed as is.

    In truth, if you can live with the look of the existing internal, which is nice clean timber, and could be sealed with a clear sealant, if you were my client, I would advise you to spend your money on the outside (as djh), and make it a really good job, not a halfway that will cost nearly as much.

    Alternative is go cheap, fill the 70mm void with a 90mm wool (my preferred is Knauf frametherm cause it's not itchy), board over the inside, and get on with using the shed. Keep the roof watertight, an occasional treatment on the outside cladding, and it'll last easy 20 years.
    • CommentAuthorGreenPaddy
    • CommentTimeJun 18th 2019
    Meant to ask, is the floor insulated?
    • CommentAuthorJT101
    • CommentTimeJun 19th 2019
    Hi both and thanks for your replies.

    Firstly to answer why insulate a summerhouse? Why not? If you're sitting in it in winter, and got the woodburning stove going, then surely saving fuel is a good thing, and should provide better comfort overall. I know I'm stating the obvious, but I see loads of people insulate summerhouses these days. Infact I don't know anyone who has had one installed and has not had it insulated.

    I might aswell explain at this point that it is my sisters summerhouse. I wouldn't have necessarily chosen to do it this way. That was her decision, but she has no clue about how to insulate and would rather not have the installers just chuck a load of cheap insulation in and forget about it, so she has asked me to do it. And usage will be social. So medium usage. Not an office space.

    To answer both your points:

    The carpentry is exceptional. I've rarely seen such good quality. Top quality timber and precision cut, probably in a factory. So to then hide this behind insulation and cladding, plus create reveals in the windows probably won't fly. Hence the internal insulation.

    And as far as a ventilation gap goes, yes I realise you are right GreenPaddy, it is not a timber frame construction because it lacks the ventilation channel. However, my assumption was that the cladding as nice as it is, is far from airtight. And will of course get worse over time. The fact that the builders recommended breathable membrane surely means that driving rain and moisture from the outside can get in, so you need to protect the the insulation from that water. Therefore moisture must be able to get out. And yes you guessed it, a load of soffit vents isn't going to fly either. It would of course ruin the nice timber on the outside.

    Installing a VCL is no hassle. If I use foil backed PIR, I just have to tape to the studs with foil tape. Couple of hours tops. Then over the top goes large sheets of painted MDF Matchboard.
    This of course raises the question of how good a non dedicated VCL.

    There must be hundreds of thousands of summerhouses constructed. I see more and more go up near me all the time. And most are insulated, from the inside. I don't know how they do it, but one merchant told me they use 70mm Celotex. None of them that I've seen have vents in the timber. So are you suggesting that all these are incorrectly insulated? I wouldn't be surprised actually.

    And no the floor is not insulated. What can I say? She should have thought about it before.

    The roof is same wooden cladding, breathable membrane, battens, roof tiles. So I would have thought it can be insulated in the same way.


    I probably wouldn't have
    • CommentAuthorGreenPaddy
    • CommentTimeJun 20th 2019
    JT101, do I know for a fact that may builders and some architects have no idea what insulation and membranes to use, but just quote things they think they've heard or seen? Yes, 30 years in this industry has shown me that.

    But look, it's just a shed. Go with the foil faced rigid insulation, foamed all round the edges of every board. The breathable membrane won't do anything for you. Wasting your time, and reducing the thermal performance of the foil face. It's PU, not going to absorb moisture or rot. The main reason breathable membranes have always been put on the outside of timber framing, is to protect it during the construction phase, but then it has to let out moisture already in the frame or passing from in-house.

    Would I bother with an internal VCL...no, there's no vapour generation internally, and the frame will get more moisture from precipitation than it would ever get from human respiration. If you want to foil tape board-to-timber, by all means, but the edge foam is the most important part of all this.

    Get on and do it, before summer starts.
    • CommentAuthorJT101
    • CommentTimeJun 21st 2019
    Thanks so much Green Paddy. Everything you and djh has made a lot of practical sense.

    Just one thing I want to know out of interest is what you mean when you say the breathable membrane will reduce the thermal performance of the foil face. Can you explain?


    And yes, I am planning to start next week. Like you say, make hay while the sun shines
    • CommentTimeJun 21st 2019
    Posted By: JT101Just one thing I want to know out of interest is what you mean when you say the breathable membrane will reduce the thermal performance of the foil face.

    Foils work by blocking radiation from the surface (directly equivalent to reflecting radiation that shines on it). To do that they need a clear space adjacent to them; I forget the exact distance but 1/4" is usually used as a safe distance, IIRC. If anything is too close, especially touching, then they don't do their job of blocking radiation and are worse than useless (since they conduct heat very well).
    • CommentAuthorJT101
    • CommentTimeJun 24th 2019
    Brilliant. Thanks djh. Understand it now. Probably won't bother with the breathable membrane now, but good to know for future reference. Just out of interest (again), what's the difference between roofing breathable membrane and house wrap?
    I found one site that said house wrap is lighter weight. So presumably you could use roofing breathable membrane on walls or roofs, but house wrap just on the walls?
    • CommentAuthorJT101
    • CommentTimeJul 31st 2019 edited
    Just some feedback for anyone else who attempts this. Don't! It is Sooooooo much work, and way beyond what anyone would be willing to pay for or put the time into for a casual use summer house. However, as an office space or somewhere generating a lot of humidity it may be useful.

    Next time I will use the rockwool option as GreenPaddy suggested. But I had to try. Luckily I was doing it for love!

    So I ended up going the full hog and tacking the breathable membrane to the cladding, then taping all the edges. Took ages.

    I then carefully drilled ventilation holes to the outside, and found some stainless steel smart looking ventilation caps which I stuffed with stainless mesh and fitted into the holes at top and bottom of the cavities. Took ages.

    If you do this, make sure you drill it where the cap will fall on a flat part of the shiplap cladding, unlike me where it ended up on a curved bit, requiring a lot more silicone.

    Each cavity, including the roof required 4 vents for cross ventilation. It was a LOT of work as Green Paddy said. It has a lot of little nooks and crannies between studs because of all the windows and doors, which in the end I didn't bother to vent because I would never have finished it.

    Then installed noggins, and more studs to catch edges of the boards. I';m not sure the noggins were necessary. Probably would have been stiff enough.

    Then installed the insulation and foamed around the edges where there were gaps. Finally foil taped to seal.

    Then clad in 9mm MDF moisture resistant grooved panel board. Routing the edges gives a nice finish. Note, before you install this, think about which direction you want the grooves to run, and how you want the wall pieces and ceiling pieces to line up.

    Finally fill all nail holes, sand, and prime in MDF primer, not regular wood primer.

    The result is nice, but I could have achieved similar results with a lot less effort if I used Rockwool since it is breathable. I would probably still install breathable membrane, but just tacked, not taped, then 90mm rockwool stuffed into 70mm gap, sheet of VCL, then board.

    Infact, by the time you allow the airgap behind PIR of 20mm, you only have 50mm PIR. 0.05 / 0.022 = 2.27 m2K/W. Rockwool of 90mm compressed into 70mm, probably gives you somewhere between i.e. 80mm so 0.08 / 0.036 = 2.22m2K/W

    So no benefit. If you have thicker walls the difference becomes more pronounced, making more of a case for PIR.

    Hope that's of use to someone.
    • CommentAuthorJT101
    • CommentTimeJul 31st 2019
    Quickly answer my previous question, you can use breathable roof membrane on the walls. As I said, the roof membrane is heavier duty and so can be used in both applications, whereas the wall membrane is lighter and can only be used on the walls.

    This does lead me onto a couple more questions.

    1) Why do you compress 90mm of rockwool into a 70mm gap?

    2) If I had to deal with a flat roof on a summer house/shed, do I need to allow a 50mm ventilation gap between the insulation and the roofing material (if I use Rockwool) as per cold roof guidelines, or are we again saying it doesn't matter because it's just a shed?

    3) Why do cold roofs require a ventilation gap of 50mm, yet with the walls with the same construction, you fill it up completely with insulation?

    Posted By: JT101
    3) Why do cold roofs require a ventilation gap of 50mm, yet with the walls with the same construction, you fill it up completely with insulation?

    My understanding is that if you have a breathable membrane then you don't need a 50mm gap with a cold roof - Can someone correct me if my understanding is wrong 'cos that's what I will be doing in a couple of months.
    • CommentTimeAug 1st 2019
    Posted By: Peter_in_HungaryMy understanding is that if you have a breathable membrane then you don't need a 50mm gap with a cold roof

    My understanding is that Building Regs require ventilation under a cold roof and that ventilation requirement is satisfied by a 50 mm gap. See e.g. https://www.swishbp.co.uk/index.php/design/building-regulations/ or https://www.roof-stores.co.uk/guides/flat-roofing/flat-roofing-ventilation/

    And to answer James question, you don't normally full fill a wall with insulation. You either partial fill (e.g. cavity) or full fill and then add battens and a rainscreen to provide a ventilated drainage plane. But it starts to get complicated and there are various cases that do use full fill. Those cases need to be supported by a suitable analysis, either individual or documented in a certificate such as a BBA.
    While it might have been a lot of hard work, it looks really nice. You'll have to keep us posted on how it performs in Winter.
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