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    •  
      CommentAuthorfostertom
    • CommentTimeNov 17th 2020
     
    I would appreciate some insights and suggestions:

    A well built Victorian Listed Building has its second and third bedrooms under the steep 50o roof slopes, which become like ovens under summer sun, and cold in winter. The roofs are natural slate in good condition, on battens, without any underslating felt, on 3"x2" raftering, on 4"x4" purlins, on one central truss across each room. All the timbering is straight and sound, excellent condition (for a change!).

    Some time fairly recently, new plasterboard and skim has replaced the original t&g boarded lining, with 50mm foil faced Cellotex between the rafters, 25mm air gap above, without any systematic through-ventilation, except via leakage between the slates.

    Though 50mm of Cellotex is a bit weak and we can improve on that for the winter, I am surprised it gets so extremely hot in summer. What would the team suggest?

    The extreme summer heating must be due to the lack of underslating felt - slates get too hot to touch under summer sun and this is radiating direct to the room, with 'only' the foil faced Cellotex to block it, and I guess the rafters get very hot too.

    I'm sure it was even worse before the plasterboarding and Cellotex. The irony is, if I had been involved then, with the good slating not to be disturbed hence no chance of underslating felt, I might have prescribed reflective Cellotex to kill the summer over heating - but this clearly hasn't worked.

    So what to do now?
    • CommentAuthorEd Davies
    • CommentTimeNov 17th 2020
     
    Pity it's the wrong time of year for extremes of either hot or cold but would it be worth checking on the temperatures of the plasterboard between and on the rafters with an IR thermometer? Maybe later in the week assuming it gets colder. See if it's really conduction through the fabric or if the slate/air gap combo is working as a great big solar-thermal collector/IR radiator via exchange of air with the room.
  1.  
    Tom, that sounds like our current 19thC house, which has no felt and the attic gets hot in summer, but the 1970s bungalow we rented previously did have felt, and its loft got even hotter. Our previous 19thC house had no felt, we converted the loft with insulation between rafters and the new attic room got hot, unless we opened the windows morning/evening.

    I don't think the felt (or lack) makes any difference tbh, it's a dark colour and radiates just as well as the slates do.

    (The issue may be the ventilation (or lack) between the slates and the insulation, some substantial ridge and eves ventilators might help restore a stack effect ?) EDIT - ignore this bit, see post below.

    Or it might be that heat rises through the 1st and 2nd floors (no insulation in floors?) but gets trapped in the (insulated, draught proofed) loft bedrooms.

    Presumably the loft bedrooms have velux style windows, which way do they face? The trad roof windows in both our 19thC houses faced North, for good reason. Do they shade the veluxes and open them overnight?

    Any scope for one of your passive solar roof jobbies with pipes circulating water into the subsoil?!?
  2.  
    Scrub my above comments re ventilation and felt. We all know that the insulating value of a 25mm air gap is insignificant compared to 50mm of PIR, irrespective whether the air gap is reflective or ventilated or felted or not.

    Some numbers:
    Let's assume 30m2 of roof, U=0.2, slates are heated to 40degC for 8 hours/day, 30m3 of room air that starts out at 20degC.

    Heat ingress through roof = 30 * 0.2 * (40-20) = 120W

    Over a day, 120W * 8h * 3600s / 1000 = 3500kJ = ~1kWh

    That's enough to heat the room air by
    3500kJ / (30m3 * 1.2kg/m3) / 1kJ/kg = 96degC !!!

    (Edit as per bhommel's comment below and correcting to U=0.5:
    600W, 5kWh, 500degC !)

    Obvs it doesn't actually get that hot as it equilibrates with the slates temperature, and some heat is absorbed by surfaces and furniture, but the picture is that the normal to-be-expected heat ingress through a mildly insulated roof in midsummer is sufficient to overheat the air in the room.

    If you add in windows there's even more heat ingress, and if you ventilate the room with air drawn in from roof level that's just as hot.

    Instead, how about adding thermal mass in the room - replace the plasterboard with thick heavy plaster, replace partition walls with brick? (Subject to structure etc)

    Or increase the roof insulation, reduce the winter heating energy, and trade that for aircon in midsummer.
  3.  
    Posted By: WillInAberdeen(The issue may be the ventilation (or lack) between the slates and the insulation, some substantial ridge and eves ventilators might help restore a stack effect ?) EDIT - ignore this bit, see post below.

    I would have thought that lots of eaves and ridge ventilation would generate a good stack effect which might eliminate all but radiated heat from the slates and the foil face (it was put in foil up wasn't it?) might take care of that. Probably eaves and ridge vents are going to be the least destructive modification so maybe worth a try.

    Posted By: WillInAberdeenPresumably the loft bedrooms have velux style windows, which way do they face?

    Good question - velux type windows can give a wonderful greenhouse effect !
    • CommentAuthortony
    • CommentTimeNov 17th 2020
     
    I am suspicious of loft to room air connections low down

    Is there a way for air to enter the floor void?
    •  
      CommentAuthorfostertom
    • CommentTimeNov 17th 2020 edited
     
    Good stuff - keep it coming!

    Just on one point
    Posted By: WillInAberdeenI don't think the felt (or lack) makes any difference tbh, it's a dark colour and radiates just as well as the slates do.
    not so sure - any opaque barrier, even a sheet of black paper, will at least halve the radiative intensity reaching the target, as it will adopt a radiant temperature exactly half way between the source and the target.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 17th 2020 edited
     
    .
    • CommentAuthorajdunlop
    • CommentTimeNov 17th 2020
     
    Make sure you have good blinds.

    We now have some of the external Velux awning blinds that we put down over summer to reduce solar gain (by 76% apparently) and in really sunny periods of the summer pull down some internal blackout blinds as well. This has made a big difference (as well as increasing the amount of insulation to keep the heat out).

    Under floor draughts can really cool things down in winter but could also be allowing the heat in in summer (as Tony says).
    • CommentAuthorCWatters
    • CommentTimeNov 17th 2020 edited
     
    Posted By: fostertomThough 50mm of Cellotex is a bit weak and we can improve on that for the winter, I am surprised it gets so extremely hot in summer.


    Our house built in 2007 has about 4 times that much and rooms in the roof still get too hot in summer, even those with no south/west facing windows. By contrast my shed which has only 100mm insulation feels cooler. Not 100% sure why but my thinking is that a well ventilated air gap helps.. eg something like..

    Outside
    Cladding
    Well ventilated air gap
    Membrane
    frame/Insulation
    Inside

    So to answer your question I'm also wondering if vents in the slate near the ridge and under the eaves would promote air flow under the slate and reduce summer heating. I'm thinking that the hot slates would promote convection cooling in summer that wouldn't occur (to the same extent) in winter when heat flow is the other way.

    In the past I've been critical of the claims made for multifoil insulation but I'm wondering if foil could also have a role in reducing summer heating. I ask because the insulation I used on the shed was cheap loft insulation that came in a bag with one side shiny foil which I put facing out. I'm thinking it might be interesting to use a membrane in the above with a shiny surface. The aluminium foil on rigid insulation isn't really shiny by comparison.
  4.  
    Posted By: meI don't think the felt (or lack) makes any difference tbh, it's a dark colour and radiates just as well as the slates do.

    the insulating value of a 25mm air gap is insignificant compared to 50mm of PIR, irrespective whether the air gap is reflective or ventilated or felted or not.
    Posted By: fostertom any opaque barrier, even a sheet of black paper, will at least halve the radiative intensity reaching the target, as it will adopt a radiant temperature exactly half way between the source and the target.

    True! But the R values (insulation value, 1/U) are:

    - radiation across unventilated gap: 0.15 m2K/W approx (less if ventilated)
    - so radiation across two such gaps: 0.3 m2K/W approx (less if ventilated)

    Versus

    - conduction through 50mm PIR:. 2.7 m2K/W

    So dividing the 25mm gap with felt into a double gap, means it still makes an insignificant contribution to the insulation, compared to the PIR. (Like in Tony's plasterboard tents, air gaps are not very good insulation compared to PIR!) Obviously, draped felt wouldn't fit in a 25mm gap..

    Convection (ventilation) worsens the insulation value of the air gap, unless it is such a gale that it can physically cool the slates layer down close to room temperature (hard to imagine) - in which case the stack effect stops, so there's nothing to drive the gale. There will already be a better convection rising up the outside of the slates, so one on the inside cannot dramatically change things.
    • CommentAuthortony
    • CommentTimeNov 17th 2020
     
    Re blinds, definitely use external blinds otherwise heat comes through the glass and is inside the room
    • CommentAuthorJeff B
    • CommentTimeNov 17th 2020
     
    We have 75mm of Celotex as IWI in our dormer bedrooms (including the sloping ceilings) plus 75mm on the cold side of the dwarf walls and 75mm between the rafters above the sloping ceilings yet these rooms still get very hot in summer. I hadn't thought about Tony's point about the "underfloor heating" effect so it will be interesting to see what effect my recent blocking off the spaces between the joists will have next summer. Certainly it has made a difference to the "underfloor cooling" we used to get - no more creaky flooring on cold windy nights and no rads turned on upstairs now. The Velux windows face more or less due west so do get hot by mid afternoon. Can't afford external blinds though.
    • CommentAuthortony
    • CommentTimeNov 17th 2020
     
    West facing windows cause the worst overheating problems roof windows are slightly less bad
    • CommentAuthorJeff B
    • CommentTimeNov 17th 2020
     
    Posted By: tonyWest facing windows cause the worst overheating problems roof windows are slightly less bad


    Why is that Tony? Intuitively I would have expected south-facing to be the worst.
  5.  
    Would an extractor fan on a thermostat, set to come on over a certain temp. help. If I recall correctly Gotanewlife, formally of this parish, had an extractor fan set to come on when inside was hotter than outside as his way of reducing the temp over night in his high thermal mass pile of stones.
    • CommentAuthorbhommels
    • CommentTimeNov 17th 2020
     
    Posted By: WillInAberdeenSome numbers:
    Let's assume 30m2 of roof, U=0.2, slates are heated to 40degC for 8 hours/day, 30m3 of room air that starts out at 20degC.

    Somebody local pointed an IR camera at their slates last summer and they reached 75 deg C(!) in the afternoon of a sunny day. Too hot to the touch is already 50-60 degrees, so I think your assumption needs to be adjusted upwards.
    The conclusion is the same though.
    • CommentAuthorRobL
    • CommentTimeNov 17th 2020
     
    PV panels would make a radical difference to the tile hence room temperature, as they are very effective at shading and ventilation. They make power too :-)
    • CommentAuthortony
    • CommentTimeNov 17th 2020
     
    West facing windows catch a lot of sun when it is getting lower and so the window catches a lot of solar radiation. Further the house has already warmed up so tends to overheat.
  6.  
    Posted By: RobLPV panels would make a radical difference to the tile hence room temperature, as they are very effective at shading and ventilation. They make power too :-)

    A nice idea
    but
    Posted By: fostertomA well built Victorian Listed Building

    so little to no chance of PP
    • CommentAuthorJeff B
    • CommentTimeNov 17th 2020
     
    Posted By: tonyWest facing windows catch a lot of sun when it is getting lower and so the window catches a lot of solar radiation. Further the house has already warmed up so tends to overheat.


    OK.
    • CommentAuthortony
    • CommentTimeNov 18th 2020
     
    I am concerned about the present case to know whether the insulation is touching the plasterboard or if there a gap between them. This will cause thermal bypass cooling the room in winter BUT this same thermal bypass will heat or overheat the room in summer

    Tis is the most likely explanation
    • CommentAuthorGreenPaddy
    • CommentTimeNov 18th 2020
     
    My thought process would be (dubious :bigsmile:)...

    1. Adding more insulation (tricky as you start getting into purlin depth / losing headroom) will reduce the heat in summer, so that there will be fewer overheating days, but you'll never eliminate it, as there will still be heat coming into an already warm room.

    2. There isn't much thermal mass to flatten those peaks.

    3. Removing the excess heat in the room with fans - you're repalcing hot air with warm air during the day. Cooler outside nightime air will help, but with low thermal mass, will soon reheat.

    4. Removing the excess heat in the room with aircon, has it's obvious cost/noise/nongreen issues, but may be part of the solution, as can also heat for winter.

    5. PV giving slate shading would be great I suspect, but listed victorian !! That's going the right direction by eliminating the source, and utilising the energy.

    6. Forced air up the underslate cavity, removing some of the heat at source, probably not that effective, and prohibitively expensive.

    So I'm thinking, what else would remove heat at source, but have some green credentials - UnderSlateHeating (as opposed to UFH), to feed a hot water cylinder.

    From the "VikingHouse" school of thought, run pipes in a serpentine fashion in the external skin - in this case the gap between slating battens and rafters, starting at the ridge, running back and forth across the roof, always falling, till you get to the eaves. Add your relfective membrane immed under (rafter face to rafter face) to help keep heat in the slate/pipe zone. Then ventilated air gap, followed by insulation.

    A simple solar thermal controller, allowing drainback (I love drainback for it's simplicity, and is what I have always installed). That removes heat at source, but also captures it for re-use.

    - The labour costs might be high/fiddly pipe install, but materials shouldn't be that great? DIY the pipe install?
    - What pipe type to withstand high slate temperatures?
    - Connecting into the existing, or adding a new hot water storage cyl might be an issue for the client but not technically difficult?

    Above could be complete rubbish, or might trigger thoughts by others.
  7.  
    A question for fostertom - what is the budget to fix this problem?
    •  
      CommentAuthorfostertom
    • CommentTimeNov 18th 2020
     
    Guys, I am overwhelmed by the response! Will answer shortly.

    Budget - enough to do whatever's necessary, I think. But scope - we won't be taking the slates off, hence lack of underslating felt will remain.
  8.  
    Hi GP,
    Re : solar thermal pipe system under the slates:
    Sounds like a great idea, I mentioned it in 3rd post of this thread based on pics FosterTom has posted previously of his earlier projects.

    But thinking about it again, a day's heat load (calculated above and as commented by Bhommel), is way too much to be absorbed by a solar hot water cylinder. And you want your solar DHW collector to be hot 80deg+ to deliver usably hot enough DHW, whereas here we need the slates to become much cooler eg 30degC.

    VH and FT previous projects had large roof solar collectors matched with large dump loops in the foundation slab or the EWI'd walls (much bigger and cooler heatsinks) - would be difficult for listed building.

    Maybe the slates' thermal collector pipe could dump heat into a ground loop or borehole in the garden (if any?), which would be reversed in winter to recover the heat into a GSHP?

    FT can't take the slates off so the pipe would have to be fixed inboard of the battens. Maybe supported in those alu UFH spreader plates? Wouldn't drainback so needs antifreeze and expansion kit.

    Otherwise a combination of more insulation and thermal mass sounds like the way to go. If the roof U value were reduced from 0.5 to 0.1 then the problem is reduced 80%. How about adding another 100-150mm of PIR and then 50-100mm of insulating plaster?
    • CommentAuthortony
    • CommentTimeNov 18th 2020
     
    In the past I have installed fans to suck warmed out of lofts in the autumn and spring

    Also air vents and big powerful 6" fans to expel warmed air from the lofts, generally by sucking outdoor air in - outdoor air is cooler than loft air
  9.  
    Posted By: RobLPV panels would make a radical difference to the tile hence room temperature, as they are very effective at shading and ventilation. They make power too :-)

    If PV helps because of the shading and ventilation they provide then would not having (very) good ventilation at eaves and ridge also work carrying away the heat between the slates and the insulation? And probably fairly cheap to apply - and combine this with better insulation inside, although this would be rather invasive.

    How much of the bedrooms is skeiling ceiling and how much is normal loft space?
    • CommentAuthorJeff B
    • CommentTimeNov 18th 2020
     
    Re: the solar thermal pipe idea: I have installed a large double radiator outside to get rid of excess heat from my solar thermal system, having no other way to disperse the excess heat. Would this help in FosterTom's situation?

    P.S. I suspect my neighbours think I am completely bonkers.
    • CommentAuthordereke
    • CommentTimeNov 19th 2020
     
    @jeffb

    after one of the super hot days this year I bought a car radiator. The plan (didn't quite get around to it) was to have the radiator and a fan in the loft conversion, pipes running out the velux windows to the paddling pool on the patio and pump the water around. Hopefully next year I'll have time to try it out.

    The other way I keep that room cooler is to wrap one of those emergency foil blankets around the outside of the window. I did measure it at the time, I think it was like 20C cooler than just having the internal blinds down. I think it would perform better with an air gap between the foil and the window, might try that next year.

    For a more useful contribution I have read that woodfibre is better than PIR to prevent overheating. Something about "decrement delay". I can't confirm this but I am very interested to hear what other people think about this. Here is one place I've read it but I do remember reading this at other sources as well https://woodfibreinsulation.co.uk/2-overheating/
   
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