How to Insulate a Victorian House Using Rigid Insulation Boards

If you live in an uninsulated Victorian house with a cellar, did you know that each room on the ground floor effectively contains an open window about 50cm square leading straight onto the (probably unheated) cellar below? No wonder it’s a bit draughty around the ankles :-)

What causes this? The gaps between the floorboards, and between the skirting and the floor, that’s what. If you have thick carpet you won’t notice so much - but if you have bare floorboards you’ll jolly well feel it. If you work out how many gaps you have, how long they are, and how wide each is (could be up to 1cm in some cases) and then calculate the open area, boom! That’s your open window onto the cellar. Brrrr!

There are a few different things you can do about that ground floor chill factor. I’m going to talk through three different techniques that we’ve tried with varying success: insulating the floor from above (the best method), insulating from below (effective but dreadful to install), and filling the gaps between the floorboards (stops draughts but doesn’t ultimately insulate).

Insulating an old house is a tricky business - I’ve written more here about the extensive research we conducted. They are designed to be a bit draughty - over-zealous prevention of air flow will lead to condensation and mould. If you insulate your ceilings and your walls and your floors and replace your windows and block up your chimney and cover up your breathable lime plaster walls with non-breathable modern plasterboard, you’ll have a problem.

We insulated our new walls when the builders put the extension in (using rigid Quinn-therm insulation boards), but we have never touched the original external walls.

You can just see the insulation sandwiched between the pillar walls. The builders also put Quinn-therm boards into the new floors.

For one thing - putting insulation against the wall and then plaster boarding over the top would make the room slightly smaller, and more importantly it would cover up the beautiful ornate cornices. Secondly, I worry that we’d end up with dripping condensation between the boards and the old walls, so we have never wanted to try it just in case. We have also never replaced the old windows, partly because of the cost, and partly because the old glass and wood is just so pretty. Instead, we use a really basic thermal film across all the windows that works a lot more effectively than you might think.

What we have done is insulate the flooring on the ground floor. We use a combination of rigid 150mm and 100mm Celotex insulation boards, and rockwool.

The first method we tried was insulating from below in the cellar. We did this because we thought taking all the boards up from above would damage them, so we thought it would be less destructive inserting the boards from below.

Wah wah wah. Insert sad trombone noise. From an insulation perspective, it works brilliantly. Our living room base temperature rose from 17 degrees to 20 degrees, which is amazing. However, it was absolute hell to install.

First of all, the plaster and lath ceiling has to come down so you can access the joists. This is a horrible job., with 145 years of dust raining down onto your head as you claw off hundreds of individually nailed on wooden strips. Grim.

Breaking Bad suit plus pink Hunter wellies. Standard.

You then have to cut the board and stuff it upwards into the gaps between the joists. Measuring is awful. Whacking into place is awful. Celotex dust hitting you in the face is awful. Coping with all the random wires is awful. I really don’t recommend taking this approach. Apart from anything else, the plaster and lath is actually pretty insulating itself, so it’s a shame to remove it. We still haven’t got round to boarding over the Celotex boards, which are still exposed in the cellar ceiling, but when we do we’ll be careful to ensure that we leave a breathable gap between the plaster board ceiling and the joists/Celotex (by using spacers) in order that the conveniently located ceiling-height air bricks can allow air to flow into the gap and circulate. If you don’t do this, and you block up the airflow entirely, you can end up with sweating joists. Not so good.

Cutting boards balanced on other boards while balancing on a step ladder.

So, what’s the alternative to insulating from below? Insulating from above, of course. The only downside is that you have to take the boards up. They come up quite easily if you prise them carefully. I recommend labelling them so you can remember what order they go back down in. When you put them back down again, you can either screw them in (quicker and cheaper, and you can take them up again easily, but doesn’t look as authentic), or hammer them in place with old fashioned nails with rectangular heads, just like the original ones (the original nails probably can’t be re-used as they get a bit bent when you pull the boards up). We tend to do a bit of both - new screws, and replica old nails. Be mindful that you are likely to snap a board or two in the process of prising them up, which you can repair with glue, or replace with a board from elsewhere in the house (or swap with a non-snapped board from the same room that would otherwise end up being hidden underneath the sofa).

Laying the new (old) boards on top of the insulation board. Kids helping out as always.

We found that you can use a patchwork of boards, squashing them together in a patchwork to fit the space, and shoving rockwool into the gaps. Every time you have a cut in the boards you obviously lower the insulation properties, but it’s a good way to get rid of the smaller offcuts instead of throwing them away.

Inserting a jigsaw of insulation board pieces and rockwool into an odd gap between the living room and the dining room. The bricks on the right are part of the wall so we didn’t fancy moving them.

A word on cutting Celotex - wear goggles, a face mask, a boiler suit, and a hat. The dust from rigid insulation board is absolutely horrid, and if it gets in your eye, it will hurt like a mother. Not an experience I plan to repeat. We found that using a sharp manual saw with smooth teeth worked well - better than an electric jigsaw. A big kitchen knife also works well - it seems to be the smoother the blade, the easier it is to cut. You can get special Celotox saws but we just used what we had already and it worked fine.

Oh, and the third technique - filling the floorboards. We have used a product called StopGap, which prevents draughts but doesn’t really insulate. If you want a warmer house, you need to do it properly by taking the floors up. StopGap is worth using though if you have kids, cos it stops them from dropping their tiny Playmobil pieces through the gaps in the floorboards!

We have now insulated the living room (from below), the playroom and boot room from above (by taking the boards up and re-laying them) and the kitchen and part of the dining room from above (while the builders were here and before the floor was laid). We still need to take up the dining room floor and the hall floor, but even with what we’ve done the temperature change is noticeable. Our house is warm and toasty and the heating bills are a lot less dreadful than they used to be. It’s not really about saving money, mind, because insulation board isn’t particularly cheap - it’s more about having a warm house instead of sitting shivering in your living room that never seems to warm up.

Any questions, ask below.